{
  "index": "1948-B-2",
  "type": "GEO",
  "tag": [
    "GEO",
    "ANA"
  ],
  "difficulty": "",
  "question": "2. \"A penny in a corner.\" A circle moves so that it is continually in contact with all three coordinate planes of an ordinary rectangular system. Find the locus of the center of the circle.",
  "solution": "Solution. As \\( \\theta \\) varies, the maximum and minimum values of \\( a \\cos \\theta+ \\) \\( b \\sin \\theta \\) are \\( \\sqrt{a^{2}+b^{2}} \\) and \\( -\\sqrt{a^{2}+b^{2}} \\). Hence, in order that zero should be an extreme value of the function\n\\[\na \\cos \\theta+b \\sin \\theta+c,\n\\]\nit is necessary and sufficient that \\( c^{2}=a^{2}+b^{2} \\).\nLet the moving circle \\( C \\) have radius \\( r \\) and center \\( \\mathrm{x}=\\left(x_{1}, x_{2}, x_{3}\\right) \\). Let \\( \\mathbf{u}, \\mathbf{v}, \\mathbf{w} \\) be mutually orthogonal unit vectors such that \\( \\mathbf{u} \\) is perpendicular to the plane of \\( C \\). Then \\( C \\) consists of points of the form\n\\[\nr \\cos \\theta \\mathbf{v}+r \\sin \\theta \\mathbf{w}+\\mathbf{x} .\n\\]\n\nSince \\( C \\) is tangent to each of the coordinate planes, each of the functions\n\\[\nr v_{i} \\cos \\theta+r w_{i} \\sin \\theta+x_{i}\n\\]\nis zero for some value of \\( \\theta \\), but otherwise takes only positive, or only negative, values. Hence\n\\[\nx_{i}{ }^{2}=r^{2} v_{i}{ }^{2}+r^{2} w_{i}{ }^{2}\n\\]\nby the result stated in the first paragraph. Therefore,\n\\[\nx_{1}{ }^{2}+x_{2}{ }^{2}+x_{3}{ }^{2}=r^{2}\\left(\\nu_{1}{ }^{2}+v_{2}{ }^{2}+\\nu_{3}{ }^{2}\\right)+r^{2}\\left(w_{1}{ }^{2}+w_{2}^{2}+w_{3}{ }^{2}\\right)=2 r^{2}\n\\]\nbecause \\( \\mathbf{v} \\) and \\( \\mathbf{w} \\) are unit vectors. Thus \\( \\mathbf{x} \\) lies on the sphere \\( S \\) with center at the origin and radius \\( r \\sqrt{2} \\). Moreover, since \\( \\mathbf{u}, \\mathbf{v}, \\mathbf{w} \\) are mutually orthogonal unit vectors,\n\\[\nu_{i}{ }^{2}+v_{i}{ }^{2}+w_{i}{ }^{2}=1 \\quad \\text { for } i=1,2,3 .\n\\]\n\nHence (3) can be written\n\\[\nx_{i}{ }^{2}=r^{2}\\left(1-u_{i}^{2}\\right),\n\\]\nand it follows that\n\\[\nx_{i}{ }^{2} \\leq r^{2} \\text { for } i=1,2,3 .\n\\]\n\nConversely, suppose \\( \\mathbf{x} \\) is any point of \\( S \\) that satisfies (5). Then we can solve (4) for \\( u_{1}, u_{2}, u_{3} \\) and \\( \\mathbf{u}=\\left(u_{1}, u_{2}, u_{3}\\right) \\) will be a unit vector. We can then choose \\( \\mathbf{v} \\) and \\( \\mathbf{w} \\) so that \\( \\mathbf{u}, \\mathbf{v}, \\mathbf{w} \\) are mutually orthogonal unit vectors, and equation (3) will hold. The functions (2) will therefore have zero as an extreme value, so the circle given parametrically by (1) will be tangent to each of the coordinate planes. (In accord with the image of a \"penny in a corner,\" we consider a circle lying wholly in a coordinate plane as tangent to that plane.)\n\nWe conclude that the required locus is that part of the sphere \\( S \\) within the cube \\( \\left|x_{i}\\right| \\leq r, i=1,2,3 \\). (If a circle lying in a coordinate plane is not regarded as tangent to that plane, then 12 points such as ( \\( \\pm r, \\pm r, 0 \\) ) must be deleted.)\n\nRemark. For most points \\( \\mathbf{x} \\) of the locus there will be eight choices of the unit vector \\( \\mathbf{u}\\left(u_{i}= \\pm \\sqrt{1-x_{i}{ }^{2} / r^{2}}\\right. \\) ) leading to four choices of a plane through \\( \\mathbf{x} \\) and thus to four circles of radius \\( r \\) with center at \\( \\mathbf{x} \\) and tangent to all three coordinate planes. Of these circles, one will be inscribed in the triangle defined in its plane by the three coordinate planes, and the other three circles will be escribed to the corresponding triangles.",
  "vars": [
    "x",
    "x_1",
    "x_2",
    "x_3",
    "x_i",
    "i",
    "u",
    "u_i",
    "u_1",
    "u_2",
    "u_3",
    "v",
    "v_i",
    "v_1",
    "v_2",
    "v_3",
    "w",
    "w_i",
    "w_1",
    "w_2",
    "w_3",
    "C",
    "\\\\theta",
    "\\\\nu",
    "\\\\nu_1",
    "\\\\nu_3"
  ],
  "params": [
    "r",
    "S"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "centervec",
        "x_1": "centerone",
        "x_2": "centertwo",
        "x_3": "centerthree",
        "x_i": "centerindex",
        "i": "indexvar",
        "u": "normvec",
        "u_i": "normindex",
        "u_1": "normone",
        "u_2": "normtwo",
        "u_3": "normthree",
        "v": "orthovecv",
        "v_i": "orthoindexv",
        "v_1": "orthoonev",
        "v_2": "orthotwov",
        "v_3": "orthothreev",
        "w": "orthovecw",
        "w_i": "orthoindexw",
        "w_1": "orthoonew",
        "w_2": "orthotwow",
        "w_3": "orthothreew",
        "C": "circleobj",
        "\\theta": "anglevar",
        "\\nu": "greeknu",
        "\\nu_1": "greeknuone",
        "\\nu_3": "greeknuthree",
        "r": "fixedradius",
        "S": "sphereset"
      },
      "question": "2. \"A penny in a corner.\" A circle moves so that it is continually in contact with all three coordinate planes of an ordinary rectangular system. Find the locus of the center of the circle.",
      "solution": "Solution. As \\( anglevar \\) varies, the maximum and minimum values of \\( a \\cos anglevar+ \\) \\( b \\sin anglevar \\) are \\( \\sqrt{a^{2}+b^{2}} \\) and \\( -\\sqrt{a^{2}+b^{2}} \\). Hence, in order that zero should be an extreme value of the function\n\\[\na \\cos anglevar+b \\sin anglevar+c,\n\\]\nit is necessary and sufficient that \\( c^{2}=a^{2}+b^{2} \\).\nLet the moving circle \\( circleobj \\) have radius \\( fixedradius \\) and center \\( \\mathrm{centervec}=\\left(centerone, centertwo, centerthree\\right) \\). Let \\( \\mathbf{normvec}, \\mathbf{orthovecv}, \\mathbf{orthovecw} \\) be mutually orthogonal unit vectors such that \\( \\mathbf{normvec} \\) is perpendicular to the plane of \\( circleobj \\). Then \\( circleobj \\) consists of points of the form\n\\[\nfixedradius \\cos anglevar \\mathbf{orthovecv}+fixedradius \\sin anglevar \\mathbf{orthovecw}+\\mathbf{centervec} .\n\\]\n\nSince \\( circleobj \\) is tangent to each of the coordinate planes, each of the functions\n\\[\nfixedradius \\, orthoindexv \\cos anglevar+fixedradius \\, orthoindexw \\sin anglevar+centerindex\n\\]\nis zero for some value of \\( anglevar \\), but otherwise takes only positive, or only negative, values. Hence\n\\[\ncenterindex^{2}=fixedradius^{2} \\, orthoindexv^{2}+fixedradius^{2} \\, orthoindexw^{2}\n\\]\nby the result stated in the first paragraph. Therefore,\n\\[\ncenterone^{2}+centertwo^{2}+centerthree^{2}=fixedradius^{2}\\left(greeknuone^{2}+orthotwov^{2}+greeknuthree^{2}\\right)+fixedradius^{2}\\left(orthoonew^{2}+orthotwow^{2}+orthothreew^{2}\\right)=2\\,fixedradius^{2}\n\\]\nbecause \\( \\mathbf{orthovecv} \\) and \\( \\mathbf{orthovecw} \\) are unit vectors. Thus \\( \\mathbf{centervec} \\) lies on the sphere \\( sphereset \\) with center at the origin and radius \\( fixedradius \\sqrt{2} \\). Moreover, since \\( \\mathbf{normvec}, \\mathbf{orthovecv}, \\mathbf{orthovecw} \\) are mutually orthogonal unit vectors,\n\\[\nnormindex^{2}+orthoindexv^{2}+orthoindexw^{2}=1 \\quad \\text { for } indexvar=1,2,3 .\n\\]\n\nHence (3) can be written\n\\[\ncenterindex^{2}=fixedradius^{2}\\left(1-normindex^{2}\\right),\n\\]\nand it follows that\n\\[\ncenterindex^{2} \\leq fixedradius^{2} \\text { for } indexvar=1,2,3 .\n\\]\n\nConversely, suppose \\( \\mathbf{centervec} \\) is any point of \\( sphereset \\) that satisfies (5). Then we can solve (4) for \\( normone, normtwo, normthree \\) and \\( \\mathbf{normvec}=\\left(normone, normtwo, normthree\\right) \\) will be a unit vector. We can then choose \\( \\mathbf{orthovecv} \\) and \\( \\mathbf{orthovecw} \\) so that \\( \\mathbf{normvec}, \\mathbf{orthovecv}, \\mathbf{orthovecw} \\) are mutually orthogonal unit vectors, and equation (3) will hold. The functions (2) will therefore have zero as an extreme value, so the circle given parametrically by (1) will be tangent to each of the coordinate planes. (In accord with the image of a \"penny in a corner,\" we consider a circle lying wholly in a coordinate plane as tangent to that plane.)\n\nWe conclude that the required locus is that part of the sphere \\( sphereset \\) within the cube \\( \\left|centerindex\\right| \\leq fixedradius, indexvar=1,2,3 \\). (If a circle lying in a coordinate plane is not regarded as tangent to that plane, then 12 points such as ( \\( \\pm fixedradius, \\pm fixedradius, 0 \\) ) must be deleted.)\n\nRemark. For most points \\( \\mathbf{centervec} \\) of the locus there will be eight choices of the unit vector \\( \\mathbf{normvec}\\left(normindex= \\pm \\sqrt{1-centerindex^{2} / fixedradius^{2}}\\right. \\) ) leading to four choices of a plane through \\( \\mathbf{centervec} \\) and thus to four circles of radius \\( fixedradius \\) with center at \\( \\mathbf{centervec} \\) and tangent to all three coordinate planes. Of these circles, one will be inscribed in the triangle defined in its plane by the three coordinate planes, and the other three circles will be escribed to the corresponding triangles."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "sandcastle",
        "x_1": "sandcastleone",
        "x_2": "sandcastletwo",
        "x_3": "sandcastlethree",
        "x_i": "sandcastleindex",
        "i": "windmill",
        "u": "daydream",
        "u_i": "daydreamindex",
        "u_1": "daydreamone",
        "u_2": "daydreamtwo",
        "u_3": "daydreamthree",
        "v": "lighthouse",
        "v_i": "lighthouseindex",
        "v_1": "lighthouseone",
        "v_2": "lighthousetwo",
        "v_3": "lighthousethree",
        "w": "treetop",
        "w_i": "treetopindex",
        "w_1": "treetopone",
        "w_2": "treetoptwo",
        "w_3": "treetopthree",
        "C": "seashell",
        "\\\\theta": "raindrop",
        "\\\\nu": "whistle",
        "\\\\nu_1": "whistleone",
        "\\\\nu_3": "whistlethree",
        "r": "compass",
        "S": "labyrinth"
      },
      "question": "2. \"A penny in a corner.\" A circle moves so that it is continually in contact with all three coordinate planes of an ordinary rectangular system. Find the locus of the center of the circle.",
      "solution": "Solution. As \\( raindrop \\) varies, the maximum and minimum values of \\( a \\cos raindrop+ \\) \\( b \\sin raindrop \\) are \\( \\sqrt{a^{2}+b^{2}} \\) and \\( -\\sqrt{a^{2}+b^{2}} \\). Hence, in order that zero should be an extreme value of the function\n\\[\na \\cos raindrop+b \\sin raindrop+c,\n\\]\nit is necessary and sufficient that \\( c^{2}=a^{2}+b^{2} \\).\n\nLet the moving circle \\( seashell \\) have radius \\( compass \\) and center \\( \\mathrm{sandcastle}=\\left(sandcastleone, sandcastletwo, sandcastlethree\\right) \\). Let \\( \\mathbf{daydream}, \\mathbf{lighthouse}, \\mathbf{treetop} \\) be mutually orthogonal unit vectors such that \\( \\mathbf{daydream} \\) is perpendicular to the plane of \\( seashell \\). Then \\( seashell \\) consists of points of the form\n\\[\ncompass \\cos raindrop \\, \\mathbf{lighthouse}+compass \\sin raindrop \\, \\mathbf{treetop}+\\mathbf{sandcastle} .\n\\]\n\nSince \\( seashell \\) is tangent to each of the coordinate planes, each of the functions\n\\[\ncompass \\, lighthouseindex \\cos raindrop+compass \\, treetopindex \\sin raindrop+sandcastleindex\n\\]\nis zero for some value of \\( raindrop \\), but otherwise takes only positive, or only negative, values. Hence\n\\[\nsandcastleindex { }^{2}=compass^{2} \\, lighthouseindex { }^{2}+compass^{2} \\, treetopindex { }^{2}\n\\]\nby the result stated in the first paragraph. Therefore,\n\\[\nsandcastleone { }^{2}+sandcastletwo { }^{2}+sandcastlethree { }^{2}=compass^{2}\\left(lighthouseone { }^{2}+lighthousetwo { }^{2}+lighthousethree { }^{2}\\right)+compass^{2}\\left(treetopone { }^{2}+treetoptwo^{2}+treetopthree { }^{2}\\right)=2 compass^{2}\n\\]\nbecause \\( \\mathbf{lighthouse} \\) and \\( \\mathbf{treetop} \\) are unit vectors. Thus \\( \\mathbf{sandcastle} \\) lies on the sphere \\( labyrinth \\) with center at the origin and radius \\( compass \\sqrt{2} \\). Moreover, since \\( \\mathbf{daydream}, \\mathbf{lighthouse}, \\mathbf{treetop} \\) are mutually orthogonal unit vectors,\n\\[\ndaydreamindex { }^{2}+lighthouseindex { }^{2}+treetopindex { }^{2}=1 \\quad \\text { for } windmill=1,2,3 .\n\\]\n\nHence (3) can be written\n\\[\nsandcastleindex { }^{2}=compass^{2}\\left(1-daydreamindex^{2}\\right),\n\\]\nand it follows that\n\\[\nsandcastleindex { }^{2} \\leq compass^{2} \\text { for } windmill=1,2,3 .\n\\]\n\nConversely, suppose \\( \\mathbf{sandcastle} \\) is any point of \\( labyrinth \\) that satisfies (5). Then we can solve (4) for \\( daydreamone, daydreamtwo, daydreamthree \\) and \\( \\mathbf{daydream}=\\left(daydreamone, daydreamtwo, daydreamthree\\right) \\) will be a unit vector. We can then choose \\( \\mathbf{lighthouse} \\) and \\( \\mathbf{treetop} \\) so that \\( \\mathbf{daydream}, \\mathbf{lighthouse}, \\mathbf{treetop} \\) are mutually orthogonal unit vectors, and equation (3) will hold. The functions (2) will therefore have zero as an extreme value, so the circle given parametrically by (1) will be tangent to each of the coordinate planes. (In accord with the image of a \"penny in a corner,\" we consider a circle lying wholly in a coordinate plane as tangent to that plane.)\n\nWe conclude that the required locus is that part of the sphere \\( labyrinth \\) within the cube \\( \\left|sandcastleindex\\right| \\leq compass, windmill=1,2,3 \\). (If a circle lying in a coordinate plane is not regarded as tangent to that plane, then 12 points such as ( \\( \\pm compass, \\pm compass, 0 \\) ) must be deleted.)\n\nRemark. For most points \\( \\mathbf{sandcastle} \\) of the locus there will be eight choices of the unit vector \\( \\mathbf{daydream}\\left(daydreamindex= \\pm \\sqrt{1-sandcastleindex { }^{2} / compass^{2}}\\right. \\) ) leading to four choices of a plane through \\( \\mathbf{sandcastle} \\) and thus to four circles of radius \\( compass \\) with center at \\( \\mathbf{sandcastle} \\) and tangent to all three coordinate planes. Of these circles, one will be inscribed in the triangle defined in its plane by the three coordinate planes, and the other three circles will be escribed to the corresponding triangles."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "boundarypoint",
        "x_1": "edgecoordone",
        "x_2": "edgecoordtwo",
        "x_3": "edgecoordthr",
        "x_i": "edgecoordidx",
        "i": "wholeindex",
        "u": "parallelvector",
        "u_i": "parallelvecidx",
        "u_1": "parallelvecone",
        "u_2": "parallelvectwo",
        "u_3": "parallelvecthr",
        "v": "normalvector",
        "v_i": "normalvecidx",
        "v_1": "normalvecone",
        "v_2": "normalvectwo",
        "v_3": "normalvecthr",
        "w": "staticvector",
        "w_i": "staticvecidx",
        "w_1": "staticvecone",
        "w_2": "staticvectwo",
        "w_3": "staticvecthr",
        "C": "squarecurve",
        "\\theta": "distanceparam",
        "\\nu": "randomvalue",
        "\\nu_1": "randomvalone",
        "\\nu_3": "randomvalthr",
        "r": "diameterlen",
        "S": "planarspace"
      },
      "question": "2. \"A penny in a corner.\" A circle moves so that it is continually in contact with all three coordinate planes of an ordinary rectangular system. Find the locus of the center of the circle.",
      "solution": "Solution. As \\( distanceparam \\) varies, the maximum and minimum values of \\( a \\cos distanceparam+ \\) \\( b \\sin distanceparam \\) are \\( \\sqrt{a^{2}+b^{2}} \\) and \\( -\\sqrt{a^{2}+b^{2}} \\). Hence, in order that zero should be an extreme value of the function\n\\[\na \\cos distanceparam+b \\sin distanceparam+c,\n\\]\nit is necessary and sufficient that \\( c^{2}=a^{2}+b^{2} \\).\nLet the moving circle \\( squarecurve \\) have radius \\( diameterlen \\) and center \\( \\mathrm{boundarypoint}=\\left(edgecoordone, edgecoordtwo, edgecoordthr\\right) \\). Let \\( \\mathbf{parallelvector}, \\mathbf{normalvector}, \\mathbf{staticvector} \\) be mutually orthogonal unit vectors such that \\( \\mathbf{parallelvector} \\) is perpendicular to the plane of \\( squarecurve \\). Then \\( squarecurve \\) consists of points of the form\n\\[\ndiameterlen \\cos distanceparam \\mathbf{normalvector}+diameterlen \\sin distanceparam \\mathbf{staticvector}+\\mathbf{boundarypoint} .\n\\]\n\nSince \\( squarecurve \\) is tangent to each of the coordinate planes, each of the functions\n\\[\ndiameterlen normalvecidx \\cos distanceparam+diameterlen staticvecidx \\sin distanceparam+edgecoordidx\n\\]\nis zero for some value of \\( distanceparam \\), but otherwise takes only positive, or only negative, values. Hence\n\\[\nedgecoordidx{ }^{2}=diameterlen^{2} normalvecidx{ }^{2}+diameterlen^{2} staticvecidx{ }^{2}\n\\]\nby the result stated in the first paragraph. Therefore,\n\\[\nedgecoordone{ }^{2}+edgecoordtwo{ }^{2}+edgecoordthr{ }^{2}=diameterlen^{2}\\left(randomvalone{ }^{2}+normalvectwo{ }^{2}+randomvalthr{ }^{2}\\right)+diameterlen^{2}\\left(staticvecone{ }^{2}+staticvectwo^{2}+staticvecthr{ }^{2}\\right)=2\\, diameterlen^{2}\n\\]\nbecause \\( \\mathbf{normalvector} \\) and \\( \\mathbf{staticvector} \\) are unit vectors. Thus \\( \\mathbf{boundarypoint} \\) lies on the sphere \\( planarspace \\) with center at the origin and radius \\( diameterlen \\sqrt{2} \\). Moreover, since \\( \\mathbf{parallelvector}, \\mathbf{normalvector}, \\mathbf{staticvector} \\) are mutually orthogonal unit vectors,\n\\[\nparallelvecidx{ }^{2}+normalvecidx{ }^{2}+staticvecidx{ }^{2}=1 \\quad \\text { for } wholeindex=1,2,3 .\n\\]\n\nHence (3) can be written\n\\[\nedgecoordidx{ }^{2}=diameterlen^{2}\\left(1-parallelvecidx^{2}\\right),\n\\]\nand it follows that\n\\[\nedgecoordidx{ }^{2} \\leq diameterlen^{2} \\text { for } wholeindex=1,2,3 .\n\\]\n\nConversely, suppose \\( \\mathbf{boundarypoint} \\) is any point of \\( planarspace \\) that satisfies (5). Then we can solve (4) for \\( parallelvecone, parallelvectwo, parallelvecthr \\) and \\( \\mathbf{parallelvector}=\\left(parallelvecone, parallelvectwo, parallelvecthr\\right) \\) will be a unit vector. We can then choose \\( \\mathbf{normalvector} \\) and \\( \\mathbf{staticvector} \\) so that \\( \\mathbf{parallelvector}, \\mathbf{normalvector}, \\mathbf{staticvector} \\) are mutually orthogonal unit vectors, and equation (3) will hold. The functions (2) will therefore have zero as an extreme value, so the circle given parametrically by (1) will be tangent to each of the coordinate planes. (In accord with the image of a \"penny in a corner,\" we consider a circle lying wholly in a coordinate plane as tangent to that plane.)\n\nWe conclude that the required locus is that part of the sphere \\( planarspace \\) within the cube \\( \\left|edgecoordidx\\right| \\leq diameterlen, wholeindex=1,2,3 \\). (If a circle lying in a coordinate plane is not regarded as tangent to that plane, then 12 points such as ( \\( \\pm diameterlen, \\pm diameterlen, 0 \\) ) must be deleted.)\n\nRemark. For most points \\( \\mathbf{boundarypoint} \\) of the locus there will be eight choices of the unit vector \\( \\mathbf{parallelvector}\\left(parallelvecidx= \\pm \\sqrt{1-edgecoordidx{ }^{2} / diameterlen^{2}}\\right. \\) ) leading to four choices of a plane through \\( \\mathbf{boundarypoint} \\) and thus to four circles of radius \\( diameterlen \\) with center at \\( \\mathbf{boundarypoint} \\) and tangent to all three coordinate planes. Of these circles, one will be inscribed in the triangle defined in its plane by the three coordinate planes, and the other three circles will be escribed to the corresponding triangles."
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "x_1": "hjgrksla",
        "x_2": "pmxudqye",
        "x_3": "nclobfzw",
        "x_i": "tsvrekmu",
        "i": "wegfnopa",
        "u": "ybzicvse",
        "u_i": "mgphdqxz",
        "u_1": "rjksuqtw",
        "u_2": "fldvzgac",
        "u_3": "kwtnephs",
        "v": "oaygtrmn",
        "v_i": "lfhqdzse",
        "v_1": "ziboqstx",
        "v_2": "ojcphnrd",
        "v_3": "gkmuylva",
        "w": "bsizqofx",
        "w_i": "cnmwpraj",
        "w_1": "exrklvut",
        "w_2": "ydqvofkm",
        "w_3": "hvasplji",
        "C": "dqwmxzrt",
        "\\theta": "pasdikuq",
        "\\nu": "kobvhjwe",
        "\\nu_1": "rqkhdpac",
        "\\nu_3": "esfyvlam",
        "r": "ztcpwelh",
        "S": "jodmnyvl"
      },
      "question": "2. \"A penny in a corner.\" A circle moves so that it is continually in contact with all three coordinate planes of an ordinary rectangular system. Find the locus of the center of the circle.",
      "solution": "Solution. As \\( pasdikuq \\) varies, the maximum and minimum values of \\( a \\cos pasdikuq+ \\) \\( b \\sin pasdikuq \\) are \\( \\sqrt{a^{2}+b^{2}} \\) and \\( -\\sqrt{a^{2}+b^{2}} \\). Hence, in order that zero should be an extreme value of the function\n\\[\na \\cos pasdikuq+b \\sin pasdikuq+c,\n\\]\nit is necessary and sufficient that \\( c^{2}=a^{2}+b^{2} \\).\n\nLet the moving circle \\( dqwmxzrt \\) have radius \\( ztcpwelh \\) and center \\( \\mathrm{qzxwvtnp}=\\left(hjgrksla, pmxudqye, nclobfzw\\right) \\). Let \\( \\mathbf{ybzicvse}, \\mathbf{oaygtrmn}, \\mathbf{bsizqofx} \\) be mutually orthogonal unit vectors such that \\( \\mathbf{ybzicvse} \\) is perpendicular to the plane of \\( dqwmxzrt \\). Then \\( dqwmxzrt \\) consists of points of the form\n\\[\nztcpwelh \\cos pasdikuq \\mathbf{oaygtrmn}+ztcpwelh \\sin pasdikuq \\mathbf{bsizqofx}+\\mathbf{qzxwvtnp} .\n\\]\n\nSince \\( dqwmxzrt \\) is tangent to each of the coordinate planes, each of the functions\n\\[\nztcpwelh \\, lfhqdzse \\cos pasdikuq+ztcpwelh \\, cnmwpraj \\sin pasdikuq+tsvrekmu\n\\]\nis zero for some value of \\( pasdikuq \\), but otherwise takes only positive, or only negative, values. Hence\n\\[\ntsvrekmu^{2}=ztcpwelh^{2} \\, lfhqdzse^{2}+ztcpwelh^{2} \\, cnmwpraj^{2}\n\\]\nby the result stated in the first paragraph. Therefore,\n\\[\nhjgrksla^{2}+pmxudqye^{2}+nclobfzw^{2}=ztcpwelh^{2}\\left(rqkhdpac^{2}+ojcphnrd^{2}+esfyvlam^{2}\\right)+ztcpwelh^{2}\\left(exrklvut^{2}+ydqvofkm^{2}+hvasplji^{2}\\right)=2 \\, ztcpwelh^{2}\n\\]\nbecause \\( \\mathbf{oaygtrmn} \\) and \\( \\mathbf{bsizqofx} \\) are unit vectors. Thus \\( \\mathbf{qzxwvtnp} \\) lies on the sphere \\( jodmnyvl \\) with center at the origin and radius \\( ztcpwelh \\sqrt{2} \\). Moreover, since \\( \\mathbf{ybzicvse}, \\mathbf{oaygtrmn}, \\mathbf{bsizqofx} \\) are mutually orthogonal unit vectors,\n\\[\nmgphdqxz^{2}+lfhqdzse^{2}+cnmwpraj^{2}=1 \\quad \\text { for } wegfnopa=1,2,3 .\n\\]\n\nHence (3) can be written\n\\[\ntsvrekmu^{2}=ztcpwelh^{2}\\left(1-mgphdqxz^{2}\\right),\n\\]\nand it follows that\n\\[\ntsvrekmu^{2} \\leq ztcpwelh^{2} \\text { for } wegfnopa=1,2,3 .\n\\]\n\nConversely, suppose \\( \\mathbf{qzxwvtnp} \\) is any point of \\( jodmnyvl \\) that satisfies (5). Then we can solve (4) for \\( rjksuqtw, fldvzgac, kwtnephs \\) and \\( \\mathbf{ybzicvse}=\\left(rjksuqtw, fldvzgac, kwtnephs\\right) \\) will be a unit vector. We can then choose \\( \\mathbf{oaygtrmn} \\) and \\( \\mathbf{bsizqofx} \\) so that \\( \\mathbf{ybzicvse}, \\mathbf{oaygtrmn}, \\mathbf{bsizqofx} \\) are mutually orthogonal unit vectors, and equation (3) will hold. The functions (2) will therefore have zero as an extreme value, so the circle given parametrically by (1) will be tangent to each of the coordinate planes. (In accord with the image of a \"penny in a corner,\" we consider a circle lying wholly in a coordinate plane as tangent to that plane.)\n\nWe conclude that the required locus is that part of the sphere \\( jodmnyvl \\) within the cube \\( \\left|tsvrekmu\\right| \\leq ztcpwelh, \\, wegfnopa=1,2,3 \\). (If a circle lying in a coordinate plane is not regarded as tangent to that plane, then 12 points such as ( \\( \\pm ztcpwelh, \\pm ztcpwelh, 0 \\) ) must be deleted.)\n\nRemark. For most points \\( \\mathbf{qzxwvtnp} \\) of the locus there will be eight choices of the unit vector \\( \\mathbf{ybzicvse}\\left(mgphdqxz= \\pm \\sqrt{1-tsvrekmu^{2} / ztcpwelh^{2}}\\right. \\) ) leading to four choices of a plane through \\( \\mathbf{qzxwvtnp} \\) and thus to four circles of radius \\( ztcpwelh \\) with center at \\( \\mathbf{qzxwvtnp} \\) and tangent to all three coordinate planes. Of these circles, one will be inscribed in the triangle defined in its plane by the three coordinate planes, and the other three circles will be escribed to the corresponding triangles."
    },
    "kernel_variant": {
      "question": "Let  \n\n\\[\nn\\ge 4,\\qquad s>0 ,\\qquad \\varphi\\in(0,\\pi/2)\n\\]\n\nbe fixed and put  \n\n\\[\n\\mathbf C=(c_{1},\\dots ,c_{n})\\in\\mathbb R^{\\,n},\n\\qquad\n\\mathbf 1=(1,\\dots ,1)\\in\\mathbb R^{\\,n},\n\\qquad\n\\Pi_{i}\\;:\\;x_{i}=c_{i}\\quad(i=1,\\dots ,n).\n\\]\n\nFurther data are  \n\n\\[\n\\mathbf p,\\ \\mathbf v\\in\\mathbb R^{\\,n},\\qquad \n\\mathbf v\\neq\\mathbf 0,\\qquad \\mathbf v\\cdot\\mathbf 1=0,\n\\]\n\nand the straight line  \n\n\\[\n\\ell=\\{\\mathbf p+t\\mathbf v\\mid t\\in\\mathbb R\\}.\n\\]\n\n1.  (Circle)  \nA \\emph{circle of radius $s$} is a set  \n\n\\[\n\\Sigma=\\{\\mathbf O+s\\mathbf u\\mid\\mathbf u\\in S^{1}\\subset(A-\\mathbf O)\\},\n\\]\n\nwhere $A$ is an affine $2$–plane in $\\mathbb R^{\\,n}$, $\\mathbf O\\in A$ is the\ncentre, $S^{1}=\\{\\mathbf u\\in A-\\mathbf O\\mid \\lVert\\mathbf u\\rVert=1\\}$ is the Euclidean unit circle of the linear plane $A-\\mathbf O$.\n\n2.  (Admissibility)  \nThe circle $\\Sigma$ is called \\emph{admissible} if, simultaneously  \n\n(i)  (tangency) for every $i$\n      the circle is tangent to $\\Pi_{i}$ and lies entirely in the same\n      closed half–space of $\\Pi_{i}$ as the point $\\mathbf C$;  \n\n(ii) (angle condition) the acute angle between the vector $\\mathbf 1$\n      and the plane $A-\\mathbf O$ equals $\\varphi$\n      (that is, if $V:=A-\\mathbf O$ and\n      $\\operatorname{proj}_{V}\\mathbf 1$ denotes the orthogonal\n      projection of $\\mathbf 1$ on $V$, then\n      $\\lVert\\operatorname{proj}_{V}\\mathbf 1\\rVert\n      =\\lVert\\mathbf 1\\rVert\\cos\\varphi=\\sqrt n\\,\\cos\\varphi$);  \n\n(iii) (incidence) $\\ell\\subset A$.\n\n(a)  Determine explicitly the set  \n\n\\[\n\\mathcal L=\\mathcal L(n,s,\\varphi,\\mathbf C,\\mathbf p,\\mathbf v)\\subset\\mathbb R^{\\,n}\n\\]\n\nof all centres $\\mathbf O$ for which an admissible circle exists.\n\n(b)  For every $\\mathbf O\\in\\mathcal L$ describe explicitly the family  \n\n\\[\n\\mathcal O(\\mathbf O)\n\\]\n\nof all affine $2$–planes $A$ such that an admissible circle of centre\n$\\mathbf O$ lies in $A$.\n\nYour answers must depend only on the fixed data\n$n,s,\\varphi,c_{1},\\dots ,c_{n},\\mathbf p,\\mathbf v$\nand the point $\\mathbf O$ that is being tested.\n\n\n\n--------------------------------------------------------------------",
      "solution": "Throughout column vectors are written as columns and ${}^{\\mathsf T}$ denotes transposition.\n\n0.  Notation  \n\n\\[\n\\mathbf v_{0}:=\\frac{\\mathbf v}{\\lVert\\mathbf v\\rVert},\\qquad\n\\mathbf d:=\\frac{\\mathbf O-\\mathbf C}{s},\\qquad\nd_{i}:=\\frac{(\\mathbf O)_{i}-c_{i}}{s}\\quad(i=1,\\dots ,n).\n\\]\n\nLet $V:=A-\\mathbf O$ be the direction space of the required circle.  \nBecause of the incidence condition (iii) we have  \n\n\\[\n\\mathbf v_{0}\\in V\n\\]\n\nand we put  \n\n\\[\n\\mathbf e_{0}:=\\mathbf v_{0}\\qquad(\\lVert\\mathbf e_{0}\\rVert=1).\n\\]\n\nWrite  \n\n\\[\n\\mathbf q:=\\mathbf p-\\mathbf O=q_{\\parallel}\\mathbf e_{0}+\\mathbf q_{\\perp},\n\\qquad\nq_{\\parallel}:=\\mathbf q\\cdot\\mathbf e_{0},\\quad\n\\mathbf q_{\\perp}\\perp\\mathbf e_{0}.\n\\]\n\nIntroduce  \n\n\\[\nm:=\\begin{cases}\n1,&\\mathbf q_{\\perp}\\neq\\mathbf 0\\quad(\\text{``generic''}),\\\\[2pt]\n0,&\\mathbf q_{\\perp}=0\\quad(\\text{``exceptional''}).\n\\end{cases}\n\\]\n\nIf $m=1$ set  \n\n\\[\n\\mathbf e_{1}:=\\dfrac{\\mathbf q_{\\perp}}{\\lVert\\mathbf q_{\\perp}\\rVert},\n\\]\n\nso that $\\{\\mathbf e_{0},\\mathbf e_{1}\\}$ is an orthonormal basis of $V$.  \nIf $m=0$ we shall construct a second unit vector\n$\\mathbf u\\perp\\mathbf e_{0}$ later on.\n\nHence  \n\n\\[\nV=\n\\begin{cases}\n\\operatorname{span}\\{\\mathbf e_{0},\\mathbf e_{1}\\}, & m=1,\\\\[6pt]\n\\operatorname{span}\\{\\mathbf e_{0},\\mathbf u\\}, & m=0.\n\\end{cases}\n\\]\n\n-------------------------------------------------\n1.  Tangency to all $\\Pi_{i}$  \n\nTake first the generic case $m=1$.  \nA point of the circle is  \n\n\\[\n\\mathbf y(\\theta)=\\mathbf O+s\\bigl(\\cos\\theta\\,\\mathbf e_{0}\n+\\sin\\theta\\,\\mathbf e_{1}\\bigr),\\qquad\\theta\\in[0,2\\pi).\n\\]\n\nFix $i$.  Then  \n\n\\[\n\\frac{(\\mathbf y(\\theta))_{i}-c_{i}}{s}\n       =d_{i}+\\cos\\theta\\,(\\mathbf e_{0})_{i}\n                 +\\sin\\theta\\,(\\mathbf e_{1})_{i}.\n\\]\n\nThe real function\n$\\theta\\mapsto\\alpha\\cos\\theta+\\beta\\sin\\theta+\\gamma$\nattains $0$ as an \\emph{endpoint} of its range\niff $|\\gamma|=\\sqrt{\\alpha^{2}+\\beta^{2}}$;  \nthe sign of $\\gamma$ determines which endpoint is $0$,\nhence on which side of the hyper–plane the whole circle lies.\nConsequently, tangency (including the ``one-side'' requirement) gives  \n\n\\[\n\\boxed{\\;\nd_{i}=\\sigma_{i}\n\\sqrt{(\\mathbf e_{0})_{i}^{2}+(\\mathbf e_{1})_{i}^{2}},\\qquad\n\\sigma_{i}\\in\\{+1,-1\\},\\;\ni=1,\\dots ,n\\quad(m=1).}\n\\tag{1.1}\n\\]\n\nRepeating the same computation with $\\mathbf e_{1}$ replaced by\n$\\mathbf u$ yields  \n\n\\[\n\\boxed{\\;\nd_{i}=\\sigma_{i}\n\\sqrt{(\\mathbf e_{0})_{i}^{2}+u_{i}^{2}},\\qquad\n\\sigma_{i}\\in\\{+1,-1\\},\\;\ni=1,\\dots ,n\\quad(m=0).}\n\\tag{1.2}\n\\]\n\nIn both cases the \\emph{sign pattern}\n$\\sigma=(\\sigma_{1},\\dots ,\\sigma_{n})$\nrecords in which half–spaces the circle is situated.\n\n-------------------------------------------------\n2.  Angle condition  \n\nBecause $\\mathbf v\\cdot\\mathbf 1=0$,\n$\\mathbf 1$ is orthogonal to $\\mathbf e_{0}$, hence projects on $V$\nexclusively via the second basis vector.  Put  \n\n\\[\n\\lambda:=\n\\begin{cases}\n\\mathbf 1\\cdot\\mathbf e_{1}, & m=1,\\\\[4pt]\n\\mathbf 1\\cdot\\mathbf u, & m=0.\n\\end{cases}\n\\]\n\nBy the definition recalled in the problem statement  \n\n\\[\n\\|\\operatorname{proj}_{V}\\mathbf 1\\|^{2}=n\\cos^{2}\\varphi,\n\\]\n\nwhence  \n\n\\[\n\\boxed{\\;\\lambda^{2}=n\\cos^{2}\\varphi.}\\tag{2.1}\n\\]\n\n-------------------------------------------------\n3.  Generic centres ($m=1$)  \n\nPut $\\mathbf w:=\\mathbf e_{1}$.  \nBecause $\\mathbf e_{0}\\perp\\mathbf w$ and\n$\\|\\mathbf w\\|=1$, the relations (1.1) can be rewritten as  \n\n\\[\nd_{i}^{2}-v_{0,i}^{2}=w_{i}^{2},\\qquad\n\\sum_{i=1}^{n}(d_{i}^{2}-v_{0,i}^{2})=\\sum_{i=1}^{n}w_{i}^{2}=1.\n\\tag{3.1}\n\\]\n\nTogether with  \n\n\\[\n(\\mathbf 1\\cdot\\mathbf w)^{2}=n\\cos^{2}\\varphi\\quad(=\\lambda^{2})\n\\tag{3.2}\n\\]\n\nthey are necessary.  \nA further relation is forced by the incidence condition:\n\n\\[\n\\boxed{\\;(\\mathbf I-\\mathbf v_{0}\\mathbf v_{0}^{\\mathsf T})(\\mathbf p-\\mathbf O)\n       =\\bigl[(\\mathbf p-\\mathbf O)\\cdot\\mathbf w\\bigr]\\mathbf w\n       \\quad\\text{with }(\\mathbf p-\\mathbf O)\\cdot\\mathbf w\\neq 0.}\n\\tag{3.3}\n\\]\n\nIndeed, $(\\mathbf p-\\mathbf O)$ must lie in the span of\n$\\{\\mathbf v_{0},\\mathbf w\\}$, and the right–hand side is the projection\nonto the direction of $\\mathbf w$.\n\nConversely, suppose a point $\\mathbf O\\notin\\ell$ satisfies the three\nsystems \\eqref{3.1}, \\eqref{3.2}, \\eqref{3.3} for\n\\emph{some} choice of sign pattern $\\sigma\\in\\{+1,-1\\}^{n}$.\nThen  \n\n\\[\nV=\\operatorname{span}\\{\\mathbf v_{0},\\mathbf w\\}\n\\quad(\\mathbf w\\neq\\pm\\mathbf v_{0})\n\\]\n\nis a $2$–dimensional linear subspace\nmaking the angle $\\varphi$ with $\\mathbf 1$,\nand condition \\eqref{3.3} guarantees $\\ell\\subset\\mathbf O+V$.\nBy \\eqref{1.1} the circle of radius $s$\nwith centre $\\mathbf O$ and lying in $A:=\\mathbf O+V$\nis tangent to every $\\Pi_{i}$ on the prescribed side.\nHence \\eqref{3.1}–\\eqref{3.3} are \\emph{necessary and sufficient}.  \n\nDefine  \n\n\\[\n\\boxed{\\;\n\\begin{aligned}\n\\mathcal L_{\\mathrm{gen}}\n  &=\\Bigl\\{\\mathbf O\\in\\mathbb R^{\\,n}\\setminus\\ell\\;\\Bigm|\\;\n        \\exists\\;\\mathbf w,\\sigma\n        \\text{ such that }\\eqref{3.1},\\eqref{3.2},\\eqref{3.3}\\text{ hold}\\Bigr\\},\\\\[6pt]\n\\mathcal O(\\mathbf O)\n  &=\\bigl\\{\\mathbf O+\\operatorname{span}\\{\\mathbf v_{0},\\mathbf w\\}\\bigr\\}\n     \\quad(\\mathbf O\\in\\mathcal L_{\\mathrm{gen}}),\n\\end{aligned}}\n\\tag{3.4}\n\\]\n\nwhere for every admissible centre $\\mathbf O$\nthe vector $\\mathbf w$ is uniquely determined (up to sign) by\n$\\mathbf w=\\pm\\dfrac{(\\mathbf I-\\mathbf v_{0}\\mathbf v_{0}^{\\mathsf T})(\\mathbf p-\\mathbf O)}\n                            {\\lVert(\\mathbf I-\\mathbf v_{0}\\mathbf v_{0}^{\\mathsf T})(\\mathbf p-\\mathbf O)\\rVert}$.\n\n-------------------------------------------------\n4.  Exceptional centres ($m=0$)  \n\nNow $\\mathbf O=\\mathbf p+t\\mathbf v$ lies on $\\ell$.\nFrom (1.2) write  \n\n\\[\na_{i}:=\\sqrt{d_{i}^{2}-v_{0,i}^{2}}\\qquad(i=1,\\dots ,n),\n\\tag{4.1}\n\\]\n\nso that  \n\n\\[\na_{i}\\ge 0,\\qquad\\sum_{i=1}^{n}a_{i}^{2}=1.\n\\tag{4.2}\n\\]\n\nThe yet unknown second basis vector $\\mathbf u$ must satisfy  \n\n\\[\n\\mathbf u\\cdot\\mathbf v_{0}=0,\\qquad\n\\lVert\\mathbf u\\rVert=1,\\qquad\n(\\mathbf 1\\cdot\\mathbf u)^{2}=n\\cos^{2}\\varphi,\n\\tag{4.3}\n\\]\n\nand, component–wise,  \n\n\\[\nu_{i}^{2}=a_{i}^{2}\\quad(i=1,\\dots ,n).\n\\tag{4.4}\n\\]\n\nPut $a:=(a_{1},\\dots ,a_{n})$ and\n$b:=(a_{1}v_{0,1},\\dots ,a_{n}v_{0,n})$.\nChoosing signs $\\varepsilon_{i}\\in\\{+1,-1\\}$ and setting  \n\n\\[\nu_{i}:=\\varepsilon_{i}a_{i}\\qquad(i=1,\\dots ,n)\n\\]\n\nturns \\eqref{4.4} into an identity.\nConsequently \\eqref{4.3} is equivalent to the \\emph{sign feasibility\nsystem}\n\n\\[\n\\boxed{\\;\n\\begin{cases}\n\\sum_{i=1}^{n}\\varepsilon_{i}a_{i}=\\sigma,\\\\[6pt]\n\\sum_{i=1}^{n}\\varepsilon_{i}a_{i}v_{0,i}=0,\n\\end{cases}\\qquad\n\\text{where }\\sigma\\in\\{+\\sqrt{n}\\cos\\varphi,\\,-\\sqrt{n}\\cos\\varphi\\}.}\n\\tag{4.5}\n\\]\n\nDefine the set of admissible sign patterns  \n\n\\[\n\\mathcal E(a):=\\Bigl\\{\\varepsilon\\in\\{+1,-1\\}^{n}\\;\\Bigm|\\;\n          \\text{\\eqref{4.5} is satisfied for some }\n          \\sigma=\\pm\\sqrt n\\cos\\varphi\\Bigr\\}.\n\\tag{4.6}\n\\]\n\n(N.B. if $a_{i}=0$ the corresponding sign $\\varepsilon_{i}$\nis irrelevant.)\n\nNecessity is clear from the derivation.  \nConversely, if $\\mathcal E(a)\\neq\\varnothing$\nchoose one $\\varepsilon\\in\\mathcal E(a)$,\ndeclare $\\mathbf u$ by $u_{i}=\\varepsilon_{i}a_{i}$,\nand set  \n\n\\[\nA=\\mathbf O+\\operatorname{span}\\{\\mathbf v_{0},\\mathbf u\\}.\n\\tag{4.7}\n\\]\n\nThe circle of centre $\\mathbf O$ and radius $s$\ncontained in $A$ then meets (i)–(iii).\nHence\n\n\\[\n\\boxed{\\;\n\\begin{aligned}\n\\mathcal L_{\\mathrm{exc}}\n   &=\\Bigl\\{\\mathbf p+t\\mathbf v\\;\\Bigm|\\;\n        t\\in\\mathbb R,\\\n        d_{i}^{2}\\ge v_{0,i}^{2}\\ (i=1,\\dots ,n),\\\n        \\sum_{i=1}^{n}(d_{i}^{2}-v_{0,i}^{2})=1,\\\n        \\mathcal E(a)\\neq\\varnothing\n        \\Bigr\\},\\\\[6pt]\n\\mathcal O(\\mathbf O)\n   &=\\Bigl\\{\\mathbf O+\\operatorname{span}\\{\\mathbf v_{0},\\mathbf u\\}\\;\\Bigm|\\;\n        \\mathbf u\\text{ arises by \\eqref{4.4} from a sign pattern }\n        \\varepsilon\\in\\mathcal E(a)\\Bigr\\}.\n\\end{aligned}}\n\\tag{4.8}\n\\]\n\n-------------------------------------------------\n5.  The total locus  \n\n\\[\n\\boxed{\\;\\mathcal L=\\mathcal L_{\\mathrm{gen}}\\;\\cup\\;\\mathcal L_{\\mathrm{exc}}\\;}\n\\tag{5.1}\n\\]\n\nThe union is disjoint because\n$\\mathcal L_{\\mathrm{gen}}\\subset\\mathbb R^{\\,n}\\setminus\\ell$\nwhile $\\mathcal L_{\\mathrm{exc}}\\subset\\ell$.\n\nAll displayed conditions involve only the prescribed data\n$n,s,\\varphi,\\mathbf C,\\mathbf p,\\mathbf v$\nand the running point $\\mathbf O$, as required.\n\n-------------------------------------------------\n6.  Verification  \n\nNecessity of the conditions that define\n$\\mathcal L_{\\mathrm{gen}}$ and $\\mathcal L_{\\mathrm{exc}}$\nwas derived in Sections 1–4.  \nConversely,\n\n– Section 3 constructs, from every\n$\\mathbf O\\in\\mathcal L_{\\mathrm{gen}}$,\na \\emph{unique} plane described in \\eqref{3.4}\nsatisfying (i)–(iii);\n\n– Section 4 constructs, from every\n$\\mathbf O\\in\\mathcal L_{\\mathrm{exc}}$,\n\\emph{all} admissible planes \\eqref{4.7}.\n\nHence parts (a) and (b) of the problem are solved.\n\n\\hfill$\\square$\n\n\n\n--------------------------------------------------------------------",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.419279",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher Dimension & More Variables:  \n   The original asks for a circle (k = 1) in 3-space (n = 3).  The variant deals with a k-sphere in arbitrary ℝⁿ (n ≥ 4, k ≤ n−2).  Both the ambient space and the dimension of the moving object are free parameters.\n\n2. Additional Constraints:  \n   Besides simultaneous tangency to n hyperplanes, the plane containing the k-sphere must (ii) make a prescribed angle with the diagonal vector 1⃗ and (iii) contain a fixed line L.  These extra orthogonality and incidence requirements introduce coupled linear–algebraic and trigonometric conditions absent from the original.\n\n3. Sophisticated Structures:  \n   The solution relies on the geometry of orthogonal decompositions, unit-sphere bundles, Stiefel and special orthogonal groups; description (19) uses fibre–bundle language to classify all admissible orientations.\n\n4. Deeper Theory & More Steps:  \n   One must generalise the “extreme-value lemma” to k dimensions, manipulate orthonormal frames in ℝⁿ, analyse cones of given axial angle, and invoke topological arguments (non-emptiness of SO bundles).  Each of these layers is absent from the original 3-D circle problem.\n\n5. Multiple Interacting Concepts:  \n   The final answer intertwines Euclidean distance (sphere of centres), coordinate inequalities (cube intersection), angular cones (condition (ii)), and incidence with a fixed line (condition (iii)), demanding holistic reasoning rather than a single geometric observation.\n\nHence the enhanced kernel variant is substantially more technical, abstract and demanding than both the original problem and the previously given kernel variant."
      }
    },
    "original_kernel_variant": {
      "question": "Let  \n\n\\[\nn\\ge 4,\\qquad s>0 ,\\qquad \\varphi\\in(0,\\pi/2)\n\\]\n\nbe fixed and put  \n\n\\[\n\\mathbf C=(c_{1},\\dots ,c_{n})\\in\\mathbb R^{\\,n},\n\\qquad\n\\mathbf 1=(1,\\dots ,1)\\in\\mathbb R^{\\,n},\n\\qquad\n\\Pi_{i}\\;:\\;x_{i}=c_{i}\\quad(i=1,\\dots ,n).\n\\]\n\nFurther data are  \n\n\\[\n\\mathbf p,\\ \\mathbf v\\in\\mathbb R^{\\,n},\\qquad \n\\mathbf v\\neq\\mathbf 0,\\qquad \\mathbf v\\cdot\\mathbf 1=0,\n\\]\n\nand the straight line  \n\n\\[\n\\ell=\\{\\mathbf p+t\\mathbf v\\mid t\\in\\mathbb R\\}.\n\\]\n\n1.  (Circle)  \nA \\emph{circle of radius $s$} is a set  \n\n\\[\n\\Sigma=\\{\\mathbf O+s\\mathbf u\\mid\\mathbf u\\in S^{1}\\subset(A-\\mathbf O)\\},\n\\]\n\nwhere $A$ is an affine $2$–plane in $\\mathbb R^{\\,n}$, $\\mathbf O\\in A$ is the\ncentre, $S^{1}=\\{\\mathbf u\\in A-\\mathbf O\\mid \\lVert\\mathbf u\\rVert=1\\}$ is the Euclidean unit circle of the linear plane $A-\\mathbf O$.\n\n2.  (Admissibility)  \nThe circle $\\Sigma$ is called \\emph{admissible} if, simultaneously  \n\n(i)  (tangency) for every $i$\n      the circle is tangent to $\\Pi_{i}$ and lies entirely in the same\n      closed half–space of $\\Pi_{i}$ as the point $\\mathbf C$;  \n\n(ii) (angle condition) the acute angle between the vector $\\mathbf 1$\n      and the plane $A-\\mathbf O$ equals $\\varphi$\n      (that is, if $V:=A-\\mathbf O$ and\n      $\\operatorname{proj}_{V}\\mathbf 1$ denotes the orthogonal\n      projection of $\\mathbf 1$ on $V$, then\n      $\\lVert\\operatorname{proj}_{V}\\mathbf 1\\rVert\n      =\\lVert\\mathbf 1\\rVert\\cos\\varphi=\\sqrt n\\,\\cos\\varphi$);  \n\n(iii) (incidence) $\\ell\\subset A$.\n\n(a)  Determine explicitly the set  \n\n\\[\n\\mathcal L=\\mathcal L(n,s,\\varphi,\\mathbf C,\\mathbf p,\\mathbf v)\\subset\\mathbb R^{\\,n}\n\\]\n\nof all centres $\\mathbf O$ for which an admissible circle exists.\n\n(b)  For every $\\mathbf O\\in\\mathcal L$ describe explicitly the family  \n\n\\[\n\\mathcal O(\\mathbf O)\n\\]\n\nof all affine $2$–planes $A$ such that an admissible circle of centre\n$\\mathbf O$ lies in $A$.\n\nYour answers must depend only on the fixed data\n$n,s,\\varphi,c_{1},\\dots ,c_{n},\\mathbf p,\\mathbf v$\nand the point $\\mathbf O$ that is being tested.\n\n\n\n--------------------------------------------------------------------",
      "solution": "Throughout column vectors are written as columns and ${}^{\\mathsf T}$ denotes transposition.\n\n0.  Notation  \n\n\\[\n\\mathbf v_{0}:=\\frac{\\mathbf v}{\\lVert\\mathbf v\\rVert},\\qquad\n\\mathbf d:=\\frac{\\mathbf O-\\mathbf C}{s},\\qquad\nd_{i}:=\\frac{(\\mathbf O)_{i}-c_{i}}{s}\\quad(i=1,\\dots ,n).\n\\]\n\nLet $V:=A-\\mathbf O$ be the direction space of the required circle.  \nBecause of the incidence condition (iii) we have  \n\n\\[\n\\mathbf v_{0}\\in V\n\\]\n\nand we put  \n\n\\[\n\\mathbf e_{0}:=\\mathbf v_{0}\\qquad(\\lVert\\mathbf e_{0}\\rVert=1).\n\\]\n\nWrite  \n\n\\[\n\\mathbf q:=\\mathbf p-\\mathbf O=q_{\\parallel}\\mathbf e_{0}+\\mathbf q_{\\perp},\n\\qquad\nq_{\\parallel}:=\\mathbf q\\cdot\\mathbf e_{0},\\quad\n\\mathbf q_{\\perp}\\perp\\mathbf e_{0}.\n\\]\n\nIntroduce  \n\n\\[\nm:=\\begin{cases}\n1,&\\mathbf q_{\\perp}\\neq\\mathbf 0\\quad(\\text{``generic''}),\\\\[2pt]\n0,&\\mathbf q_{\\perp}=0\\quad(\\text{``exceptional''}).\n\\end{cases}\n\\]\n\nIf $m=1$ set  \n\n\\[\n\\mathbf e_{1}:=\\dfrac{\\mathbf q_{\\perp}}{\\lVert\\mathbf q_{\\perp}\\rVert},\n\\]\n\nso that $\\{\\mathbf e_{0},\\mathbf e_{1}\\}$ is an orthonormal basis of $V$.  \nIf $m=0$ we shall construct a second unit vector\n$\\mathbf u\\perp\\mathbf e_{0}$ later on.\n\nHence  \n\n\\[\nV=\n\\begin{cases}\n\\operatorname{span}\\{\\mathbf e_{0},\\mathbf e_{1}\\}, & m=1,\\\\[6pt]\n\\operatorname{span}\\{\\mathbf e_{0},\\mathbf u\\}, & m=0.\n\\end{cases}\n\\]\n\n-------------------------------------------------\n1.  Tangency to all $\\Pi_{i}$  \n\nTake first the generic case $m=1$.  \nA point of the circle is  \n\n\\[\n\\mathbf y(\\theta)=\\mathbf O+s\\bigl(\\cos\\theta\\,\\mathbf e_{0}\n+\\sin\\theta\\,\\mathbf e_{1}\\bigr),\\qquad\\theta\\in[0,2\\pi).\n\\]\n\nFix $i$.  Then  \n\n\\[\n\\frac{(\\mathbf y(\\theta))_{i}-c_{i}}{s}\n       =d_{i}+\\cos\\theta\\,(\\mathbf e_{0})_{i}\n                 +\\sin\\theta\\,(\\mathbf e_{1})_{i}.\n\\]\n\nThe real function\n$\\theta\\mapsto\\alpha\\cos\\theta+\\beta\\sin\\theta+\\gamma$\nattains $0$ as an \\emph{endpoint} of its range\niff $|\\gamma|=\\sqrt{\\alpha^{2}+\\beta^{2}}$;  \nthe sign of $\\gamma$ determines which endpoint is $0$,\nhence on which side of the hyper–plane the whole circle lies.\nConsequently, tangency (including the ``one-side'' requirement) gives  \n\n\\[\n\\boxed{\\;\nd_{i}=\\sigma_{i}\n\\sqrt{(\\mathbf e_{0})_{i}^{2}+(\\mathbf e_{1})_{i}^{2}},\\qquad\n\\sigma_{i}\\in\\{+1,-1\\},\\;\ni=1,\\dots ,n\\quad(m=1).}\n\\tag{1.1}\n\\]\n\nRepeating the same computation with $\\mathbf e_{1}$ replaced by\n$\\mathbf u$ yields  \n\n\\[\n\\boxed{\\;\nd_{i}=\\sigma_{i}\n\\sqrt{(\\mathbf e_{0})_{i}^{2}+u_{i}^{2}},\\qquad\n\\sigma_{i}\\in\\{+1,-1\\},\\;\ni=1,\\dots ,n\\quad(m=0).}\n\\tag{1.2}\n\\]\n\nIn both cases the \\emph{sign pattern}\n$\\sigma=(\\sigma_{1},\\dots ,\\sigma_{n})$\nrecords in which half–spaces the circle is situated.\n\n-------------------------------------------------\n2.  Angle condition  \n\nBecause $\\mathbf v\\cdot\\mathbf 1=0$,\n$\\mathbf 1$ is orthogonal to $\\mathbf e_{0}$, hence projects on $V$\nexclusively via the second basis vector.  Put  \n\n\\[\n\\lambda:=\n\\begin{cases}\n\\mathbf 1\\cdot\\mathbf e_{1}, & m=1,\\\\[4pt]\n\\mathbf 1\\cdot\\mathbf u, & m=0.\n\\end{cases}\n\\]\n\nBy the definition recalled in the problem statement  \n\n\\[\n\\|\\operatorname{proj}_{V}\\mathbf 1\\|^{2}=n\\cos^{2}\\varphi,\n\\]\n\nwhence  \n\n\\[\n\\boxed{\\;\\lambda^{2}=n\\cos^{2}\\varphi.}\\tag{2.1}\n\\]\n\n-------------------------------------------------\n3.  Generic centres ($m=1$)  \n\nPut $\\mathbf w:=\\mathbf e_{1}$.  \nBecause $\\mathbf e_{0}\\perp\\mathbf w$ and\n$\\|\\mathbf w\\|=1$, the relations (1.1) can be rewritten as  \n\n\\[\nd_{i}^{2}-v_{0,i}^{2}=w_{i}^{2},\\qquad\n\\sum_{i=1}^{n}(d_{i}^{2}-v_{0,i}^{2})=\\sum_{i=1}^{n}w_{i}^{2}=1.\n\\tag{3.1}\n\\]\n\nTogether with  \n\n\\[\n(\\mathbf 1\\cdot\\mathbf w)^{2}=n\\cos^{2}\\varphi\\quad(=\\lambda^{2})\n\\tag{3.2}\n\\]\n\nthey are necessary.  \nA further relation is forced by the incidence condition:\n\n\\[\n\\boxed{\\;(\\mathbf I-\\mathbf v_{0}\\mathbf v_{0}^{\\mathsf T})(\\mathbf p-\\mathbf O)\n       =\\bigl[(\\mathbf p-\\mathbf O)\\cdot\\mathbf w\\bigr]\\mathbf w\n       \\quad\\text{with }(\\mathbf p-\\mathbf O)\\cdot\\mathbf w\\neq 0.}\n\\tag{3.3}\n\\]\n\nIndeed, $(\\mathbf p-\\mathbf O)$ must lie in the span of\n$\\{\\mathbf v_{0},\\mathbf w\\}$, and the right–hand side is the projection\nonto the direction of $\\mathbf w$.\n\nConversely, suppose a point $\\mathbf O\\notin\\ell$ satisfies the three\nsystems \\eqref{3.1}, \\eqref{3.2}, \\eqref{3.3} for\n\\emph{some} choice of sign pattern $\\sigma\\in\\{+1,-1\\}^{n}$.\nThen  \n\n\\[\nV=\\operatorname{span}\\{\\mathbf v_{0},\\mathbf w\\}\n\\quad(\\mathbf w\\neq\\pm\\mathbf v_{0})\n\\]\n\nis a $2$–dimensional linear subspace\nmaking the angle $\\varphi$ with $\\mathbf 1$,\nand condition \\eqref{3.3} guarantees $\\ell\\subset\\mathbf O+V$.\nBy \\eqref{1.1} the circle of radius $s$\nwith centre $\\mathbf O$ and lying in $A:=\\mathbf O+V$\nis tangent to every $\\Pi_{i}$ on the prescribed side.\nHence \\eqref{3.1}–\\eqref{3.3} are \\emph{necessary and sufficient}.  \n\nDefine  \n\n\\[\n\\boxed{\\;\n\\begin{aligned}\n\\mathcal L_{\\mathrm{gen}}\n  &=\\Bigl\\{\\mathbf O\\in\\mathbb R^{\\,n}\\setminus\\ell\\;\\Bigm|\\;\n        \\exists\\;\\mathbf w,\\sigma\n        \\text{ such that }\\eqref{3.1},\\eqref{3.2},\\eqref{3.3}\\text{ hold}\\Bigr\\},\\\\[6pt]\n\\mathcal O(\\mathbf O)\n  &=\\bigl\\{\\mathbf O+\\operatorname{span}\\{\\mathbf v_{0},\\mathbf w\\}\\bigr\\}\n     \\quad(\\mathbf O\\in\\mathcal L_{\\mathrm{gen}}),\n\\end{aligned}}\n\\tag{3.4}\n\\]\n\nwhere for every admissible centre $\\mathbf O$\nthe vector $\\mathbf w$ is uniquely determined (up to sign) by\n$\\mathbf w=\\pm\\dfrac{(\\mathbf I-\\mathbf v_{0}\\mathbf v_{0}^{\\mathsf T})(\\mathbf p-\\mathbf O)}\n                            {\\lVert(\\mathbf I-\\mathbf v_{0}\\mathbf v_{0}^{\\mathsf T})(\\mathbf p-\\mathbf O)\\rVert}$.\n\n-------------------------------------------------\n4.  Exceptional centres ($m=0$)  \n\nNow $\\mathbf O=\\mathbf p+t\\mathbf v$ lies on $\\ell$.\nFrom (1.2) write  \n\n\\[\na_{i}:=\\sqrt{d_{i}^{2}-v_{0,i}^{2}}\\qquad(i=1,\\dots ,n),\n\\tag{4.1}\n\\]\n\nso that  \n\n\\[\na_{i}\\ge 0,\\qquad\\sum_{i=1}^{n}a_{i}^{2}=1.\n\\tag{4.2}\n\\]\n\nThe yet unknown second basis vector $\\mathbf u$ must satisfy  \n\n\\[\n\\mathbf u\\cdot\\mathbf v_{0}=0,\\qquad\n\\lVert\\mathbf u\\rVert=1,\\qquad\n(\\mathbf 1\\cdot\\mathbf u)^{2}=n\\cos^{2}\\varphi,\n\\tag{4.3}\n\\]\n\nand, component–wise,  \n\n\\[\nu_{i}^{2}=a_{i}^{2}\\quad(i=1,\\dots ,n).\n\\tag{4.4}\n\\]\n\nPut $a:=(a_{1},\\dots ,a_{n})$ and\n$b:=(a_{1}v_{0,1},\\dots ,a_{n}v_{0,n})$.\nChoosing signs $\\varepsilon_{i}\\in\\{+1,-1\\}$ and setting  \n\n\\[\nu_{i}:=\\varepsilon_{i}a_{i}\\qquad(i=1,\\dots ,n)\n\\]\n\nturns \\eqref{4.4} into an identity.\nConsequently \\eqref{4.3} is equivalent to the \\emph{sign feasibility\nsystem}\n\n\\[\n\\boxed{\\;\n\\begin{cases}\n\\sum_{i=1}^{n}\\varepsilon_{i}a_{i}=\\sigma,\\\\[6pt]\n\\sum_{i=1}^{n}\\varepsilon_{i}a_{i}v_{0,i}=0,\n\\end{cases}\\qquad\n\\text{where }\\sigma\\in\\{+\\sqrt{n}\\cos\\varphi,\\,-\\sqrt{n}\\cos\\varphi\\}.}\n\\tag{4.5}\n\\]\n\nDefine the set of admissible sign patterns  \n\n\\[\n\\mathcal E(a):=\\Bigl\\{\\varepsilon\\in\\{+1,-1\\}^{n}\\;\\Bigm|\\;\n          \\text{\\eqref{4.5} is satisfied for some }\n          \\sigma=\\pm\\sqrt n\\cos\\varphi\\Bigr\\}.\n\\tag{4.6}\n\\]\n\n(N.B. if $a_{i}=0$ the corresponding sign $\\varepsilon_{i}$\nis irrelevant.)\n\nNecessity is clear from the derivation.  \nConversely, if $\\mathcal E(a)\\neq\\varnothing$\nchoose one $\\varepsilon\\in\\mathcal E(a)$,\ndeclare $\\mathbf u$ by $u_{i}=\\varepsilon_{i}a_{i}$,\nand set  \n\n\\[\nA=\\mathbf O+\\operatorname{span}\\{\\mathbf v_{0},\\mathbf u\\}.\n\\tag{4.7}\n\\]\n\nThe circle of centre $\\mathbf O$ and radius $s$\ncontained in $A$ then meets (i)–(iii).\nHence\n\n\\[\n\\boxed{\\;\n\\begin{aligned}\n\\mathcal L_{\\mathrm{exc}}\n   &=\\Bigl\\{\\mathbf p+t\\mathbf v\\;\\Bigm|\\;\n        t\\in\\mathbb R,\\\n        d_{i}^{2}\\ge v_{0,i}^{2}\\ (i=1,\\dots ,n),\\\n        \\sum_{i=1}^{n}(d_{i}^{2}-v_{0,i}^{2})=1,\\\n        \\mathcal E(a)\\neq\\varnothing\n        \\Bigr\\},\\\\[6pt]\n\\mathcal O(\\mathbf O)\n   &=\\Bigl\\{\\mathbf O+\\operatorname{span}\\{\\mathbf v_{0},\\mathbf u\\}\\;\\Bigm|\\;\n        \\mathbf u\\text{ arises by \\eqref{4.4} from a sign pattern }\n        \\varepsilon\\in\\mathcal E(a)\\Bigr\\}.\n\\end{aligned}}\n\\tag{4.8}\n\\]\n\n-------------------------------------------------\n5.  The total locus  \n\n\\[\n\\boxed{\\;\\mathcal L=\\mathcal L_{\\mathrm{gen}}\\;\\cup\\;\\mathcal L_{\\mathrm{exc}}\\;}\n\\tag{5.1}\n\\]\n\nThe union is disjoint because\n$\\mathcal L_{\\mathrm{gen}}\\subset\\mathbb R^{\\,n}\\setminus\\ell$\nwhile $\\mathcal L_{\\mathrm{exc}}\\subset\\ell$.\n\nAll displayed conditions involve only the prescribed data\n$n,s,\\varphi,\\mathbf C,\\mathbf p,\\mathbf v$\nand the running point $\\mathbf O$, as required.\n\n-------------------------------------------------\n6.  Verification  \n\nNecessity of the conditions that define\n$\\mathcal L_{\\mathrm{gen}}$ and $\\mathcal L_{\\mathrm{exc}}$\nwas derived in Sections 1–4.  \nConversely,\n\n– Section 3 constructs, from every\n$\\mathbf O\\in\\mathcal L_{\\mathrm{gen}}$,\na \\emph{unique} plane described in \\eqref{3.4}\nsatisfying (i)–(iii);\n\n– Section 4 constructs, from every\n$\\mathbf O\\in\\mathcal L_{\\mathrm{exc}}$,\n\\emph{all} admissible planes \\eqref{4.7}.\n\nHence parts (a) and (b) of the problem are solved.\n\n\\hfill$\\square$\n\n\n\n--------------------------------------------------------------------",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.364790",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher Dimension & More Variables:  \n   The original asks for a circle (k = 1) in 3-space (n = 3).  The variant deals with a k-sphere in arbitrary ℝⁿ (n ≥ 4, k ≤ n−2).  Both the ambient space and the dimension of the moving object are free parameters.\n\n2. Additional Constraints:  \n   Besides simultaneous tangency to n hyperplanes, the plane containing the k-sphere must (ii) make a prescribed angle with the diagonal vector 1⃗ and (iii) contain a fixed line L.  These extra orthogonality and incidence requirements introduce coupled linear–algebraic and trigonometric conditions absent from the original.\n\n3. Sophisticated Structures:  \n   The solution relies on the geometry of orthogonal decompositions, unit-sphere bundles, Stiefel and special orthogonal groups; description (19) uses fibre–bundle language to classify all admissible orientations.\n\n4. Deeper Theory & More Steps:  \n   One must generalise the “extreme-value lemma” to k dimensions, manipulate orthonormal frames in ℝⁿ, analyse cones of given axial angle, and invoke topological arguments (non-emptiness of SO bundles).  Each of these layers is absent from the original 3-D circle problem.\n\n5. Multiple Interacting Concepts:  \n   The final answer intertwines Euclidean distance (sphere of centres), coordinate inequalities (cube intersection), angular cones (condition (ii)), and incidence with a fixed line (condition (iii)), demanding holistic reasoning rather than a single geometric observation.\n\nHence the enhanced kernel variant is substantially more technical, abstract and demanding than both the original problem and the previously given kernel variant."
      }
    }
  },
  "checked": true,
  "problem_type": "calculation"
}