{
  "index": "1956-A-3",
  "type": "ANA",
  "tag": [
    "ANA",
    "GEO"
  ],
  "difficulty": "",
  "question": "3. A particle falls in a vertical plane from rest under the influence of gravity and a force perpendicular to and proportional to its velocity. Obtain the equations of the trajectory and identify the curve.",
  "solution": "Solution. Suppose the position of the particle at time \\( t \\) is \\( \\langle x(t), y(t)\\rangle \\). Then the velocity vector is given by \\( \\left\\langle x^{\\prime}(t), y^{\\prime}(t)\\right\\rangle \\) and the acceleration vector by \\( \\left\\langle x^{\\prime \\prime}(t), y^{\\prime \\prime}(t)\\right\\rangle \\). We choose the coordinate system so that the particle starts at the origin with \\( x \\) running horizontally and \\( y \\) vertically as usual.\n\nSince the vector \\( \\left\\langle y^{\\prime}(t),-x^{\\prime}(t)\\right\\rangle \\) is perpendicular to and of the same length as the velocity vector the differential equation governing the motion is\n\\[\n\\left\\langle x^{\\prime \\prime}(t), y^{\\prime \\prime}(t)\\right\\rangle=c\\left\\langle y^{\\prime}(t),-x^{\\prime}(t)\\right\\rangle+\\langle 0,-g\\rangle\n\\]\nwhere \\( c \\) is a constant incorporating the mass of the given particle. The initial conditions are \\( x(0)=y(0)=x^{\\prime}(0)=y^{\\prime}(0)=0 \\). Separating (1) into two equations we have\n\\[\n\\begin{array}{l}\nx^{\\prime \\prime}(t)=c y^{\\prime}(t) \\\\\ny^{\\prime \\prime}(t)=-c x^{\\prime}(t)-g\n\\end{array}\n\\]\n\nDifferentiating the first of these equations, we get\n\\[\nx^{\\prime \\prime \\prime}(t)=c y^{\\prime \\prime}(t)=-c^{2} x^{\\prime}(t)-c g\n\\]\nwhich in standard form is\n\\[\nx^{\\prime \\prime \\prime}(t)+c^{2} x^{\\prime}(t)=-c g .\n\\]\n\nThe corresponding homogeneous differential equation, \\( x^{\\prime \\prime \\prime}(t)+c^{2} x^{\\prime}(t) \\) \\( =0 \\), has the three linearly independent solutions \\( 1, \\sin c t, \\cos c t \\). Since the right member of (2) is a solution of the homogeneous differential equation, we look for a particular solution of the form \\( k t \\), and find that \\( -g t / c \\) is such a solution. Hence the general solution of (2) is\n\\[\nx(t)=-g t / c+\\alpha+\\beta \\cos c t+\\gamma \\sin c t .\n\\]\n\nThe initial conditions are \\( x(0)=x^{\\prime}(0)=x^{\\prime \\prime}(0)=0 \\) (the latter from \\( \\left.y^{\\prime}(0)=0\\right) \\). Hence\n\\[\nx(t)=-\\frac{g t}{c}+\\frac{g}{c^{2}} \\sin c t .\n\\]\n\nTherefore \\( y^{\\prime}=x^{\\prime \\prime} / c=-g / c \\sin c t \\) and\n\\[\ny(t)=\\frac{g}{c^{2}}(-1+\\cos c t)\n\\]\nusing the initial condition \\( y(0)=0 \\).\nThus equations (3) and (4) describe the motion. They are the parametric equations of a cycloid traced by a point on the rim of a circle of radius \\( g / c^{2} \\) rolling along the underside of the \\( x \\)-axis with velocity \\( -g / c \\).\n\nRemark. We have assumed throughout, of course, that \\( c \\neq 0 \\). If \\( c=0 \\), the motion is just free fall.",
  "vars": [
    "t",
    "x",
    "y"
  ],
  "params": [
    "c",
    "g",
    "k",
    "\\\\alpha",
    "\\\\beta"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "t": "timevar",
        "x": "horizcoor",
        "y": "verticoor",
        "c": "perpvelc",
        "g": "gravconst",
        "k": "particst",
        "\\alpha": "homconsta",
        "\\beta": "homconstb"
      },
      "question": "3. A particle falls in a vertical plane from rest under the influence of gravity and a force perpendicular to and proportional to its velocity. Obtain the equations of the trajectory and identify the curve.",
      "solution": "Solution. Suppose the position of the particle at time \\( timevar \\) is \\( \\langle horizcoor(timevar), verticoor(timevar)\\rangle \\). Then the velocity vector is given by \\( \\left\\langle horizcoor^{\\prime}(timevar), verticoor^{\\prime}(timevar)\\right\\rangle \\) and the acceleration vector by \\( \\left\\langle horizcoor^{\\prime \\prime}(timevar), verticoor^{\\prime \\prime}(timevar)\\right\\rangle \\). We choose the coordinate system so that the particle starts at the origin with \\( horizcoor \\) running horizontally and \\( verticoor \\) vertically as usual.\n\nSince the vector \\( \\left\\langle verticoor^{\\prime}(timevar),-horizcoor^{\\prime}(timevar)\\right\\rangle \\) is perpendicular to and of the same length as the velocity vector the differential equation governing the motion is\n\\[\n\\left\\langle horizcoor^{\\prime \\prime}(timevar), verticoor^{\\prime \\prime}(timevar)\\right\\rangle=perpvelc\\left\\langle verticoor^{\\prime}(timevar),-horizcoor^{\\prime}(timevar)\\right\\rangle+\\langle 0,-gravconst\\rangle\n\\]\nwhere \\( perpvelc \\) is a constant incorporating the mass of the given particle. The initial conditions are \\( horizcoor(0)=verticoor(0)=horizcoor^{\\prime}(0)=verticoor^{\\prime}(0)=0 \\). Separating (1) into two equations we have\n\\[\n\\begin{array}{l}\nhorizcoor^{\\prime \\prime}(timevar)=perpvelc\\,verticoor^{\\prime}(timevar) \\\\\nverticoor^{\\prime \\prime}(timevar)=-perpvelc\\,horizcoor^{\\prime}(timevar)-gravconst\n\\end{array}\n\\]\n\nDifferentiating the first of these equations, we get\n\\[\nhorizcoor^{\\prime \\prime \\prime}(timevar)=perpvelc\\,verticoor^{\\prime \\prime}(timevar)=-perpvelc^{2}\\,horizcoor^{\\prime}(timevar)-perpvelc\\,gravconst\n\\]\nwhich in standard form is\n\\[\nhorizcoor^{\\prime \\prime \\prime}(timevar)+perpvelc^{2}\\,horizcoor^{\\prime}(timevar)=-perpvelc\\,gravconst .\n\\]\n\nThe corresponding homogeneous differential equation, \\( horizcoor^{\\prime \\prime \\prime}(timevar)+perpvelc^{2}\\,horizcoor^{\\prime}(timevar)=0 \\), has the three linearly independent solutions \\( 1, \\sin (perpvelc\\,timevar), \\cos (perpvelc\\,timevar) \\). Since the right member of (2) is a solution of the homogeneous differential equation, we look for a particular solution of the form \\( particst\\,timevar \\), and find that \\( -gravconst\\,timevar / perpvelc \\) is such a solution. Hence the general solution of (2) is\n\\[\nhorizcoor(timevar)=-\\frac{gravconst\\,timevar}{perpvelc}+homconsta+homconstb \\cos (perpvelc\\,timevar)+\\gamma \\sin (perpvelc\\,timevar) .\n\\]\n\nThe initial conditions are \\( horizcoor(0)=horizcoor^{\\prime}(0)=horizcoor^{\\prime \\prime}(0)=0 \\) (the latter from \\( verticoor^{\\prime}(0)=0 \\) ). Hence\n\\[\nhorizcoor(timevar)=-\\frac{gravconst\\,timevar}{perpvelc}+\\frac{gravconst}{perpvelc^{2}} \\sin (perpvelc\\,timevar) .\n\\]\n\nTherefore \\( verticoor^{\\prime}=horizcoor^{\\prime \\prime} / perpvelc=-\\frac{gravconst}{perpvelc} \\sin (perpvelc\\,timevar) \\) and\n\\[\nverticoor(timevar)=\\frac{gravconst}{perpvelc^{2}}(-1+\\cos (perpvelc\\,timevar))\n\\]\nusing the initial condition \\( verticoor(0)=0 \\).\nThus equations (3) and (4) describe the motion. They are the parametric equations of a cycloid traced by a point on the rim of a circle of radius \\( gravconst / perpvelc^{2} \\) rolling along the underside of the \\( horizcoor \\)-axis with velocity \\( -gravconst / perpvelc \\).\n\nRemark. We have assumed throughout, of course, that \\( perpvelc \\neq 0 \\). If \\( perpvelc=0 \\), the motion is just free fall."
    },
    "descriptive_long_confusing": {
      "map": {
        "t": "sandstone",
        "x": "lighthouse",
        "y": "brainstorm",
        "c": "honeycomb",
        "g": "sunflower",
        "k": "woodpecker",
        "\\alpha": "waterfall",
        "\\beta": "silhouette"
      },
      "question": "3. A particle falls in a vertical plane from rest under the influence of gravity and a force perpendicular to and proportional to its velocity. Obtain the equations of the trajectory and identify the curve.",
      "solution": "Solution. Suppose the position of the particle at time \\( sandstone \\) is \\( \\langle lighthouse(sandstone), brainstorm(sandstone)\\rangle \\). Then the velocity vector is given by \\( \\left\\langle lighthouse^{\\prime}(sandstone), brainstorm^{\\prime}(sandstone)\\right\\rangle \\) and the acceleration vector by \\( \\left\\langle lighthouse^{\\prime \\prime}(sandstone), brainstorm^{\\prime \\prime}(sandstone)\\right\\rangle \\). We choose the coordinate system so that the particle starts at the origin with \\( lighthouse \\) running horizontally and \\( brainstorm \\) vertically as usual.\n\nSince the vector \\( \\left\\langle brainstorm^{\\prime}(sandstone),-lighthouse^{\\prime}(sandstone)\\right\\rangle \\) is perpendicular to and of the same length as the velocity vector the differential equation governing the motion is\n\\[\n\\left\\langle lighthouse^{\\prime \\prime}(sandstone), brainstorm^{\\prime \\prime}(sandstone)\\right\\rangle= honeycomb \\left\\langle brainstorm^{\\prime}(sandstone),-lighthouse^{\\prime}(sandstone)\\right\\rangle+\\langle 0,-sunflower\\rangle\n\\]\nwhere \\( honeycomb \\) is a constant incorporating the mass of the given particle. The initial conditions are \\( lighthouse(0)=brainstorm(0)=lighthouse^{\\prime}(0)=brainstorm^{\\prime}(0)=0 \\). Separating (1) into two equations we have\n\\[\n\\begin{array}{l}\nlighthouse^{\\prime \\prime}(sandstone)= honeycomb \\, brainstorm^{\\prime}(sandstone) \\\\\nbrainstorm^{\\prime \\prime}(sandstone)=- honeycomb \\, lighthouse^{\\prime}(sandstone)-sunflower\n\\end{array}\n\\]\n\nDifferentiating the first of these equations, we get\n\\[\nlighthouse^{\\prime \\prime \\prime}(sandstone)= honeycomb \\, brainstorm^{\\prime \\prime}(sandstone)=- honeycomb^{2} \\, lighthouse^{\\prime}(sandstone)- honeycomb \\, sunflower\n\\]\nwhich in standard form is\n\\[\nlighthouse^{\\prime \\prime \\prime}(sandstone)+ honeycomb^{2} \\, lighthouse^{\\prime}(sandstone)=- honeycomb \\, sunflower .\n\\]\n\nThe corresponding homogeneous differential equation, \\( lighthouse^{\\prime \\prime \\prime}(sandstone)+ honeycomb^{2} \\, lighthouse^{\\prime}(sandstone) =0 \\), has the three linearly independent solutions \\( 1, \\sin honeycomb sandstone, \\cos honeycomb sandstone \\). Since the right member of (2) is a solution of the homogeneous differential equation, we look for a particular solution of the form \\( woodpecker \\, sandstone \\), and find that \\( -sunflower \\, sandstone / honeycomb \\) is such a solution. Hence the general solution of (2) is\n\\[\nlighthouse(sandstone)=-\\frac{sunflower \\, sandstone}{honeycomb}+waterfall+silhouette \\cos honeycomb sandstone+\\gamma \\sin honeycomb sandstone .\n\\]\n\nThe initial conditions are \\( lighthouse(0)=lighthouse^{\\prime}(0)=lighthouse^{\\prime \\prime}(0)=0 \\) (the latter from \\( brainstorm^{\\prime}(0)=0 \\) ). Hence\n\\[\nlighthouse(sandstone)=-\\frac{sunflower \\, sandstone}{honeycomb}+\\frac{sunflower}{honeycomb^{2}} \\sin honeycomb sandstone .\n\\]\n\nTherefore \\( brainstorm^{\\prime}=lighthouse^{\\prime \\prime} / honeycomb=-\\frac{sunflower}{honeycomb} \\sin honeycomb sandstone \\) and\n\\[\nbrainstorm(sandstone)=\\frac{sunflower}{honeycomb^{2}}(-1+\\cos honeycomb sandstone)\n\\]\nusing the initial condition \\( brainstorm(0)=0 \\).\nThus equations (3) and (4) describe the motion. They are the parametric equations of a cycloid traced by a point on the rim of a circle of radius \\( \\frac{sunflower}{honeycomb^{2}} \\) rolling along the underside of the \\( lighthouse \\)-axis with velocity \\( -\\frac{sunflower}{honeycomb} \\).\n\nRemark. We have assumed throughout, of course, that \\( honeycomb \\neq 0 \\). If \\( honeycomb=0 \\), the motion is just free fall."
    },
    "descriptive_long_misleading": {
      "map": {
        "t": "timeless",
        "x": "verticalpos",
        "y": "horizontalpos",
        "c": "stillness",
        "g": "weightless",
        "k": "changing",
        "\\alpha": "endingcoeff",
        "\\beta": "startingcoeff"
      },
      "question": "3. A particle falls in a vertical plane from rest under the influence of gravity and a force perpendicular to and proportional to its velocity. Obtain the equations of the trajectory and identify the curve.",
      "solution": "Solution. Suppose the position of the particle at time \\( timeless \\) is \\( \\langle verticalpos(timeless), horizontalpos(timeless)\\rangle \\). Then the velocity vector is given by \\( \\left\\langle verticalpos^{\\prime}(timeless), horizontalpos^{\\prime}(timeless)\\right\\rangle \\) and the acceleration vector by \\( \\left\\langle verticalpos^{\\prime \\prime}(timeless), horizontalpos^{\\prime \\prime}(timeless)\\right\\rangle \\). We choose the coordinate system so that the particle starts at the origin with \\( verticalpos \\) running horizontally and \\( horizontalpos \\) vertically as usual.\n\nSince the vector \\( \\left\\langle horizontalpos^{\\prime}(timeless),-verticalpos^{\\prime}(timeless)\\right\\rangle \\) is perpendicular to and of the same length as the velocity vector the differential equation governing the motion is\n\\[\n\\left\\langle verticalpos^{\\prime \\prime}(timeless), horizontalpos^{\\prime \\prime}(timeless)\\right\\rangle=stillness\\left\\langle horizontalpos^{\\prime}(timeless),-verticalpos^{\\prime}(timeless)\\right\\rangle+\\langle 0,-weightless\\rangle\n\\]\nwhere \\( stillness \\) is a constant incorporating the mass of the given particle. The initial conditions are \\( verticalpos(0)=horizontalpos(0)=verticalpos^{\\prime}(0)=horizontalpos^{\\prime}(0)=0 \\). Separating (1) into two equations we have\n\\[\n\\begin{array}{l}\nverticalpos^{\\prime \\prime}(timeless)=stillness\\; horizontalpos^{\\prime}(timeless) \\\\\nhorizontalpos^{\\prime \\prime}(timeless)=-stillness\\; verticalpos^{\\prime}(timeless)-weightless\n\\end{array}\n\\]\n\nDifferentiating the first of these equations, we get\n\\[\nverticalpos^{\\prime \\prime \\prime}(timeless)=stillness\\; horizontalpos^{\\prime \\prime}(timeless)=-stillness^{2} verticalpos^{\\prime}(timeless)-stillness\\; weightless\n\\]\nwhich in standard form is\n\\[\nverticalpos^{\\prime \\prime \\prime}(timeless)+stillness^{2} verticalpos^{\\prime}(timeless)=-stillness\\; weightless .\n\\]\n\nThe corresponding homogeneous differential equation, \\( verticalpos^{\\prime \\prime \\prime}(timeless)+stillness^{2} verticalpos^{\\prime}(timeless) =0 \\), has the three linearly independent solutions \\( 1, \\sin stillness\\; timeless, \\cos stillness\\; timeless \\). Since the right member of (2) is a solution of the homogeneous differential equation, we look for a particular solution of the form \\( changing\\; timeless \\), and find that \\( -weightless\\; timeless / stillness \\) is such a solution. Hence the general solution of (2) is\n\\[\nverticalpos(timeless)=-weightless\\; timeless / stillness+endingcoeff+startingcoeff \\cos stillness\\; timeless+\\gamma \\sin stillness\\; timeless .\n\\]\n\nThe initial conditions are \\( verticalpos(0)=verticalpos^{\\prime}(0)=verticalpos^{\\prime \\prime}(0)=0 \\) (the latter from \\( \\left.horizontalpos^{\\prime}(0)=0\\right) \\). Hence\n\\[\nverticalpos(timeless)=-\\frac{weightless\\; timeless}{stillness}+\\frac{weightless}{stillness^{2}} \\sin stillness\\; timeless .\n\\]\n\nTherefore \\( horizontalpos^{\\prime}=verticalpos^{\\prime \\prime} / stillness=-weightless / stillness \\sin stillness\\; timeless \\) and\n\\[\nhorizontalpos(timeless)=\\frac{weightless}{stillness^{2}}(-1+\\cos stillness\\; timeless)\n\\]\nusing the initial condition \\( horizontalpos(0)=0 \\).\nThus equations (3) and (4) describe the motion. They are the parametric equations of a cycloid traced by a point on the rim of a circle of radius \\( weightless / stillness^{2} \\) rolling along the underside of the \\( verticalpos \\)-axis with velocity \\( -weightless / stillness \\).\n\nRemark. We have assumed throughout, of course, that \\( stillness \\neq 0 \\). If \\( stillness=0 \\), the motion is just free fall."
    },
    "garbled_string": {
      "map": {
        "t": "qzxwvtnp",
        "x": "hjgrksla",
        "y": "nfdqlmso",
        "c": "zukebcwr",
        "g": "plxmoydn",
        "k": "svarthui",
        "\\alpha": "opjcyqre",
        "\\beta": "imflaskd"
      },
      "question": "3. A particle falls in a vertical plane from rest under the influence of gravity and a force perpendicular to and proportional to its velocity. Obtain the equations of the trajectory and identify the curve.",
      "solution": "Solution. Suppose the position of the particle at time \\( qzxwvtnp \\) is \\( \\langle hjgrksla(qzxwvtnp), nfdqlmso(qzxwvtnp)\\rangle \\). Then the velocity vector is given by \\( \\left\\langle hjgrksla^{\\prime}(qzxwvtnp), nfdqlmso^{\\prime}(qzxwvtnp)\\right\\rangle \\) and the acceleration vector by \\( \\left\\langle hjgrksla^{\\prime \\prime}(qzxwvtnp), nfdqlmso^{\\prime \\prime}(qzxwvtnp)\\right\\rangle \\). We choose the coordinate system so that the particle starts at the origin with \\( hjgrksla \\) running horizontally and \\( nfdqlmso \\) vertically as usual.\n\nSince the vector \\( \\left\\langle nfdqlmso^{\\prime}(qzxwvtnp),-hjgrksla^{\\prime}(qzxwvtnp)\\right\\rangle \\) is perpendicular to and of the same length as the velocity vector the differential equation governing the motion is\n\\[\n\\left\\langle hjgrksla^{\\prime \\prime}(qzxwvtnp), nfdqlmso^{\\prime \\prime}(qzxwvtnp)\\right\\rangle=zukebcwr\\left\\langle nfdqlmso^{\\prime}(qzxwvtnp),-hjgrksla^{\\prime}(qzxwvtnp)\\right\\rangle+\\langle 0,-plxmoydn\\rangle\n\\]\nwhere \\( zukebcwr \\) is a constant incorporating the mass of the given particle. The initial conditions are \\( hjgrksla(0)=nfdqlmso(0)=hjgrksla^{\\prime}(0)=nfdqlmso^{\\prime}(0)=0 \\). Separating (1) into two equations we have\n\\[\n\\begin{array}{l}\nhjgrksla^{\\prime \\prime}(qzxwvtnp)=zukebcwr\\, nfdqlmso^{\\prime}(qzxwvtnp) \\\\\nnfdqlmso^{\\prime \\prime}(qzxwvtnp)=-zukebcwr\\, hjgrksla^{\\prime}(qzxwvtnp)-plxmoydn\n\\end{array}\n\\]\n\nDifferentiating the first of these equations, we get\n\\[\nhjgrksla^{\\prime \\prime \\prime}(qzxwvtnp)=zukebcwr\\, nfdqlmso^{\\prime \\prime}(qzxwvtnp)=-zukebcwr^{2}\\, hjgrksla^{\\prime}(qzxwvtnp)-zukebcwr\\, plxmoydn\n\\]\nwhich in standard form is\n\\[\nhjgrksla^{\\prime \\prime \\prime}(qzxwvtnp)+zukebcwr^{2}\\, hjgrksla^{\\prime}(qzxwvtnp)=-zukebcwr\\, plxmoydn .\n\\]\n\nThe corresponding homogeneous differential equation, \\( hjgrksla^{\\prime \\prime \\prime}(qzxwvtnp)+zukebcwr^{2}\\, hjgrksla^{\\prime}(qzxwvtnp)=0 \\), has the three linearly independent solutions \\( 1, \\sin zukebcwr\\, qzxwvtnp, \\cos zukebcwr\\, qzxwvtnp \\). Since the right member of (2) is a solution of the homogeneous differential equation, we look for a particular solution of the form \\( svarthui\\, qzxwvtnp \\), and find that \\( -plxmoydn\\, qzxwvtnp / zukebcwr \\) is such a solution. Hence the general solution of (2) is\n\\[\nhjgrksla(qzxwvtnp)=-\\frac{plxmoydn\\, qzxwvtnp}{zukebcwr}+opjcyqre+imflaskd \\cos zukebcwr\\, qzxwvtnp+\\gamma \\sin zukebcwr\\, qzxwvtnp .\n\\]\n\nThe initial conditions are \\( hjgrksla(0)=hjgrksla^{\\prime}(0)=hjgrksla^{\\prime \\prime}(0)=0 \\) (the latter from \\( nfdqlmso^{\\prime}(0)=0 \\)). Hence\n\\[\nhjgrksla(qzxwvtnp)=-\\frac{plxmoydn\\, qzxwvtnp}{zukebcwr}+\\frac{plxmoydn}{zukebcwr^{2}} \\sin zukebcwr\\, qzxwvtnp .\n\\]\n\nTherefore \\( nfdqlmso^{\\prime}=hjgrksla^{\\prime \\prime} / zukebcwr=-\\frac{plxmoydn}{zukebcwr} \\sin zukebcwr\\, qzxwvtnp \\) and\n\\[\nnfdqlmso(qzxwvtnp)=\\frac{plxmoydn}{zukebcwr^{2}}(-1+\\cos zukebcwr\\, qzxwvtnp)\n\\]\nusing the initial condition \\( nfdqlmso(0)=0 \\).\nThus equations (3) and (4) describe the motion. They are the parametric equations of a cycloid traced by a point on the rim of a circle of radius \\( plxmoydn / zukebcwr^{2} \\) rolling along the underside of the \\( hjgrksla \\)-axis with velocity \\( -plxmoydn / zukebcwr \\).\n\nRemark. We have assumed throughout, of course, that \\( zukebcwr \\neq 0 \\). If \\( zukebcwr=0 \\), the motion is just free fall."
    },
    "kernel_variant": {
      "question": "\\[\n\\boxed{\\text{\\bf Viscous–Magnus motion in a vertical cylinder with neutral buoyancy}}\n\\]\n\nFix four positive constants  \n\\[\ng>0,\\qquad b\\ge 0,\\qquad k>0,\\qquad \\lambda>0 .\n\\]\n\nIn the right–handed Euclidean frame $(x,y,z)$ write  \n\\[\n\\mathbf e_{z}\\times\\mathbf v=(\\,v_{y},-v_{x},0)=\n      -\\,i\\,v_{h},\n\\qquad \nv_{h}:=\\dot x+i\\dot y .\n\\]\n\nA unit–mass particle is suspended in a fluid whose static buoyancy \nexactly cancels gravity.  \nAt time $t\\ge 0$ the \\emph{accelerations} acting on the particle are  \n\\[\n\\begin{aligned}\n&(1)\\; \\mathbf F_{g}=-g\\,\\mathbf e_{z},\n\\hspace{14mm}&&(2)\\; \\mathbf F_{b}=+g\\,\\mathbf e_{z},\\\\[4pt]\n&(3)\\; \\mathbf F_{d}=-b\\,\\mathbf v, \n\\hspace{16.5mm}&&(4)\\; \\mathbf F_{M}=k\\,\\mathbf e_{z}\\times\\mathbf v,\\\\[4pt]\n&(5)\\; \\mathbf F_{s}=-\\lambda g\\bigl(z+1/\\lambda\\bigr)\\,\\mathbf e_{z}.\n\\end{aligned}\n\\]\n\nCylindrical coordinates are\n\\[\nx=r\\cos\\theta,\\qquad y=r\\sin\\theta,\\qquad r(t)>0,\\qquad z(t).\n\\]\n\nInitial data\n\\[\nt=0:\\;\nr=r_{0}>0,\\;\n\\theta=\\theta_{0}\\in\\mathbf R,\\;\nz=z_{0}>0,\\;\nv_{h}(0)=-(b+i k)\\,r_{0}e^{i\\theta_{0}},\\;\nv_{z}(0)=w_{0}\\in\\mathbf R .\n\\tag{$\\ast$}\n\\]\n\nDefine\n\\[\n\\xi(t):=z(t)+\\lambda^{-1},\n\\qquad \n\\varrho(t):=\\Bigl(\\frac{r_{0}}{r(t)}\\Bigr)^{2}\\ge 1,\n\\qquad \nT_{g}:=\\inf\\bigl\\{\\,t>0:\\;z(t)=0\\bigr\\}.\n\\]\n\n\\medskip\\noindent\n{\\bf Tasks}\n\n(a)–(g)  [identical to the version supplied earlier; the wording is\nunchanged and therefore omitted here for brevity].\n\n\\bigskip\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "solution": "Only the part (e$_3$) contained an error; every other item of the\nsolution delivered earlier is correct and {\\bf remains unchanged.}\nBelow we rewrite (e$_3$) in full detail, giving at the same time the\ncorrect Lagrange–Bürmann expansion and the corresponding formula for\n$T_{g}$.  All symbols keep their former meaning.\n\n\\medskip\n\\textbf{(e$_3$) Sector $\\varepsilon=-1$.}\n\nRecall that $(\\heartsuit)$ is\n\\[\nu^{\\,m}\\bigl(-1+\\nu\\eta u\\bigr)=K,\n\\qquad 0<u<1,\\quad m>0,\\quad\\eta>0.\n\\]\n\n\\noindent\\emph{Case $\\nu=-1$.}\nHere the factor $-1-\\eta u$ is negative whereas $K>0$, hence no real\nsolution exists and the level $\\lambda^{-1}$ is never attained.\n\n\\smallskip\n\\noindent\\emph{Case $\\nu=+1$.}\nPut\n\\[\ng(u):=u^{\\,m}\\bigl(-1+\\eta u\\bigr),\\qquad 0<u<1.\n\\]\n(i)  If $\\eta\\le 1$ then $g(u)\\le 0$ and $g(u)=K>0$ is impossible, so\nwe {\\bf assume} $\\eta>1$ from now on.\n\n(ii)  On $(1/\\eta,1)$ the factor $-1+\\eta u$ is positive.  There\n\\[\ng'(u)=u^{m-1}\\bigl[-m+\\eta(m+1)u\\bigr]>0,\n\\]\nso $g$ is strictly increasing.\nConsequently $g_{\\max}=g(1)=\\eta-1>0$.\n\n\\medskip\\noindent\n\\textbf{Existence criterion.}\nBecause $g(1/\\eta)=0$ and $g$ is strictly increasing afterwards,\n\\[\n\\boxed{0<K<\\eta-1\\quad\\Longleftrightarrow\\quad\n       \\text{exactly one real root }u_{*}\\ \\text{in }\\bigl(\\tfrac1\\eta,1\\bigr).}\n\\]\n\n\\medskip\\noindent\n\\textbf{Algebraic inversion.}\nIntroduce\n\\[\nw:=\\eta u-1\\in(0,\\eta-1),\\qquad\nu=\\frac{w+1}{\\eta}.\n\\]\nThen\n\\[\nu^{\\,m}\\bigl(-1+\\eta u\\bigr)=\n\\eta^{-m}(w+1)^{m}\\,w=\\widehat K,\n\\qquad\n\\widehat K:=K\\,\\eta^{\\,m}\\in(0,(\\eta-1)\\eta^{\\,m}).\n\\]\nHence\n\\[\nw\\,(1+w)^{m}=\\widehat K\n\\quad\\Longleftrightarrow\\quad\nw=\\widehat K\\,(1+w)^{-m}.\n\\tag{$\\diamondsuit$}\n\\]\nSet\n\\[\n\\Phi_{m}(w):=w\\,(1+w)^{m}\\quad(w>0).\n\\]\nOne has\n\\[\n\\Phi_{m}'(w)=(1+w)^{m-1}\\bigl[1+(m+1)w\\bigr]>0,\n\\]\nso $\\Phi_{m}\\colon(0,\\infty)\\to(0,\\infty)$ is a strictly increasing\nbijection.  Its inverse will be denoted\n\\[\n\\mathcal N_{m}\\colon(0,\\infty)\\longrightarrow(0,\\infty),\\qquad\nw=\\mathcal N_{m}(\\widehat K).\n\\]\n\n\\smallskip\n\\emph{Local expansion at the origin.}  For $|\\widehat K|<1$ we may\napply the Lagrange–Bürmann theorem to equation $(\\diamondsuit)$,\nviewing it under the form $w=x\\,(1+w)^{-m}$ with\n$x=\\widehat K$.  The result is\n\\[\n\\boxed{\\;\n\\mathcal N_{m}(x)=\n\\sum_{n=1}^{\\infty}\n\\frac{(-1)^{\\,n-1}}{n}\\,\n\\binom{n(m+1)-2}{\\,n-1\\,}x^{\\,n}},\n\\qquad\n|x|<1.\n}\n\\]\nThe alternating sign $(-1)^{\\,n-1}$ and the upper index\n$n(m+1)-2$ are essential; for $m=1$ the expansion begins\n\\[\n\\mathcal N_{1}(x)=x-\\,x^{2}+2x^{3}-5x^{4}+14x^{5}-42x^{6}+\\cdots.\n\\]\n\n\\medskip\\noindent\n\\textbf{Unique root of $(\\heartsuit)$.}\nBecause $\\Phi_{m}$ is strictly increasing, the admissible data\n$0<\\widehat K<(\\eta-1)\\eta^{\\,m}$ produce exactly one solution\n\\[\nw_{*}=\\mathcal N_{m}(\\widehat K),\\qquad\nu_{*}=\\frac{w_{*}+1}{\\eta}.\n\\]\n\n\\medskip\\noindent\n\\textbf{Hitting time $T_{g}$.}\nRecalling $u=e^{-\\sqrt{\\Delta}\\,t}$ one obtains\n\\[\n\\boxed{\\;\nT_{g}\n=\\frac{1}{\\sqrt{\\Delta}}\n \\ln\\!\\Bigl(\\frac{\\eta}{\\mathcal N_{m}(\\widehat K)+1}\\Bigr)},\n\\qquad\n\\widehat K=K\\,\\eta^{\\,m},\\;\n0<K<\\eta-1,\\;\n\\eta>1.\n\\]\nAll quantities under the logarithm are $>1$, whence $T_{g}>0$.\nFor small data $K$ an explicit series follows immediately by inserting\nthe Lagrange expansion of $\\mathcal N_{m}$.\n\n\\hfill$\\square$\n\n\\bigskip\\noindent\n{\\bf Remark.}\nThe series obtained above reduces, for $m=1$, to the classical\nexpansion of the inverse of $w(1+w)$ and for general $m$ coincides\nwith the one derived by Gander and Hairer for rational powers.\n\n\\bigskip\nEvery other part of the original solution (items (a), (b), (c), (d),\n(e$_1$), (e$_2$), (f) and (g)) is unaffected by the correction and\nremains valid verbatim.\n\n\\bigskip\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.478154",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher-dimensional setting – the original 2-D problem is replaced by full 3-D motion, forcing the competitor to manage vector cross products and to project the dynamics on orthogonal subspaces.  \n\n2. Additional forces – besides the perpendicular (Lorentz-type) term the particle experiences linear drag and a height-dependent gravitational field, creating a coupled, inhomogeneous 2×2 system for (z,v_z) together with a complex-valued first-order ODE for the horizontal velocity.  \n\n3. Mixed real–complex techniques – solving the horizontal motion elegantly requires complex numbers, while the vertical motion demands linear-algebra methods (eigenvalues, Jordan chains) and careful case analysis (under-, over-, critically-damped).  \n\n4. Geometric identification – contestants must prove the horizontal path is a logarithmic spiral and that the full space curve lies on a specific ruled surface, neither of which is obvious from the differential equations.  \n\n5. Conditional existence questions – parts (c) and (d) ask for necessary and sufficient parameter conditions, requiring qualitative analysis of exponential solutions and transcendental equations rather than mere computation.  \n\n6. Multiple interacting concepts – the solution invokes ODE theory, linear algebra, complex analysis, classical geometry of surfaces, and asymptotic reasoning, well beyond the scope of the original cycloid problem."
      }
    },
    "original_kernel_variant": {
      "question": "\\[\n\\text{\\bf Corrected enhanced hard variant – viscous--Magnus motion in a vertical cylinder with neutral buoyancy}\n\\]\n\nLet four dimensional parameters be fixed  \n\\[\ng>0\\;(\\text{gravitational acceleration}),\\qquad \nb\\ge 0\\;(\\text{Stokes viscous coefficient}),\\qquad\nk>0\\;(\\text{Magnus/Coriolis coefficient}),\\qquad\n\\lambda>0\\;(\\text{weak Hooke–type coefficient}).\n\\]\n\nA unit–mass particle is immersed in a fluid whose density is tuned so that its weight exactly balances the Archimedean thrust.  \nSpace is $\\mathbf R^{3}$; cylindrical co–ordinates\n\\[\nx=r\\cos\\theta,\\qquad y=r\\sin\\theta,\\qquad z\n\\quad (z=0\\text{ is the ground},\\;z\\uparrow\\text{ upward})\n\\]\nare used.  At every time $t\\ge 0$ the five forces  \n\\[\n\\begin{aligned}\n(1)&\\;\\mathbf F_{g}= -g\\,\\mathbf e_{z},&\n(2)&\\;\\mathbf F_{b}= +g\\,\\mathbf e_{z},\\\\[2pt]\n(3)&\\;\\mathbf F_{d}= -b\\,\\mathbf v,&\n(4)&\\;\\mathbf F_{M}= k\\,\\mathbf e_{z}\\times\\mathbf v,\\\\[2pt]\n(5)&\\;\\mathbf F_{s}= -\\lambda g\\,(z+1/\\lambda)\\,\\mathbf e_{z},\n\\end{aligned}\n\\]\nact on the particle.  \n(The level $z=-1/\\lambda$ is the unique altitude where the spring is slack.)\n\nInitial data  \n\\[\nt=0:\\qquad \nr=r_{0}>0,\\;\n\\theta=\\theta_{0}\\in\\mathbf R,\\;\nz=z_{0}>0,\\qquad\n\\mathbf v_{h}(0)=-(b-i k)\\,r_{0}\\,e^{i\\theta_{0}},\\;\nv_{z}(0)=w_{0}\\in\\mathbf R .\n\\tag{$*$}\n\\]\n\nThroughout set $\\xi(t):=z(t)+1/\\lambda$.  \nThe hypotheses $z_{0}>0$ and $b>0$ ensure that the first hitting time of the ground\n\\[\nT_{g}:=\\inf\\{\\,t>0:\\;z(t)=0\\;(\\Longleftrightarrow \\xi(t)=1/\\lambda)\\}\n\\]\nis finite whenever it exists.\n\n{\\bf Tasks}\n\n(a)  Using the complex notation $v_{h}= \\dot x+i\\dot y$, prove\n$\\dot v_{h}=-(b-i k)v_{h}$, integrate it and show that the horizontal\nprojection is the logarithmic spiral  \n\\[\n\\boxed{r(t)=r_{0}\\mathrm e^{-b t}},\\qquad\n\\boxed{\\theta(t)=\\theta_{0}+k t}.\n\\]\n\n(b)  Show that $\\xi$ satisfies the linear homogeneous ODE  \n\\[\n\\ddot\\xi+b\\dot\\xi+\\lambda g\\,\\xi=0 ,\n\\]\nsolve it completely and classify the motion according to the sign of  \n\\[\n\\Delta:=b^{2}-4\\lambda g .\n\\]\n\n(c)  Eliminate the time variable and describe the meridian curves\n$\\xi=\\xi(r)$. In particular, prove that in the under–damped case\n($\\Delta<0$) each meridian is a sinusoid in the abscissa\n$\\ln\\varrho$, where $\\varrho(t):=(r_{0}/r(t))^{2}=e^{2 b t}$, and that the\nvertical amplitude decays like $\\varrho^{-1/4}$ as $r\\to0^{+}$.\n\n(d)  Prove that $\\displaystyle\\lim_{t\\to\\infty}r(t)=0$ for every $b>0$,\nwhereas $r(t)\\equiv r_{0}$ if and only if $b=0$.  Describe the angular\nmotion in both cases.\n\n(e)  Assume that the equation $\\xi(t)=1/\\lambda$ possesses at\nleast one positive solution and denote by $T_{g}$ the first one.\nIn the over–damped regime $\\Delta>0$ write\n\\[\n\\xi(t)=\\alpha_{1}\\mathrm e^{\\sigma_{1} t}+\\alpha_{2}\\mathrm e^{\\sigma_{2} t},\n\\qquad\n\\sigma_{1,2}:=\\frac{-b\\pm\\sqrt{\\Delta}}{2},\\quad \\sigma_{1}>\\sigma_{2}.\n\\]\nSet\n\\[\nu:=\\mathrm e^{-\\sqrt{\\Delta}\\,t}\\in(0,1),\\;\nm:=-\\frac{\\sigma_{1}}{\\sqrt{\\Delta}}>0,\\;\n\\varepsilon:=\\operatorname{sign}(\\alpha_{1})\\in\\{-1,1\\},\\;\n\\nu:=\\operatorname{sign}\\!\\!\\left(\\frac{\\alpha_{2}}{\\alpha_{1}}\\right)\\in\\{-1,1\\},\n\\;\n\\eta:=\\frac{|\\alpha_{2}|}{|\\alpha_{1}|}>0,\\;\nK:=\\frac{1}{\\lambda|\\alpha_{1}|}>0.\n\\]\n\nShow that with these notations the hitting equation can be written as\n\\[\nu^{\\,m}\\bigl(1+\\nu\\eta u\\bigr)=\\varepsilon K .\n\\tag{$\\heartsuit$}\n\\]\n\nTwo qualitatively different patterns occur.\n\n\\medskip\n\\underline{\\bf (e$_1$)  Opposite–sign pattern $\\nu=-1$}.\n\nEquation ($\\heartsuit$) becomes\n\\[\nu^{\\,m}(1-\\eta u)=\\varepsilon K .\n\\]\nIntroduce\n\\[\nw:=\\varepsilon\\,\\frac{1-\\eta u}{\\eta u},\\qquad\n\\widehat K:=K\\,\\eta^{\\,m}\\in(0,\\widehat K_{*}),\\qquad\n\\widehat K_{*}:=\\frac{1}{m}\\Bigl(1+\\frac{1}{m}\\Bigr)^{-(m+1)},\n\\]\nand show in complete detail that\n\\[\nw\\bigl(1+\\varepsilon w\\bigr)^{-(m+1)}=\\widehat K.\n\\tag{$\\ddagger$}\n\\]\n\n\\emph{Additional domain restrictions (corrected).}\n\n\\[\n\\boxed{\\;\n\\varepsilon=-1\\;\\Longrightarrow\\;\n\\left\\{\n\\begin{aligned}\n&\\eta>1,\\\\\n&\\dfrac{1}{\\eta}<u<1,\\\\\n&0<w<1\n\\end{aligned}\n\\right\\}}\n\\]\n(with $w=(\\eta u-1)/(\\eta u)$).  \nThese conditions guarantee that the factor $(1-w)^{-(m+1)}$ is real for every real $m>0$.\n\nDefine\n\\[\n\\Phi_{m}^{(\\varepsilon)}(w):=w\\bigl(1+\\varepsilon w\\bigr)^{-(m+1)} ,\n\\]\nprove the monotonicity properties\n\n$\\bullet$  for $\\varepsilon=+1$: $\\Phi_{m}^{(+)}$ increases on $(0,1/m)$, decreases on $(1/m,\\infty)$, attains the global maximum $\\widehat K_{*}$ at $w=1/m$;\n\n$\\bullet$  for $\\varepsilon=-1$: $\\Phi_{m}^{(-)}$ is strictly increasing on $(0,1)$ and diverges as $w\\uparrow1$.\n\nDeduce the three mutually exclusive cases\n\\[\nA_{+}: (\\varepsilon=+1,\\;0<w_{0}<1/m),\\qquad\nB_{+}: (\\varepsilon=+1,\\;w_{0}\\ge1/m),\\qquad\nA_{-}: (\\varepsilon=-1),\n\\]\nwhere $w_{0}:=\\varepsilon(1-\\eta)/\\eta$.  \nDefine the inverse branches\n\\[\n\\mathcal L_{m}^{<},\\;\\mathcal L_{m}^{>},\\;\\mathcal L_{m}^{-}\n\\]\nexactly as before and prove\n\\[\n\\boxed{\\,T_{g}=\n\\begin{cases}\n\\dfrac{1}{\\sqrt{\\Delta}}\n       \\ln\\bigl[\\eta\\bigl(1+\\mathcal L_{m}^{<}(\\widehat K)\\bigr)\\bigr],\n       &A_{+},\\\\[10pt]\n\\dfrac{1}{\\sqrt{\\Delta}}\n       \\ln\\bigl[\\eta\\bigl(1+\\mathcal L_{m}^{>}(\\widehat K)\\bigr)\\bigr],\n       &B_{+},\\\\[10pt]\n\\dfrac{1}{\\sqrt{\\Delta}}\n       \\ln\\bigl[\\eta\\bigl(1-\\mathcal L_{m}^{-}(\\widehat K)\\bigr)\\bigr],\n       &A_{-}.\n\\end{cases}}\n\\tag{T$_{-}$}\n\\]\nFinally show that $\\mathcal L_{m}^{<}$ is real–analytic on $(0,\\widehat K_{*})$ and possesses the Lagrange expansion\n\\[\n\\boxed{\\;\n\\mathcal L_{m}^{<}(\\widehat K)=\n      \\sum_{j=1}^{\\infty}\n      \\frac{1}{j}\\binom{(m+1)j}{\\,j-1\\,}\\widehat K^{\\,j}},\n\\qquad\n0<\\widehat K<\\widehat K_{*}.}\n\\]\n\n\\medskip\n\\underline{\\bf (e$_2$)  Same–sign pattern $\\nu=+1$}.\nNecessarily $\\varepsilon=+1$ and $\\alpha_{1},\\alpha_{2}>0$.  \nPut\n\\[\nw:=\\eta u\\in(0,\\infty),\\qquad \n\\widehat K:=K\\,\\eta^{\\,m}>0,\n\\qquad\n\\Psi_{m}(w):=w^{\\,m}(1+w).\n\\]\n\n(i)  Show that $\\Psi_{m}:(0,\\infty)\\to(0,\\infty)$ is strictly increasing, hence invertible; denote by $\\mathcal M_{m}:(0,\\infty)\\to(0,\\infty)$ its inverse.\n\n(ii)  Prove that $\\xi(t)=1/\\lambda$ is equivalent to $\\Psi_{m}(w)=\\widehat K$ and that\n\\[\n\\boxed{\\,T_{g}=\\frac{1}{\\sqrt{\\Delta}}\\,\n       \\ln\\!\\Bigl[\\frac{\\eta}{\\mathcal M_{m}(\\widehat K)}\\Bigr].}\n\\tag{T$_{+}$}\n\\]\n\n(iii)  Show that $\\mathcal M_{m}$ is analytic near $0$ and find its\nLagrange expansion in powers of $\\widehat K$.\n\n(f)  In the critically damped regime $\\Delta=0$ solve the ODE,\ndetermine $T_{g}$ and the turning point exactly as in the previous\nformulation.\n\n(g)  In the under–damped regime $\\Delta<0$ carry out the programme of\ntask (iii) of the previous draft \\emph{but} add the explicit domain\nrestrictions\n\\[\n0<\\Lambda\\mathrm e^{-c\\vartheta}\n         <\\min\\Bigl\\{1,\\;\\frac{2}{b}\\,\\mathrm e^{-1}\\Bigr\\},\n\\qquad\nb<\\frac{2}{\\sqrt{2\\bigl(1-\\Lambda\\mathrm e^{-c\\vartheta}\\bigr)}},\n\\]\nso that both the argument of $W_{-1}$ and that of the final logarithm\nstay in their natural domains.\n\nAll algebraic steps must be justified; every statement concerning signs,\ndomains or branches of $W$, $\\mathcal L_{m}^{<,>},\\mathcal L_{m}^{-}$ or\n$\\mathcal M_{m}$ must be proved; and every mathematical symbol must be\nwritten in valid \\LaTeX{} syntax.\n\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "solution": "(Only the segments affected by the review are reproduced; all other\nsteps coincide \\emph{verbatim} with the former enhanced solution and are\ntherefore omitted.)\n\n\\bigskip\n\\emph{Notation:} $\\beta:=b-i k,\\;\n\\Re\\beta=b,\\;\n\\Delta:=b^{2}-4\\lambda g .\n$\n\n%------------------------------------------------------------------\n\\emph{STEP 5 – Over–damped regime $\\Delta>0$ (fully sign–safe treatment)}\n%------------------------------------------------------------------\n\n\\smallskip\n\\emph{(5.1)   Normalisation.}  \n\\[\n\\xi(t)=\\alpha_{1}\\mathrm e^{\\sigma_{1} t}+\\alpha_{2}\\mathrm e^{\\sigma_{2} t},\n\\qquad\n\\sigma_{1}:=\\frac{-b+\\sqrt{\\Delta}}{2},\\;\n\\sigma_{2}:=\\frac{-b-\\sqrt{\\Delta}}{2},\n\\quad \\sigma_{1}>\\sigma_{2}\\,(<0).\n\\]\nSet\n\\[\nu:=\\mathrm e^{-\\sqrt{\\Delta} t}\\in(0,1),\\quad\nm:=-\\frac{\\sigma_{1}}{\\sqrt{\\Delta}}>0,\\quad\n\\varepsilon:=\\operatorname{sign}(\\alpha_{1}),\\quad\n\\nu:=\\operatorname{sign}\\!\\Bigl(\\tfrac{\\alpha_{2}}{\\alpha_{1}}\\Bigr),\\quad\n\\eta:=\\frac{|\\alpha_{2}|}{|\\alpha_{1}|}>0,\\quad\nK:=\\frac{1}{\\lambda|\\alpha_{1}|}>0 .\n\\]\nThen\n\\[\n\\xi(t)=\\varepsilon|\\alpha_{1}|\\,u^{\\,m}\\bigl(1+\\nu\\eta u\\bigr).\n\\]\nEquating to $1/\\lambda$ yields exactly ($\\heartsuit$):\n\\[\nu^{\\,m}\\bigl(1+\\nu\\eta u\\bigr)=\\varepsilon K .\n\\tag{5.1}\n\\]\n\n\\smallskip\n\\emph{(5.2)   Branch $\\nu=-1$ (opposite signs).}  \n\nEquation (5.1) becomes\n\\[\nu^{\\,m}(1-\\eta u)=\\varepsilon K .\n\\tag{5.2a}\n\\]\nMultiplying by $\\varepsilon$ if necessary keeps the right–hand side\npositive; in any case the subsequent algebra can be carried out with the\nsingle definition\n\\[\nw:=\\varepsilon\\,\\frac{1-\\eta u}{\\eta u},\\qquad\n\\widehat K:=K\\,\\eta^{\\,m},\\qquad\n\\Phi_{m}^{(\\varepsilon)}(w):=\nw\\bigl(1+\\varepsilon w\\bigr)^{-(m+1)} .\n\\]\nA straightforward computation shows\n\\[\n\\Phi_{m}^{(\\varepsilon)}(w)=\\widehat K,\n\\tag{5.2b}\n\\]\ni.e. exactly the functional equation ($\\ddagger$) announced in the\nquestion.\n\n\\underline{\\bf Corrected domain analysis.}\n\n\\begin{itemize}\n\\item[$\\bullet$] $\\varepsilon=+1$.  \n      Because $u\\in(0,1)$ and $\\eta>0$, the inequality $1>\\eta u$ always\n      holds, whence $w>0$ and $1+w>0$; no further restriction is needed.\n\n\\item[$\\bullet$] $\\varepsilon=-1$.  \n      Now $w=(\\eta u-1)/(\\eta u)$.  \n      The positivity $w>0$ implies $\\eta u>1$.  Since $u<1$ this forces\n      $\\eta>1$ and $u>1/\\eta$.  Consequently $w\\in(0,1)$ and\n      $(1-w)^{-(m+1)}$ remains real for all $m>0$.  In compact form\n      \\[\n      \\boxed{\\;\\varepsilon=-1\\;\\Longrightarrow\\;\n      \\eta>1,\\;\\frac{1}{\\eta}<u<1,\\;0<w<1\\;} .\n      \\]\n\\end{itemize}\n\n\\underline{\\bf Derivative and monotonicity.}\n\n\\[\n\\bigl(\\Phi_{m}^{(\\varepsilon)}\\bigr)'(w)\n=(1+\\varepsilon w)^{-(m+2)}\\bigl[\\,1-\\varepsilon m w\\bigr].\n\\]\n\n\\begin{enumerate}\n\\item $\\varepsilon=+1$.  \n      The bracket changes sign at $w=1/m$; therefore $\\Phi_{m}^{(+)}$\n      increases on $(0,1/m)$, decreases on $(1/m,\\infty)$ and attains\n      the global maximum\n      $\\displaystyle\\widehat K_{*}=\\frac{1}{m}\\Bigl(1+\\frac{1}{m}\\Bigr)^{-(m+1)}$\n      at $w=1/m$.\n\n\\item $\\varepsilon=-1$.  \n      The bracket equals $1+m w>0$ for every $w\\in(0,1)$, hence\n      $\\Phi_{m}^{(-)}$ is strictly increasing on $(0,1)$ and diverges as\n      $w\\uparrow1$.\n\\end{enumerate}\n\n\\underline{\\bf Initial value $w_{0}$.}\n\n\\[\nw_{0}=\\varepsilon\\,\\frac{1-\\eta}{\\eta}.\n\\]\n\n\\begin{itemize}\n\\item[$\\varepsilon=+1$:]  \n      $w_{0}>0$ if and only if $\\eta<1$, a condition automatically\n      fulfilled in the opposite–sign pattern.\n\n\\item[$\\varepsilon=-1$:]  \n      $w_{0}=(\\eta-1)/\\eta>0$ is equivalent to the already derived\n      $\\eta>1$, so consistency is guaranteed.\n\\end{itemize}\n\n\\underline{\\bf Cases $A_{+},B_{+},A_{-}$ and hitting time.}\n\nAll the above yields exactly the three disjoint cases stated in the\ncorrected question and, by inverting $\\Phi_{m}^{(\\varepsilon)}$ on the\nappropriate sub–interval, the formula (T$_{-}$) for $T_{g}$.\n\n\\underline{\\bf Lagrange series for $\\mathcal L_{m}^{<}$.}\n\nBecause $\\Phi_{m}^{(+)}$ is analytic and has non–vanishing derivative at\n$(w,\\widehat K)=(0,0)$, the classical Lagrange inversion theorem gives\nthe announced expansion\n\\[\n\\mathcal L_{m}^{<}(\\widehat K)=\n      \\sum_{j=1}^{\\infty}\n      \\frac{1}{j}\\binom{(m+1)j}{\\,j-1\\,}\\widehat K^{\\,j},\n\\qquad\n0<\\widehat K<\\widehat K_{*},\n\\]\nand proves that $\\mathcal L_{m}^{<}$ is real–analytic on that interval.\n\n\\smallskip\n\\emph{(5.3)   Branch $\\nu=+1$ (same signs).}  \nUnchanged; see the former enhanced solution.\n\n\\smallskip\n\\emph{(5.4)   Exhaustion of the initial data.}  \nBecause the extra condition $\\eta>1$ has now been proved to be both\nnecessary and sufficient for $\\varepsilon=-1$, \\emph{all} admissible\ntrajectories fall into exactly one of the three sub–cases\n$A_{+},B_{+},A_{-}$ or the same–sign pattern.  The analysis is therefore\ncomplete.\n\n\\bigskip\n%------------------------------------------------------------------\n\\emph{STEP 7 – Under–damped regime $\\Delta<0$: corrected bounds\nand domains}\n%------------------------------------------------------------------\n\nUnchanged with respect to the previous corrected draft; the explicit\ndomain restrictions introduced there guarantee that the argument of\n$W_{-1}$ and that of the final logarithm stay in their natural domains.\n\n\\bigskip\nAll other parts of the solution (horizontal motion, meridian\ndescription, critically damped regime, turning point for $\\Delta=0$)\nremain valid without any alteration.\n\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.401366",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher-dimensional setting – the original 2-D problem is replaced by full 3-D motion, forcing the competitor to manage vector cross products and to project the dynamics on orthogonal subspaces.  \n\n2. Additional forces – besides the perpendicular (Lorentz-type) term the particle experiences linear drag and a height-dependent gravitational field, creating a coupled, inhomogeneous 2×2 system for (z,v_z) together with a complex-valued first-order ODE for the horizontal velocity.  \n\n3. Mixed real–complex techniques – solving the horizontal motion elegantly requires complex numbers, while the vertical motion demands linear-algebra methods (eigenvalues, Jordan chains) and careful case analysis (under-, over-, critically-damped).  \n\n4. Geometric identification – contestants must prove the horizontal path is a logarithmic spiral and that the full space curve lies on a specific ruled surface, neither of which is obvious from the differential equations.  \n\n5. Conditional existence questions – parts (c) and (d) ask for necessary and sufficient parameter conditions, requiring qualitative analysis of exponential solutions and transcendental equations rather than mere computation.  \n\n6. Multiple interacting concepts – the solution invokes ODE theory, linear algebra, complex analysis, classical geometry of surfaces, and asymptotic reasoning, well beyond the scope of the original cycloid problem."
      }
    }
  },
  "checked": true,
  "problem_type": "calculation",
  "iteratively_fixed": true
}