{
  "index": "1998-A-2",
  "type": "GEO",
  "tag": [
    "GEO",
    "ANA"
  ],
  "difficulty": "",
  "question": "Let $s$ be any arc of the unit circle lying entirely in the first\nquadrant.  Let $A$ be the area of the region lying below $s$ and\nabove the $x$-axis and let $B$ be the area of the region lying to the\nright of the $y$-axis and to the left of $s$.  Prove that $A+B$ depends\nonly on the arc length, and not on the position, of $s$.",
  "solution": "First solution:\nto fix notation, let $A$ be the area\nof region $DEFG$, and $B$ be the area of $DEIH$; further\nlet $C$ denote the area of sector $ODE$, which only depends on the\narc length of $s$.  If $[XYZ]$ denotes the area of triangle\n$[XYZ]$, then we have\n$A = C + [OEG] - [ODF]$ and $B = C + [ODH] - [OEI]$.  But\nclearly $[OEG] = [OEI]$ and $[ODF] = [ODH]$, and so\n$A + B = 2C$.\n\n\\begin{center}\n\\begin{tikzpicture}\n\\draw (0,0) circle (2);\n\\draw (0,2) -- (0,0) -- (2,0);\n\\draw (1.732,0) -- (1.732,1) -- (0,1);\n\\draw (.7,0) -- (.7,1.873) -- (0,1.873);\n\\draw (1.732,1) -- (0,0) -- (.7,1.873);\n\\draw (0,0) node[anchor=north east] {$O$};\n\\draw (.7,0) node[anchor=north] {$F$};\n\\draw (1.732,0) node[anchor=north] {$G$};\n\\draw (1.732,1) node[anchor=south west] {$E$};\n\\draw (.7,1.873) node[anchor=south west] {$D$};\n\\draw (0,1) node[anchor=east] {$I$};\n\\draw (0,1.7) node[anchor=east] {$H$};\n\\end{tikzpicture}\n\\end{center}\n\nSecond solution: We may parametrize a point in $s$ by any of\n$x$, $y$, or $\\theta = \\tan^{-1} (y/x)$.  Then $A$ and $B$ are\njust the integrals of $y\\,dx$ and $x\\,dy$ over the appropriate\nintervals; thus $A+B$ is the integral of $x\\,dy - y\\,dx$\n(minus because the limits of integration are reversed).\nBut $d\\theta = x\\,dy - y\\,dx$, and so $A+B = \\Delta \\theta$\nis precisely the radian measure of $s$. (Of course, one can perfectly well\ndo this problem by computing the two integrals\nseparately. But what's the fun in that?)",
  "vars": [
    "s",
    "x",
    "y",
    "\\\\theta",
    "A",
    "B",
    "C",
    "D",
    "E",
    "F",
    "G",
    "H",
    "I",
    "O"
  ],
  "params": [],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "s": "quadrantarc",
        "x": "horizcoord",
        "y": "vertcoord",
        "\\theta": "angleparam",
        "A": "firstarea",
        "B": "secondarea",
        "C": "sectorarea",
        "D": "pointdelta",
        "E": "pointecho",
        "F": "pointfoxtrot",
        "G": "pointgolf",
        "H": "pointhotel",
        "I": "pointindia",
        "O": "originpoint"
      },
      "question": "Let $quadrantarc$ be any arc of the unit circle lying entirely in the first\nquadrant.  Let $firstarea$ be the area of the region lying below $quadrantarc$ and\nabove the $horizcoord$-axis and let $secondarea$ be the area of the region lying to the\nright of the $vertcoord$-axis and to the left of $quadrantarc$.  Prove that $firstarea+secondarea$ depends\nonly on the arc length, and not on the position, of $quadrantarc$.",
      "solution": "First solution:\nto fix notation, let $firstarea$ be the area\nof region $pointdelta pointecho pointfoxtrot pointgolf$, and $secondarea$ be the area of $pointdelta pointecho pointindia pointhotel$; further\nlet $sectorarea$ denote the area of sector $originpoint pointdelta pointecho$, which only depends on the\narc length of $quadrantarc$.  If $[XYZ]$ denotes the area of triangle\n$[XYZ]$, then we have\n$firstarea = sectorarea + [originpoint pointecho pointgolf] - [originpoint pointdelta pointfoxtrot]$ and $secondarea = sectorarea + [originpoint pointdelta pointhotel] - [originpoint pointecho pointindia]$.  But\nclearly $[originpoint pointecho pointgolf] = [originpoint pointecho pointindia]$ and $[originpoint pointdelta pointfoxtrot] = [originpoint pointdelta pointhotel]$, and so\n$firstarea + secondarea = 2\\,sectorarea$.\n\n\\begin{center}\n\\begin{tikzpicture}\n\\draw (0,0) circle (2);\n\\draw (0,2) -- (0,0) -- (2,0);\n\\draw (1.732,0) -- (1.732,1) -- (0,1);\n\\draw (.7,0) -- (.7,1.873) -- (0,1.873);\n\\draw (1.732,1) -- (0,0) -- (.7,1.873);\n\\draw (0,0) node[anchor=north east] {$originpoint$};\n\\draw (.7,0) node[anchor=north] {$pointfoxtrot$};\n\\draw (1.732,0) node[anchor=north] {$pointgolf$};\n\\draw (1.732,1) node[anchor=south west] {$pointecho$};\n\\draw (.7,1.873) node[anchor=south west] {$pointdelta$};\n\\draw (0,1) node[anchor=east] {$pointindia$};\n\\draw (0,1.7) node[anchor=east] {$pointhotel$};\n\\end{tikzpicture}\n\\end{center}\n\nSecond solution: We may parametrize a point in $quadrantarc$ by any of\n$horizcoord$, $vertcoord$, or $angleparam = \\tan^{-1} (vertcoord/horizcoord)$.  Then $firstarea$ and $secondarea$ are\njust the integrals of $vertcoord\\,dhorizcoord$ and $horizcoord\\,dvertcoord$ over the appropriate\nintervals; thus $firstarea+secondarea$ is the integral of $horizcoord\\,dvertcoord - vertcoord\\,dhorizcoord$\n(minus because the limits of integration are reversed).\nBut $dangleparam = horizcoord\\,dvertcoord - vertcoord\\,dhorizcoord$, and so $firstarea+secondarea = \\Delta angleparam$\nis precisely the radian measure of $quadrantarc$. (Of course, one can perfectly well\ndo this problem by computing the two integrals\nseparately. But what's the fun in that?)"
    },
    "descriptive_long_confusing": {
      "map": {
        "s": "horizonex",
        "x": "paradoxer",
        "y": "labyrinth",
        "\\theta": "ignition",
        "A": "nebulasix",
        "B": "galactus",
        "C": "quartzite",
        "D": "sapphire",
        "E": "emeralden",
        "F": "flamingo",
        "G": "gondolium",
        "H": "harpooner",
        "I": "insignia",
        "O": "oblivion"
      },
      "question": "Let $horizonex$ be any arc of the unit circle lying entirely in the first quadrant.  Let $nebulasix$ be the area of the region lying below $horizonex$ and above the $paradoxer$-axis and let $galactus$ be the area of the region lying to the right of the $labyrinth$-axis and to the left of $horizonex$.  Prove that $nebulasix+galactus$ depends only on the arc length, and not on the position, of $horizonex$.",
      "solution": "First solution:\nto fix notation, let $nebulasix$ be the area\nof region $sapphireemeraldenflamingogondolium$, and $galactus$ be the area of $sapphireemeraldeninsigniaharpooner$; further\nlet $quartzite$ denote the area of sector $oblivionsapphireemeralden$, which only depends on the\narc length of $horizonex$.  If $[XYZ]$ denotes the area of triangle\n$[XYZ]$, then we have\n$nebulasix = quartzite + [oblivionemeraldengondolium] - [oblivionsapphireflamingo]$ and $galactus = quartzite + [oblivionsapphireharpooner] - [oblivionemeraldeninsignia]$.  But\nclearly $[oblivionemeraldengondolium] = [oblivionemeraldeninsignia]$ and $[oblivionsapphireflamingo] = [oblivionsapphireharpooner]$, and so\n$nebulasix + galactus = 2quartzite$.\n\n\\begin{center}\n\\begin{tikzpicture}\n\\draw (0,0) circle (2);\n\\draw (0,2) -- (0,0) -- (2,0);\n\\draw (1.732,0) -- (1.732,1) -- (0,1);\n\\draw (.7,0) -- (.7,1.873) -- (0,1.873);\n\\draw (1.732,1) -- (0,0) -- (.7,1.873);\n\\draw (0,0) node[anchor=north east] {$oblivion$};\n\\draw (.7,0) node[anchor=north] {$flamingo$};\n\\draw (1.732,0) node[anchor=north] {$gondolium$};\n\\draw (1.732,1) node[anchor=south west] {$emeralden$};\n\\draw (.7,1.873) node[anchor=south west] {$sapphire$};\n\\draw (0,1) node[anchor=east] {$insignia$};\n\\draw (0,1.7) node[anchor=east] {$harpooner$};\n\\end{tikzpicture}\n\\end{center}\n\nSecond solution: We may parametrize a point in $horizonex$ by any of\n$paradoxer$, $labyrinth$, or $ignition = \\tan^{-1} (labyrinth/paradoxer)$.  Then $nebulasix$ and $galactus$ are\njust the integrals of $labyrinth\\,dparadoxer$ and $paradoxer\\,dlabyrinth$ over the appropriate\nintervals; thus $nebulasix+galactus$ is the integral of $paradoxer\\,dlabyrinth - labyrinth\\,dparadoxer$\n(minus because the limits of integration are reversed).\nBut $dignition = paradoxer\\,dlabyrinth - labyrinth\\,dparadoxer$, and so $nebulasix+galactus = \\Delta ignition$\nis precisely the radian measure of $horizonex$. (Of course, one can perfectly well\ndo this problem by computing the two integrals\nseparately. But what's the fun in that?)"
    },
    "descriptive_long_misleading": {
      "map": {
        "s": "straightline",
        "x": "verticalcoord",
        "y": "horizontalcoord",
        "\\theta": "linearparam",
        "A": "perimeter",
        "B": "circumference",
        "C": "anglemeasure",
        "D": "nonpointd",
        "E": "nonpointe",
        "F": "nonpointf",
        "G": "nonpointg",
        "H": "nonpointh",
        "I": "nonpointi",
        "O": "nonpointo"
      },
      "question": "Let $straightline$ be any arc of the unit circle lying entirely in the first\nquadrant.  Let $perimeter$ be the area of the region lying below $straightline$ and\nabove the $verticalcoord$-axis and let $circumference$ be the area of the region lying to the\nright of the $horizontalcoord$-axis and to the left of $straightline$.  Prove that $perimeter+circumference$ depends\nonly on the arc length, and not on the position, of $straightline$.",
      "solution": "First solution:\nto fix notation, let $perimeter$ be the area\nof region $nonpointdnonpointenonpointfnonpointg$, and $circumference$ be the area of $nonpointdnonpointenonpointinonpointh$; further\nlet $anglemeasure$ denote the area of sector $nonpointononpointdnonpointe$, which only depends on the\narc length of $straightline$.  If $[XYZ]$ denotes the area of triangle\n$[XYZ]$, then we have\n$perimeter = anglemeasure + [nonpointononpointenonpointg] - [nonpointononpointdnonpointf]$ and $circumference = anglemeasure + [nonpointononpointdnonpointh] - [nonpointononpointenonpointi]$.  But\nclearly $[nonpointononpointenonpointg] = [nonpointononpointenonpointi]$ and $[nonpointononpointdnonpointf] = [nonpointononpointdnonpointh]$, and so\n$perimeter + circumference = 2anglemeasure$.\n\n\\begin{center}\n\\begin{tikzpicture}\n\\draw (0,0) circle (2);\n\\draw (0,2) -- (0,0) -- (2,0);\n\\draw (1.732,0) -- (1.732,1) -- (0,1);\n\\draw (.7,0) -- (.7,1.873) -- (0,1.873);\n\\draw (1.732,1) -- (0,0) -- (.7,1.873);\n\\draw (0,0) node[anchor=north east] {$nonpointo$};\n\\draw (.7,0) node[anchor=north] {$nonpointf$};\n\\draw (1.732,0) node[anchor=north] {$nonpointg$};\n\\draw (1.732,1) node[anchor=south west] {$nonpointe$};\n\\draw (.7,1.873) node[anchor=south west] {$nonpointd$};\n\\draw (0,1) node[anchor=east] {$nonpointi$};\n\\draw (0,1.7) node[anchor=east] {$nonpointh$};\n\\end{tikzpicture}\n\\end{center}\n\nSecond solution: We may parametrize a point in $straightline$ by any of\n$verticalcoord$, $horizontalcoord$, or $linearparam = \\tan^{-1} (horizontalcoord/verticalcoord)$.  Then $perimeter$ and $circumference$ are\njust the integrals of $horizontalcoord\\,dverticalcoord$ and $verticalcoord\\,dhorizontalcoord$ over the appropriate\nintervals; thus $perimeter+circumference$ is the integral of $verticalcoord\\,dhorizontalcoord - horizontalcoord\\,dverticalcoord$\n(minus because the limits of integration are reversed).\nBut $dlinearparam = verticalcoord\\,dhorizontalcoord - horizontalcoord\\,dverticalcoord$, and so $perimeter+circumference = \\Delta linearparam$\nis precisely the radian measure of $straightline$. (Of course, one can perfectly well\ndo this problem by computing the two integrals\nseparately. But what's the fun in that?)"
    },
    "garbled_string": {
      "map": {
        "s": "vtnqwsaz",
        "x": "hjgrksla",
        "y": "mzpqowne",
        "\\theta": "qxeumadl",
        "A": "fcklzbta",
        "B": "wmqensod",
        "C": "yrpkdjbo",
        "D": "ukwasnqe",
        "E": "tbqvnzmx",
        "F": "lucsgemh",
        "G": "pzkchtav",
        "H": "xwrgeomd",
        "I": "nbfzqhel",
        "O": "jxcsqmia"
      },
      "question": "Let $vtnqwsaz$ be any arc of the unit circle lying entirely in the first\nquadrant.  Let $fcklzbta$ be the area of the region lying below $vtnqwsaz$ and\nabove the $hjgrksla$-axis and let $wmqensod$ be the area of the region lying to the\nright of the $mzpqowne$-axis and to the left of $vtnqwsaz$.  Prove that $fcklzbta+wmqensod$ depends\nonly on the arc length, and not on the position, of $vtnqwsaz$.",
      "solution": "First solution:\nto fix notation, let $fcklzbta$ be the area\nof region $ukwasnqetbqvnzmxlucsgemhpzkchtav$, and $wmqensod$ be the area of $ukwasnqetbqvnzmxnbfzqhelxwrgeomd$; further\nlet $yrpkdjbo$ denote the area of sector $jxcsqmia ukwasnqe tbqvnzmx$, which only depends on the\narc length of $vtnqwsaz$.  If $[XYZ]$ denotes the area of triangle\n$[XYZ]$, then we have\n$fcklzbta = yrpkdjbo + [jxcsqmiatbqvnzmxpzkchtav] - [jxcsqmiaukwasnqelucsgemh]$ and $wmqensod = yrpkdjbo + [jxcsqmiaukwasnqexwrgeomd] - [jxcsqmiatbqvnzmxnbfzqhel]$.  But\nclearly $[jxcsqmiatbqvnzmxpzkchtav] = [jxcsqmiatbqvnzmxnbfzqhel]$ and $[jxcsqmiaukwasnqelucsgemh] = [jxcsqmiaukwasnqexwrgeomd]$, and so\n$fcklzbta + wmqensod = 2yrpkdjbo$.\n\n\\begin{center}\n\\begin{tikzpicture}\n\\draw (0,0) circle (2);\n\\draw (0,2) -- (0,0) -- (2,0);\n\\draw (1.732,0) -- (1.732,1) -- (0,1);\n\\draw (.7,0) -- (.7,1.873) -- (0,1.873);\n\\draw (1.732,1) -- (0,0) -- (.7,1.873);\n\\draw (0,0) node[anchor=north east] {$jxcsqmia$};\n\\draw (.7,0) node[anchor=north] {$lucsgemh$};\n\\draw (1.732,0) node[anchor=north] {$pzkchtav$};\n\\draw (1.732,1) node[anchor=south west] {$tbqvnzmx$};\n\\draw (.7,1.873) node[anchor=south west] {$ukwasnqe$};\n\\draw (0,1) node[anchor=east] {$nbfzqhel$};\n\\draw (0,1.7) node[anchor=east] {$xwrgeomd$};\n\\end{tikzpicture}\n\\end{center}\n\nSecond solution: We may parametrize a point in $vtnqwsaz$ by any of\n$hjgrksla$, $mzpqowne$, or $qxeumadl = \\tan^{-1} (mzpqowne/hjgrksla)$.  Then $fcklzbta$ and $wmqensod$ are\njust the integrals of $mzpqowne\\,dhjgrksla$ and $hjgrksla\\,dmzpqowne$ over the appropriate\nintervals; thus $fcklzbta+wmqensod$ is the integral of $hjgrksla\\,dmzpqowne - mzpqowne\\,dhjgrksla$\n(minus because the limits of integration are reversed).\nBut $dqxeumadl = hjgrksla\\,dmzpqowne - mzpqowne\\,dhjgrksla$, and so $fcklzbta+wmqensod = \\Delta qxeumadl$\nis precisely the radian measure of $vtnqwsaz$. (Of course, one can perfectly well\ndo this problem by computing the two integrals\nseparately. But what's the fun in that?)"
    },
    "kernel_variant": {
      "question": "Fix a radius $R>0$ and an acute angle $\\varphi$ with $0<\\varphi<\\pi$.  \nIn polar coordinates $(r,\\theta)$ put  \n\\[\n W(\\varphi):=\\{(r,\\theta)\\colon 0<r<R,\\;0<\\theta<\\varphi\\}.\n\\]\n\nLet  \n\\[\n \\rho\\colon[0,R]\\times[0,\\varphi]\\longrightarrow(0,\\infty)\n\\]\nbe a continuous \\emph{area–density} function.  \nGiven numbers $0<\\alpha<\\beta<\\varphi$ consider the circular arc  \n\\[\n s\\colon r=R,\\quad\\theta\\in[\\alpha,\\beta]\n\\]\nand the three regions that $s$ carves out of $W(\\varphi)$:\n\\[\n\\begin{aligned}\n A(\\alpha)      &=\\{(r,\\theta)\\colon 0<r<R,\\;0<\\theta<\\alpha\\},\\\\[2mm]\n \\widehat C(\\alpha,\\beta) &=\\{(r,\\theta)\\colon 0<r<R,\\;\\alpha\\le\\theta\\le\\beta\\},\\\\[2mm]\n B(\\beta)       &=\\{(r,\\theta)\\colon 0<r<R,\\;\\beta<\\theta<\\varphi\\}. \n\\end{aligned}\n\\]\n\nFor every Jordan set $X\\subset W(\\varphi)$ put  \n\\[\n M_\\rho(X):=\\iint_{X}\\rho(r,\\theta)\\,dA\n \\qquad\\text{(the $\\rho$–weighted area).}\n\\]\n\nFor a real parameter $k$ define the functional  \n\\[\n S_{\\rho,k}(\\alpha,\\beta):=k\\,M_\\rho\\!\\bigl(A(\\alpha)\\bigr)+M_\\rho\\!\\bigl(B(\\beta)\\bigr).\n\\]\n\nDenote by  \n\\[\n L:=R(\\beta-\\alpha)\n\\]\nthe geometric length of the arc $s$.  \nWe call the density $\\rho$ \\emph{$\\theta$–balanced} if  \n\\[\n \\int_{0}^{R}\\rho(r,\\theta)\\,r\\,dr\n \\quad\\text{is independent of }\\theta.\n\\]\n\nThe tasks are:\n\n(A)  Determine all continuous densities $\\rho$ for which there exists a real constant \n$k=k(\\rho,\\varphi)$ such that  \n\\[\n S_{\\rho,k}(\\alpha,\\beta)\n \\quad\\text{depends on $s$ only through its length }L\n\\]\n(hence is completely independent of $\\alpha$) for \\emph{every} pair $0<\\alpha<\\beta<\\varphi$ (equivalently, for every $L\\in(0,\\varphi R)$).\n\n(B)  For every such admissible $\\rho$ find the unique constant $k(\\rho,\\varphi)$ and give an explicit formula for $S_{\\rho,k}$ in terms of $L$.\n\n(C)  Prove that if $\\rho$ is \\emph{not} admissible, then for \\emph{every} real $k$ one can find two arcs of equal length whose values of $S_{\\rho,k}$ are different.\n\n(D)  (Sharpness)  For real numbers $(\\lambda,\\mu)$ not both zero consider  \n\\[\n T_{\\lambda,\\mu}(\\alpha,\\beta):=\\lambda\\,M_\\rho\\!\\bigl(A(\\alpha)\\bigr)+\n                                 \\mu\\,M_\\rho\\!\\bigl(B(\\beta)\\bigr).\n\\]\nShow that $T_{\\lambda,\\mu}$ can be a function of $L$ alone \\emph{iff}  \n(i) $\\rho$ is admissible, and  \n(ii) $\\lambda=\\mu$ (up to an overall non–zero scalar factor).\n\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "solution": "Throughout write  \n\\[\n F(\\theta):=\\int_{0}^{R}\\rho(r,\\theta)\\,r\\,dr\\qquad(0\\le\\theta\\le\\varphi).\n \\tag{1}\n\\]\nBecause $\\rho$ is continuous on the compact closure of $W(\\varphi)$, $F$ is continuous (and positive) on $[0,\\varphi]$.  Polar integration gives  \n\\[\n M_\\rho\\!\\bigl(A(\\alpha)\\bigr)=\\int_{0}^{\\alpha}F(t)\\,dt,\n \\qquad\n M_\\rho\\!\\bigl(B(\\beta)\\bigr)=\\int_{\\beta}^{\\varphi}F(t)\\,dt.\n \\tag{2}\n\\]\n\n--------------------------------------------------------------------\nStep 1 – Reduction to a functional equation.  \nFix a length $L\\in(0,\\varphi R)$ and vary the arc by keeping $\\beta:=\\alpha+L/R$.  \nDefine  \n\\[\n H(\\alpha):=S_{\\rho,k}\\bigl(\\alpha,\\alpha+L/R\\bigr)\n           =k\\int_{0}^{\\alpha}F(t)\\,dt+\\int_{\\alpha+L/R}^{\\varphi}F(t)\\,dt,\n \\tag{3}\n\\]\nfor $\\alpha\\in(0,\\varphi-L/R)$.  \nThe hypothesis that $S_{\\rho,k}$ depends on $L$ alone (for every $L$) is equivalent to $H(\\alpha)\\equiv\\text{const}$.  Differentiating (3) gives  \n\\[\n H'(\\alpha)=k\\,F(\\alpha)-F(\\alpha+h),\\qquad h:=L/R>0.\n \\tag{4}\n\\]\n\nThus $H'\\equiv0$ for \\emph{all} admissible pairs $(\\alpha,h)$ iff  \n\\[\n F(\\alpha+h)=k\\,F(\\alpha)\\qquad\n \\bigl(0<\\alpha,\\;0<h,\\;\\alpha+h<\\varphi\\bigr).\n \\tag{5}\n\\]\nWe now analyse continuous $F$ satisfying (5).\n\n--------------------------------------------------------------------\nStep 2 – Solving the functional equation.\n\n2.1  Necessarily $|k|=1$.  \nIterating (5) $m$ times,\n\\[\n F(\\alpha+mh)=k^{\\,m}\\,F(\\alpha)\n \\tag{6}\n\\]\nwhenever $\\alpha+mh<\\varphi$.  Boundedness of $F$ on $[0,\\varphi]$ forces $|k|\\le1$ and, by reversing (5), also $|k|\\ge1$; hence $|k|=1$.\n\n2.2  Excluding $k=-1$.  \nAssume $k=-1$.  Then $F(\\alpha+2h)=F(\\alpha)$ for every admissible $(\\alpha,h)$, so $F$ is periodic with arbitrarily small positive period and by continuity must be constant.  Substituting into (5) with $k=-1$ yields $C=-C$, hence $C=0$, contradicting the positivity of $F$.  Therefore  \n\\[\n k=1.\n \\tag{7}\n\\]\n\n2.3  With $k=1$, (5) becomes $F(\\alpha+h)=F(\\alpha)$, i.e.\\ $F$ is \\emph{constant} on $(0,\\varphi)$.  Write  \n\\[\n F(\\theta)\\equiv C>0.\n \\tag{8}\n\\]\n\n--------------------------------------------------------------------\nStep 3 – Characterisation of admissible densities.\n\nNecessity:  (7)–(8) show that the length–only property implies $F$ is constant, i.e.\\ $\\rho$ is $\\theta$–balanced.\n\nSufficiency:  Conversely, if $\\rho$ is $\\theta$–balanced, then $F\\equiv C$ and by (2)\n\\[\n M_\\rho\\!\\bigl(A(\\alpha)\\bigr)=C\\alpha,\\qquad\n M_\\rho\\!\\bigl(B(\\beta)\\bigr)=C(\\varphi-\\beta).\n\\]\nTaking $k=1$ (the only admissible value) we obtain  \n\\[\n S_{\\rho,1}(\\alpha,\\beta)=C\\bigl(\\alpha+\\varphi-\\beta\\bigr)\n                         =C\\Bigl(\\varphi-\\dfrac{L}{R}\\Bigr),\n \\tag{9}\n\\]\nwhich indeed depends on $s$ only through its length $L$.  \nThus the admissible densities are exactly the $\\theta$–balanced ones, and\n\\[\n k(\\rho,\\varphi)=1,\\qquad\n S_{\\rho,1}(L)=C\\Bigl(\\varphi-\\dfrac{L}{R}\\Bigr).\n \\tag{10}\n\\]\nThis completes (A) and (B).\n\n--------------------------------------------------------------------\nStep 4 – The non–admissible case (proof of (C)).  \n\nAssume now that $\\rho$ is \\emph{not} $\\theta$–balanced, so $F$ is \\emph{not} constant.  Fix an arbitrary real $k$.\n\nFor $h\\in(0,\\varphi)$ define the \\emph{quadratic discrepancy}  \n\\[\n \\Phi(h):=\\int_{0}^{\\varphi-h}\\!\\bigl(F(\\alpha+h)-k\\,F(\\alpha)\\bigr)^{2}\\,d\\alpha.\n \\tag{11}\n\\]\n\n(i)  $\\Phi$ is continuous on $(0,\\varphi)$ because the integrand depends continuously on $(\\alpha,h)$ over a compact set.\n\n(ii)  If $\\Phi(h)=0$ for all $h\\in(0,\\varphi)$, then for every such $h$ we have $F(\\alpha+h)=k\\,F(\\alpha)$ for \\emph{all} $\\alpha$ with $\\alpha,\\alpha+h\\in(0,\\varphi)$.  Fix two distinct values $h_{1},h_{2}\\in(0,\\varphi)$; combining the two equalities gives  \n\\[\n F(\\alpha+h_{1}+h_{2})=k^{\\,2}F(\\alpha)\n \\quad\\text{and}\\quad\n F(\\alpha+h_{1}+h_{2})=k\\,F(\\alpha+h_{1})=k^{\\,2}F(\\alpha).\n\\]\nIterating we see that $F$ assumes only the values $k^{m}F(\\alpha)$, hence is bounded exactly as in Step 2.1, which forces $|k|=1$.  If $k=-1$, the same contradiction as in Step 2.2 arises; if $k=1$ then $F(\\alpha+h)=F(\\alpha)$ for every $h$, so $F$ is constant—contradicting the non–admissibility assumption.  Consequently  \n\\[\n \\text{there exists at least one }h_{0}\\in(0,\\varphi)\\text{ with }\\Phi(h_{0})>0.\n \\tag{12}\n\\]\n\nFix such an $h_{0}$ and abbreviate $h_{0}=h$.  Define  \n\\[\n G(\\alpha):=S_{\\rho,k}(\\alpha,\\alpha+h)\n           =k\\int_{0}^{\\alpha}F(t)\\,dt+\\int_{\\alpha+h}^{\\varphi}F(t)\\,dt\n           \\qquad(0<\\alpha<\\varphi-h).\n \\tag{13}\n\\]\nAs before,\n\\[\n G'(\\alpha)=k\\,F(\\alpha)-F(\\alpha+h).\n\\]\nThe assumption $\\Phi(h)>0$ means that $G'(\\alpha)$ is not identically $0$ on $(0,\\varphi-h)$, so $G$ is \\emph{not} constant.  Hence there exist $\\alpha_{1},\\alpha_{2}\\in(0,\\varphi-h)$ such that  \n\\[\n S_{\\rho,k}(\\alpha_{1},\\alpha_{1}+h)\\ne\n S_{\\rho,k}(\\alpha_{2},\\alpha_{2}+h),\n \\]\nwhile both arcs have the same length $L=hR$.  This establishes (C).\n\n--------------------------------------------------------------------\nStep 5 – Sharpness for arbitrary linear combinations (part (D)).  \n\nLet $\\lambda,\\mu$ be real, not both zero, and suppose  \n\\[\n T_{\\lambda,\\mu}(\\alpha,\\beta)\n        =\\lambda\\,M_\\rho\\!\\bigl(A(\\alpha)\\bigr)+\\mu\\,M_\\rho\\!\\bigl(B(\\beta)\\bigr)\n\\]\ndepends on $L$ alone.  Necessarily $\\mu\\ne0$ (otherwise $T_{\\lambda,\\mu}$ would depend on $\\alpha$).  Write  \n\\[\n T_{\\lambda,\\mu}=\\mu\\,S_{\\rho,k}\\quad\\text{with }k:=\\lambda/\\mu.\n \\tag{14}\n\\]\nBy assumption $S_{\\rho,k}$ is length–only, so by (A)–(B) the density is admissible and $k=1$, i.e.\\ $\\lambda=\\mu$.  Conversely, if $\\rho$ is admissible and $\\lambda=\\mu\\ne0$, then from (9)\n\\[\n T_{\\lambda,\\lambda}(\\alpha,\\beta)=\\lambda\\,C\\Bigl(\\varphi-\\dfrac{L}{R}\\Bigr)\n\\]\ndepends only on $L$.  Thus (D) is proved.\n\n--------------------------------------------------------------------\nThe problem is therefore completely resolved.\n\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.757475",
        "was_fixed": false,
        "difficulty_analysis": "1. Generalises the kernel problem from the uniform density ρ≡1 to an arbitrary continuous density, forcing the solver to CHARACTERISE all densities for which a location-invariant functional can exist.  \n2. Introduces functional equations (Step 2) whose resolution requires a qualitative study of continuous solutions rather than mere algebraic cancellation.  \n3. The existence/uniqueness part demands differentiability arguments and a global “solid argument’’ (varying L continuously) – well beyond the elementary triangle-area tricks of the original.  \n4. Non-admissibility (part C) calls for a stability/perturbation argument: producing explicit counter-examples for every k when ρ is not radial.  \n5. Part D shows the obtained class is MAXIMAL, completing a full if-and-only-if classification – a depth absent from the original and current variants. All together the problem blends integral geometry, functional equations and real analysis, demanding several advanced techniques instead of a single Green-theorem swipe."
      }
    },
    "original_kernel_variant": {
      "question": "Fix a radius $R>0$ and an acute angle $\\varphi$ with $0<\\varphi<\\pi$.  \nIn polar coordinates $(r,\\theta)$ put  \n\\[\n W(\\varphi):=\\{(r,\\theta)\\colon 0<r<R,\\;0<\\theta<\\varphi\\}.\n\\]\n\nLet  \n\\[\n \\rho\\colon[0,R]\\times[0,\\varphi]\\longrightarrow(0,\\infty)\n\\]\nbe a continuous \\emph{area–density} function.  \nGiven numbers $0<\\alpha<\\beta<\\varphi$ consider the circular arc  \n\\[\n s\\colon r=R,\\quad\\theta\\in[\\alpha,\\beta]\n\\]\nand the three regions that $s$ carves out of $W(\\varphi)$:\n\\[\n\\begin{aligned}\n A(\\alpha)      &=\\{(r,\\theta)\\colon 0<r<R,\\;0<\\theta<\\alpha\\},\\\\[2mm]\n \\widehat C(\\alpha,\\beta) &=\\{(r,\\theta)\\colon 0<r<R,\\;\\alpha\\le\\theta\\le\\beta\\},\\\\[2mm]\n B(\\beta)       &=\\{(r,\\theta)\\colon 0<r<R,\\;\\beta<\\theta<\\varphi\\}. \n\\end{aligned}\n\\]\n\nFor every Jordan set $X\\subset W(\\varphi)$ put  \n\\[\n M_\\rho(X):=\\iint_{X}\\rho(r,\\theta)\\,dA\n \\qquad\\text{(the $\\rho$–weighted area).}\n\\]\n\nFor a real parameter $k$ define the functional  \n\\[\n S_{\\rho,k}(\\alpha,\\beta):=k\\,M_\\rho\\!\\bigl(A(\\alpha)\\bigr)+M_\\rho\\!\\bigl(B(\\beta)\\bigr).\n\\]\n\nDenote by  \n\\[\n L:=R(\\beta-\\alpha)\n\\]\nthe geometric length of the arc $s$.  \nWe call the density $\\rho$ \\emph{$\\theta$–balanced} if  \n\\[\n \\int_{0}^{R}\\rho(r,\\theta)\\,r\\,dr\n \\quad\\text{is independent of }\\theta.\n\\]\n\nThe tasks are:\n\n(A)  Determine all continuous densities $\\rho$ for which there exists a real constant \n$k=k(\\rho,\\varphi)$ such that  \n\\[\n S_{\\rho,k}(\\alpha,\\beta)\n \\quad\\text{depends on $s$ only through its length }L\n\\]\n(hence is completely independent of $\\alpha$) for \\emph{every} pair $0<\\alpha<\\beta<\\varphi$ (equivalently, for every $L\\in(0,\\varphi R)$).\n\n(B)  For every such admissible $\\rho$ find the unique constant $k(\\rho,\\varphi)$ and give an explicit formula for $S_{\\rho,k}$ in terms of $L$.\n\n(C)  Prove that if $\\rho$ is \\emph{not} admissible, then for \\emph{every} real $k$ one can find two arcs of equal length whose values of $S_{\\rho,k}$ are different.\n\n(D)  (Sharpness)  For real numbers $(\\lambda,\\mu)$ not both zero consider  \n\\[\n T_{\\lambda,\\mu}(\\alpha,\\beta):=\\lambda\\,M_\\rho\\!\\bigl(A(\\alpha)\\bigr)+\n                                 \\mu\\,M_\\rho\\!\\bigl(B(\\beta)\\bigr).\n\\]\nShow that $T_{\\lambda,\\mu}$ can be a function of $L$ alone \\emph{iff}  \n(i) $\\rho$ is admissible, and  \n(ii) $\\lambda=\\mu$ (up to an overall non–zero scalar factor).\n\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "solution": "Throughout write  \n\\[\n F(\\theta):=\\int_{0}^{R}\\rho(r,\\theta)\\,r\\,dr\\qquad(0\\le\\theta\\le\\varphi).\n \\tag{1}\n\\]\nBecause $\\rho$ is continuous on the compact closure of $W(\\varphi)$, $F$ is continuous (and positive) on $[0,\\varphi]$.  Polar integration gives  \n\\[\n M_\\rho\\!\\bigl(A(\\alpha)\\bigr)=\\int_{0}^{\\alpha}F(t)\\,dt,\n \\qquad\n M_\\rho\\!\\bigl(B(\\beta)\\bigr)=\\int_{\\beta}^{\\varphi}F(t)\\,dt.\n \\tag{2}\n\\]\n\n--------------------------------------------------------------------\nStep 1 – Reduction to a functional equation.  \nFix a length $L\\in(0,\\varphi R)$ and vary the arc by keeping $\\beta:=\\alpha+L/R$.  \nDefine  \n\\[\n H(\\alpha):=S_{\\rho,k}\\bigl(\\alpha,\\alpha+L/R\\bigr)\n           =k\\int_{0}^{\\alpha}F(t)\\,dt+\\int_{\\alpha+L/R}^{\\varphi}F(t)\\,dt,\n \\tag{3}\n\\]\nfor $\\alpha\\in(0,\\varphi-L/R)$.  \nThe hypothesis that $S_{\\rho,k}$ depends on $L$ alone (for every $L$) is equivalent to $H(\\alpha)\\equiv\\text{const}$.  Differentiating (3) gives  \n\\[\n H'(\\alpha)=k\\,F(\\alpha)-F(\\alpha+h),\\qquad h:=L/R>0.\n \\tag{4}\n\\]\n\nThus $H'\\equiv0$ for \\emph{all} admissible pairs $(\\alpha,h)$ iff  \n\\[\n F(\\alpha+h)=k\\,F(\\alpha)\\qquad\n \\bigl(0<\\alpha,\\;0<h,\\;\\alpha+h<\\varphi\\bigr).\n \\tag{5}\n\\]\nWe now analyse continuous $F$ satisfying (5).\n\n--------------------------------------------------------------------\nStep 2 – Solving the functional equation.\n\n2.1  Necessarily $|k|=1$.  \nIterating (5) $m$ times,\n\\[\n F(\\alpha+mh)=k^{\\,m}\\,F(\\alpha)\n \\tag{6}\n\\]\nwhenever $\\alpha+mh<\\varphi$.  Boundedness of $F$ on $[0,\\varphi]$ forces $|k|\\le1$ and, by reversing (5), also $|k|\\ge1$; hence $|k|=1$.\n\n2.2  Excluding $k=-1$.  \nAssume $k=-1$.  Then $F(\\alpha+2h)=F(\\alpha)$ for every admissible $(\\alpha,h)$, so $F$ is periodic with arbitrarily small positive period and by continuity must be constant.  Substituting into (5) with $k=-1$ yields $C=-C$, hence $C=0$, contradicting the positivity of $F$.  Therefore  \n\\[\n k=1.\n \\tag{7}\n\\]\n\n2.3  With $k=1$, (5) becomes $F(\\alpha+h)=F(\\alpha)$, i.e.\\ $F$ is \\emph{constant} on $(0,\\varphi)$.  Write  \n\\[\n F(\\theta)\\equiv C>0.\n \\tag{8}\n\\]\n\n--------------------------------------------------------------------\nStep 3 – Characterisation of admissible densities.\n\nNecessity:  (7)–(8) show that the length–only property implies $F$ is constant, i.e.\\ $\\rho$ is $\\theta$–balanced.\n\nSufficiency:  Conversely, if $\\rho$ is $\\theta$–balanced, then $F\\equiv C$ and by (2)\n\\[\n M_\\rho\\!\\bigl(A(\\alpha)\\bigr)=C\\alpha,\\qquad\n M_\\rho\\!\\bigl(B(\\beta)\\bigr)=C(\\varphi-\\beta).\n\\]\nTaking $k=1$ (the only admissible value) we obtain  \n\\[\n S_{\\rho,1}(\\alpha,\\beta)=C\\bigl(\\alpha+\\varphi-\\beta\\bigr)\n                         =C\\Bigl(\\varphi-\\dfrac{L}{R}\\Bigr),\n \\tag{9}\n\\]\nwhich indeed depends on $s$ only through its length $L$.  \nThus the admissible densities are exactly the $\\theta$–balanced ones, and\n\\[\n k(\\rho,\\varphi)=1,\\qquad\n S_{\\rho,1}(L)=C\\Bigl(\\varphi-\\dfrac{L}{R}\\Bigr).\n \\tag{10}\n\\]\nThis completes (A) and (B).\n\n--------------------------------------------------------------------\nStep 4 – The non–admissible case (proof of (C)).  \n\nAssume now that $\\rho$ is \\emph{not} $\\theta$–balanced, so $F$ is \\emph{not} constant.  Fix an arbitrary real $k$.\n\nFor $h\\in(0,\\varphi)$ define the \\emph{quadratic discrepancy}  \n\\[\n \\Phi(h):=\\int_{0}^{\\varphi-h}\\!\\bigl(F(\\alpha+h)-k\\,F(\\alpha)\\bigr)^{2}\\,d\\alpha.\n \\tag{11}\n\\]\n\n(i)  $\\Phi$ is continuous on $(0,\\varphi)$ because the integrand depends continuously on $(\\alpha,h)$ over a compact set.\n\n(ii)  If $\\Phi(h)=0$ for all $h\\in(0,\\varphi)$, then for every such $h$ we have $F(\\alpha+h)=k\\,F(\\alpha)$ for \\emph{all} $\\alpha$ with $\\alpha,\\alpha+h\\in(0,\\varphi)$.  Fix two distinct values $h_{1},h_{2}\\in(0,\\varphi)$; combining the two equalities gives  \n\\[\n F(\\alpha+h_{1}+h_{2})=k^{\\,2}F(\\alpha)\n \\quad\\text{and}\\quad\n F(\\alpha+h_{1}+h_{2})=k\\,F(\\alpha+h_{1})=k^{\\,2}F(\\alpha).\n\\]\nIterating we see that $F$ assumes only the values $k^{m}F(\\alpha)$, hence is bounded exactly as in Step 2.1, which forces $|k|=1$.  If $k=-1$, the same contradiction as in Step 2.2 arises; if $k=1$ then $F(\\alpha+h)=F(\\alpha)$ for every $h$, so $F$ is constant—contradicting the non–admissibility assumption.  Consequently  \n\\[\n \\text{there exists at least one }h_{0}\\in(0,\\varphi)\\text{ with }\\Phi(h_{0})>0.\n \\tag{12}\n\\]\n\nFix such an $h_{0}$ and abbreviate $h_{0}=h$.  Define  \n\\[\n G(\\alpha):=S_{\\rho,k}(\\alpha,\\alpha+h)\n           =k\\int_{0}^{\\alpha}F(t)\\,dt+\\int_{\\alpha+h}^{\\varphi}F(t)\\,dt\n           \\qquad(0<\\alpha<\\varphi-h).\n \\tag{13}\n\\]\nAs before,\n\\[\n G'(\\alpha)=k\\,F(\\alpha)-F(\\alpha+h).\n\\]\nThe assumption $\\Phi(h)>0$ means that $G'(\\alpha)$ is not identically $0$ on $(0,\\varphi-h)$, so $G$ is \\emph{not} constant.  Hence there exist $\\alpha_{1},\\alpha_{2}\\in(0,\\varphi-h)$ such that  \n\\[\n S_{\\rho,k}(\\alpha_{1},\\alpha_{1}+h)\\ne\n S_{\\rho,k}(\\alpha_{2},\\alpha_{2}+h),\n \\]\nwhile both arcs have the same length $L=hR$.  This establishes (C).\n\n--------------------------------------------------------------------\nStep 5 – Sharpness for arbitrary linear combinations (part (D)).  \n\nLet $\\lambda,\\mu$ be real, not both zero, and suppose  \n\\[\n T_{\\lambda,\\mu}(\\alpha,\\beta)\n        =\\lambda\\,M_\\rho\\!\\bigl(A(\\alpha)\\bigr)+\\mu\\,M_\\rho\\!\\bigl(B(\\beta)\\bigr)\n\\]\ndepends on $L$ alone.  Necessarily $\\mu\\ne0$ (otherwise $T_{\\lambda,\\mu}$ would depend on $\\alpha$).  Write  \n\\[\n T_{\\lambda,\\mu}=\\mu\\,S_{\\rho,k}\\quad\\text{with }k:=\\lambda/\\mu.\n \\tag{14}\n\\]\nBy assumption $S_{\\rho,k}$ is length–only, so by (A)–(B) the density is admissible and $k=1$, i.e.\\ $\\lambda=\\mu$.  Conversely, if $\\rho$ is admissible and $\\lambda=\\mu\\ne0$, then from (9)\n\\[\n T_{\\lambda,\\lambda}(\\alpha,\\beta)=\\lambda\\,C\\Bigl(\\varphi-\\dfrac{L}{R}\\Bigr)\n\\]\ndepends only on $L$.  Thus (D) is proved.\n\n--------------------------------------------------------------------\nThe problem is therefore completely resolved.\n\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.582387",
        "was_fixed": false,
        "difficulty_analysis": "1. Generalises the kernel problem from the uniform density ρ≡1 to an arbitrary continuous density, forcing the solver to CHARACTERISE all densities for which a location-invariant functional can exist.  \n2. Introduces functional equations (Step 2) whose resolution requires a qualitative study of continuous solutions rather than mere algebraic cancellation.  \n3. The existence/uniqueness part demands differentiability arguments and a global “solid argument’’ (varying L continuously) – well beyond the elementary triangle-area tricks of the original.  \n4. Non-admissibility (part C) calls for a stability/perturbation argument: producing explicit counter-examples for every k when ρ is not radial.  \n5. Part D shows the obtained class is MAXIMAL, completing a full if-and-only-if classification – a depth absent from the original and current variants. All together the problem blends integral geometry, functional equations and real analysis, demanding several advanced techniques instead of a single Green-theorem swipe."
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}