{
  "index": "1949-B-6",
  "type": "GEO",
  "tag": [
    "GEO",
    "ANA"
  ],
  "difficulty": "",
  "question": "6. Let \\( C \\) be a closed convex curve with a continuously turning tangent and let \\( O \\) be a point inside \\( C \\). With each point \\( P \\) on \\( C \\) we associate the point \\( T(P) \\) on \\( C \\) which is defined as follows: Draw the tangent to \\( C \\) at \\( P \\) and from \\( O \\) drop the perpendicular to that tangent. \\( T(P) \\) is then the point at which this perpendicular intersects the curve \\( C \\).\n\nStarting now with a point \\( P_{0} \\) on \\( C \\), we define points \\( P_{n} \\) by the formula \\( P_{n}= \\) \\( T\\left(P_{n-1}\\right), n \\geq 1 \\). Prove that the points \\( P_{n} \\) approach a limit, and characterize those points which can be limits of sequences \\( P_{n} \\). (You may consider the facts that \\( T \\) is a continuous transformation and that a convex curve lies on one side of each of its tangents as not requiring proofs.)",
  "solution": "Solution. The phrase \"with a continuously turning tangent\" means that \\( C \\) is a curve of class \\( C^{1} \\).\n\nLet \\( g(P) \\) be the distance from \\( O \\) to a point \\( P \\); this defines a differentiable function on the whole plane except for the point \\( O \\). Since \\( C \\) does not pass through \\( O \\), the restriction of \\( g \\) to \\( C \\) is differentiable. It has a critical point at \\( P \\) if and only if the gradient of \\( g \\) at \\( P \\) is normal to \\( C \\) at \\( P \\), that is the line \\( \\overleftrightarrow{O P} \\) is perpendicular to the tangent to \\( C \\) at \\( P \\). This is precisely the condition that \\( T(P)=P \\).\n\nSince \\( T \\) is a continuous map of \\( C \\) into itself, the set \\( F \\) of fixed points of \\( T \\) is a closed set, and \\( C-F \\), an open set relative to \\( C \\), falls into components each of which is an open arc bounded by two members of \\( F \\). The function \\( g \\) is strictly monotonic on any such are since it has no critical points there.\n\nSuppose \\( P \\) is not a fixed point of \\( T \\). Consider the diagram. \\( \\overleftrightarrow{P Q} \\) is tangent to the curve \\( C \\) at \\( P \\), and \\( \\overparen{O Q} \\) is perpendicular to \\( \\overleftrightarrow{P Q} \\). Since \\( T(P) \\neq P, P \\neq \\) \\( Q \\), so \\( |O P|>|O Q| \\).\n\nSince the origin \\( O \\) and all of \\( C \\) lie on the same side of the tangent \\( \\stackrel{\\leftrightarrow Q}{P Q} \\), and \\( R=T(P) \\) is on the ray \\( \\overrightarrow{O Q} \\), we have \\( |O Q| \\geq|O R| \\). Hence\n\\[\ng(T(P))<g(P) .\n\\]\n\nMoreover, there can be no fixed point \\( S \\) of \\( T \\) on the part of \\( C \\) lying in the interior of \\( \\angle P O Q \\). For such a point \\( S \\) would lie in the closed triangular region POQ, and the perpendicular \\( l \\) to \\( O S \\) at \\( S \\) would contain a point of the interior of segment \\( O P \\), and this point would lie in the interior of \\( C \\). But if \\( S=T(S) \\), then \\( l \\) would be tangent to \\( C \\) at \\( S \\) and could not contain a point interior to \\( C \\). Hence we conclude that the open arc \\( \\overparen{P R} \\) of \\( C \\) lies in \\( C-F \\) and therefore either \\( R=T(P) \\) is in the same component of \\( C-F \\) as \\( P \\) or \\( T(P) \\) is one endpoint of that component and \\( T(P) \\in F \\). [Note that \\( C \\) might contain the whole segment \\( P Q \\), in which case \\( T(P)=Q \\) and \\( T(Q) \\) \\( =Q \\).\n\nNow suppose \\( P_{0} \\) is given and \\( P_{n}=T\\left(P_{n-1}\\right) \\) for \\( n \\geq 1 \\). If \\( P_{0} \\in F \\), then clearly \\( P_{0}=P_{1}=P_{2}=\\cdots \\) and the sequence converges to \\( P_{0} \\). If \\( P_{0} \\& F \\), let \\( A \\) and \\( B \\) be the endpoints of the component \\( D \\) of \\( C-F \\) containing \\( P_{0} \\), and choose the notation so that\n\\[\ng(A)<g\\left(P_{0}\\right)<g(B) .\n\\]\n(This is possible since \\( g \\) is strictly monotonic on \\( D \\).) We have seen that \\( g\\left(P_{1}\\right)<g\\left(P_{0}\\right) \\) and either \\( P_{1} \\in D \\) or \\( P_{1} \\) is an endpoint of \\( D \\), in which case \\( P_{1}=A \\). Repeating this argument, it follows that either the points \\( P_{0}, P_{1} \\), \\( P_{2}, \\ldots \\) are all in \\( D \\) or eventually \\( P_{k}=A \\) for some \\( k \\), and then \\( P_{n}=A \\) for all \\( n \\geq k \\). In the latter case, we clearly have \\( P_{n} \\rightarrow A \\). In the former case, \\( P_{0}, P_{1}, P_{2}, \\ldots \\) is a monotonic sequence in \\( D \\) since \\( g\\left(P_{0}\\right)>g\\left(P_{1}\\right)>g\\left(P_{2}\\right) \\) \\( >\\cdots \\), so it converges to some point \\( X \\) in \\( \\bar{D} \\).\n\nNow \\( T(X)=T\\left(\\lim _{n-\\infty} P_{n}\\right)=\\lim _{n-\\infty} T\\left(P_{n}\\right)=\\lim _{n-\\infty} P_{n+1}=X \\). Thus \\( X \\) is a fixed point of \\( T \\) and \\( X \\in \\bar{D} \\). So \\( X=A \\) or \\( X=B \\). But \\( g(X)=\\lim \\) \\( g\\left(P_{n}\\right) \\leq g\\left(P_{0}\\right)<g(B) \\), so \\( X=A \\). Thus we have shown that, in any case, \\( P_{n} \\rightarrow A \\), i.e., if \\( P_{0} \\in C-F \\), then \\( P_{n} \\) converges to that endpoint of the component of \\( C-F \\) containing \\( P_{0} \\) which is closer to \\( O \\). And \\( \\lim P_{n} \\) is a fixed point of \\( T \\).\n\nIt is clear that any fixed point \\( Y \\) of \\( T \\) is the limit of such a sequence: Take \\( P_{0}=Y \\).",
  "vars": [
    "P",
    "P_0",
    "P_1",
    "P_2",
    "P_k",
    "P_n",
    "P_n-1",
    "Q",
    "R",
    "S",
    "X",
    "Y",
    "n",
    "k"
  ],
  "params": [
    "C",
    "O",
    "T",
    "g",
    "F",
    "A",
    "B",
    "D"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "C": "boundarycurve",
        "O": "innerpoint",
        "T": "projectionmap",
        "g": "distancefun",
        "F": "fixedset",
        "A": "nearerend",
        "B": "furtherend",
        "D": "openarcset",
        "P": "startpoint",
        "P_0": "pointzero",
        "P_1": "pointone",
        "P_2": "pointtwo",
        "P_k": "pointkay",
        "P_n": "pointenn",
        "P_n-1": "pointprev",
        "Q": "auxpointq",
        "R": "auxpointr",
        "S": "auxpoints",
        "X": "limitpointx",
        "Y": "limitpointy",
        "n": "indexenn",
        "k": "indexkay"
      },
      "question": "6. Let \\( boundarycurve \\) be a closed convex curve with a continuously turning tangent and let \\( innerpoint \\) be a point inside \\( boundarycurve \\). With each point \\( startpoint \\) on \\( boundarycurve \\) we associate the point \\( projectionmap(startpoint) \\) on \\( boundarycurve \\) which is defined as follows: Draw the tangent to \\( boundarycurve \\) at \\( startpoint \\) and from \\( innerpoint \\) drop the perpendicular to that tangent. \\( projectionmap(startpoint) \\) is then the point at which this perpendicular intersects the curve \\( boundarycurve \\).\n\nStarting now with a point \\( pointzero \\) on \\( boundarycurve \\), we define points \\( pointenn \\) by the formula \\( pointenn = projectionmap\\left(pointprev\\right), indexenn \\geq 1 \\). Prove that the points \\( pointenn \\) approach a limit, and characterize those points which can be limits of sequences \\( pointenn \\). (You may consider the facts that \\( projectionmap \\) is a continuous transformation and that a convex curve lies on one side of each of its tangents as not requiring proofs.)",
      "solution": "Solution. The phrase \"with a continuously turning tangent\" means that \\( boundarycurve \\) is a curve of class \\( boundarycurve^{1} \\).\n\nLet \\( distancefun(startpoint) \\) be the distance from \\( innerpoint \\) to a point \\( startpoint \\); this defines a differentiable function on the whole plane except for the point \\( innerpoint \\). Since \\( boundarycurve \\) does not pass through \\( innerpoint \\), the restriction of \\( distancefun \\) to \\( boundarycurve \\) is differentiable. It has a critical point at \\( startpoint \\) if and only if the gradient of \\( distancefun \\) at \\( startpoint \\) is normal to \\( boundarycurve \\) at \\( startpoint \\), that is the line \\( \\overleftrightarrow{innerpoint\\, startpoint} \\) is perpendicular to the tangent to \\( boundarycurve \\) at \\( startpoint \\). This is precisely the condition that \\( projectionmap(startpoint)=startpoint \\).\n\nSince \\( projectionmap \\) is a continuous map of \\( boundarycurve \\) into itself, the set \\( fixedset \\) of fixed points of \\( projectionmap \\) is a closed set, and \\( boundarycurve-fixedset \\), an open set relative to \\( boundarycurve \\), falls into components each of which is an open arc bounded by two members of \\( fixedset \\). The function \\( distancefun \\) is strictly monotonic on any such are since it has no critical points there.\n\nSuppose \\( startpoint \\) is not a fixed point of \\( projectionmap \\). Consider the diagram. \\( \\overleftrightarrow{startpoint\\, auxpointq} \\) is tangent to the curve \\( boundarycurve \\) at \\( startpoint \\), and \\( \\overparen{innerpoint\\, auxpointq} \\) is perpendicular to \\( \\overleftrightarrow{startpoint\\, auxpointq} \\). Since \\( projectionmap(startpoint) \\neq startpoint, startpoint \\neq auxpointq \\), we have \\( |innerpoint\\, startpoint|>|innerpoint\\, auxpointq| \\).\n\nSince the origin \\( innerpoint \\) and all of \\( boundarycurve \\) lie on the same side of the tangent \\( \\stackrel{\\leftrightarrow{auxpointq}}{startpoint\\, auxpointq} \\), and \\( auxpointr = projectionmap(startpoint) \\) is on the ray \\( \\overrightarrow{innerpoint\\, auxpointq} \\), we have \\( |innerpoint\\, auxpointq| \\ge |innerpoint\\, auxpointr| \\). Hence\n\\[\n distancefun(projectionmap(startpoint)) < distancefun(startpoint) .\n\\]\n\nMoreover, there can be no fixed point \\( auxpoints \\) of \\( projectionmap \\) on the part of \\( boundarycurve \\) lying in the interior of \\( \\angle startpoint\\, innerpoint\\, auxpointq \\). For such a point \\( auxpoints \\) would lie in the closed triangular region startpoint–innerpoint–auxpointq, and the perpendicular \\( l \\) to \\( innerpoint\\, auxpoints \\) at \\( auxpoints \\) would contain a point of the interior of segment \\( innerpoint\\, startpoint \\), and this point would lie in the interior of \\( boundarycurve \\). But if \\( auxpoints = projectionmap(auxpoints) \\), then \\( l \\) would be tangent to \\( boundarycurve \\) at \\( auxpoints \\) and could not contain a point interior to \\( boundarycurve \\). Hence we conclude that the open arc \\( \\overparen{startpoint\\, auxpointr} \\) of \\( boundarycurve \\) lies in \\( boundarycurve-fixedset \\), and therefore either \\( auxpointr = projectionmap(startpoint) \\) is in the same component of \\( boundarycurve-fixedset \\) as \\( startpoint \\) or \\( projectionmap(startpoint) \\) is one endpoint of that component and \\( projectionmap(startpoint) \\in fixedset \\). (Note that \\( boundarycurve \\) might contain the whole segment \\( startpoint\\, auxpointq \\), in which case \\( projectionmap(startpoint)=auxpointq \\) and \\( projectionmap(auxpointq)=auxpointq \\).)\n\nNow suppose \\( pointzero \\) is given and \\( pointenn = projectionmap\\left(pointprev\\right) \\) for \\( indexenn \\ge 1 \\). If \\( pointzero \\in fixedset \\), then clearly \\( pointzero = pointone = pointtwo = \\cdots \\) and the sequence converges to \\( pointzero \\). If \\( pointzero \\& fixedset \\), let \\( nearerend \\) and \\( furtherend \\) be the endpoints of the component \\( openarcset \\) of \\( boundarycurve-fixedset \\) containing \\( pointzero \\), and choose the notation so that\n\\[\n distancefun(nearerend) < distancefun\\left(pointzero\\right) < distancefun(furtherend) .\n\\]\n(This is possible since \\( distancefun \\) is strictly monotonic on \\( openarcset \\).) We have seen that \\( distancefun\\left(pointone\\right) < distancefun\\left(pointzero\\right) \\) and either \\( pointone \\in openarcset \\) or \\( pointone \\) is an endpoint of \\( openarcset \\), in which case \\( pointone = nearerend \\). Repeating this argument, it follows that either the points \\( pointzero, pointone, pointtwo, \\ldots \\) all lie in \\( openarcset \\) or eventually \\( pointkay = nearerend \\) for some \\( indexkay \\), and then \\( pointenn = nearerend \\) for all \\( indexenn \\ge indexkay \\). In the latter case we clearly have \\( pointenn \\to nearerend \\). In the former case, \\( pointzero, pointone, pointtwo, \\ldots \\) is a monotonic sequence in \\( openarcset \\) since \\( distancefun\\left(pointzero\\right) > distancefun\\left(pointone\\right) > distancefun\\left(pointtwo\\right) > \\cdots \\), so it converges to some point \\( limitpointx \\) in \\( \\bar{openarcset} \\).\n\nNow \\( projectionmap(limitpointx) = projectionmap\\bigl(\\lim_{indexenn\\to\\infty} pointenn\\bigr) = \\lim_{indexenn\\to\\infty} projectionmap(pointenn) = \\lim_{indexenn\\to\\infty} pointenn+1 = limitpointx \\). Thus \\( limitpointx \\) is a fixed point of \\( projectionmap \\) and \\( limitpointx \\in \\bar{openarcset} \\). So \\( limitpointx = nearerend \\) or \\( limitpointx = furtherend \\). But \\( distancefun(limitpointx) = \\lim distancefun(pointenn) \\le distancefun(pointzero) < distancefun(furtherend) \\), so \\( limitpointx = nearerend \\). Thus we have shown that, in any case, \\( pointenn \\to nearerend \\); i.e., if \\( pointzero \\in boundarycurve-fixedset \\), then \\( pointenn \\) converges to that endpoint of the component of \\( boundarycurve-fixedset \\) containing \\( pointzero \\) which is closer to \\( innerpoint \\). Moreover, \\( \\lim pointenn \\) is a fixed point of \\( projectionmap \\).\n\nIt is clear that any fixed point \\( limitpointy \\) of \\( projectionmap \\) is the limit of such a sequence: take \\( pointzero = limitpointy \\)."
    },
    "descriptive_long_confusing": {
      "map": {
        "P": "lighthouse",
        "P_0": "marigolds",
        "P_1": "tangerine",
        "P_2": "silhouette",
        "P_k": "accordion",
        "P_n": "buttercup",
        "P_n-1": "harmonica",
        "Q": "gazeboing",
        "R": "sapphirey",
        "S": "wilderness",
        "X": "pendulous",
        "Y": "cinnamonl",
        "n": "dandelion",
        "k": "firebrick",
        "C": "quartzite",
        "O": "asteroid",
        "T": "cerulean",
        "g": "paperclip",
        "F": "euphoria",
        "A": "honeycomb",
        "B": "waterfall",
        "D": "partridge"
      },
      "question": "6. Let \\( quartzite \\) be a closed convex curve with a continuously turning tangent and let \\( asteroid \\) be a point inside \\( quartzite \\). With each point \\( lighthouse \\) on \\( quartzite \\) we associate the point \\( cerulean(lighthouse) \\) on \\( quartzite \\) which is defined as follows: Draw the tangent to \\( quartzite \\) at \\( lighthouse \\) and from \\( asteroid \\) drop the perpendicular to that tangent. \\( cerulean(lighthouse) \\) is then the point at which this perpendicular intersects the curve \\( quartzite \\).\n\nStarting now with a point \\( marigolds \\) on \\( quartzite \\), we define points \\( buttercup \\) by the formula \\( buttercup = cerulean\\left(harmonica\\right), dandelion \\geq 1 \\). Prove that the points \\( buttercup \\) approach a limit, and characterize those points which can be limits of sequences \\( buttercup \\). (You may consider the facts that \\( cerulean \\) is a continuous transformation and that a convex curve lies on one side of each of its tangents as not requiring proofs.)",
      "solution": "Solution. The phrase \"with a continuously turning tangent\" means that \\( quartzite \\) is a curve of class \\( quartzite^{1} \\).\n\nLet \\( paperclip(lighthouse) \\) be the distance from \\( asteroid \\) to a point \\( lighthouse \\); this defines a differentiable function on the whole plane except for the point \\( asteroid \\). Since \\( quartzite \\) does not pass through \\( asteroid \\), the restriction of \\( paperclip \\) to \\( quartzite \\) is differentiable. It has a critical point at \\( lighthouse \\) if and only if the gradient of \\( paperclip \\) at \\( lighthouse \\) is normal to \\( quartzite \\) at \\( lighthouse \\), that is the line \\( \\overleftrightarrow{asteroid\\, lighthouse} \\) is perpendicular to the tangent to \\( quartzite \\) at \\( lighthouse \\). This is precisely the condition that \\( cerulean(lighthouse)=lighthouse \\).\n\nSince \\( cerulean \\) is a continuous map of \\( quartzite \\) into itself, the set \\( euphoria \\) of fixed points of \\( cerulean \\) is a closed set, and \\( quartzite-euphoria \\), an open set relative to \\( quartzite \\), falls into components each of which is an open arc bounded by two members of \\( euphoria \\). The function \\( paperclip \\) is strictly monotonic on any such are since it has no critical points there.\n\nSuppose \\( lighthouse \\) is not a fixed point of \\( cerulean \\). Consider the diagram. \\( \\overleftrightarrow{lighthouse\\, gazeboing} \\) is tangent to the curve \\( quartzite \\) at \\( lighthouse \\), and \\( \\overparen{asteroid\\, gazeboing} \\) is perpendicular to \\( \\overleftrightarrow{lighthouse\\, gazeboing} \\). Since \\( cerulean(lighthouse) \\neq lighthouse, lighthouse \\neq gazeboing \\), so \\( |asteroid\\, lighthouse|>|asteroid\\, gazeboing| \\).\n\nSince the origin \\( asteroid \\) and all of \\( quartzite \\) lie on the same side of the tangent \\( \\stackrel{\\leftrightarrow{gazeboing}}{lighthouse\\, gazeboing} \\), and \\( sapphirey=\\cerulean(lighthouse) \\) is on the ray \\( \\overrightarrow{asteroid\\, gazeboing} \\), we have \\( |asteroid\\, gazeboing| \\geq|asteroid\\, sapphirey| \\). Hence\n\\[\npaperclip(\\cerulean(lighthouse))<paperclip(lighthouse) .\n\\]\n\nMoreover, there can be no fixed point \\( wilderness \\) of \\( cerulean \\) on the part of \\( quartzite \\) lying in the interior of \\( \\angle lighthouse\\, asteroid\\, gazeboing \\). For such a point \\( wilderness \\) would lie in the closed triangular region lighthouse–asteroid–gazeboing, and the perpendicular \\( l \\) to \\( asteroid\\, wilderness \\) at \\( wilderness \\) would contain a point of the interior of segment \\( asteroid\\, lighthouse \\), and this point would lie in the interior of \\( quartzite \\). But if \\( wilderness=\\cerulean(wilderness) \\), then \\( l \\) would be tangent to \\( quartzite \\) at \\( wilderness \\) and could not contain a point interior to \\( quartzite \\). Hence we conclude that the open arc \\( \\overparen{lighthouse\\, sapphirey} \\) of \\( quartzite \\) lies in \\( quartzite-euphoria \\) and therefore either \\( sapphirey=\\cerulean(lighthouse) \\) is in the same component of \\( quartzite-euphoria \\) as \\( lighthouse \\) or \\( \\cerulean(lighthouse) \\) is one endpoint of that component and \\( \\cerulean(lighthouse) \\in euphoria \\). [Note that \\( quartzite \\) might contain the whole segment lighthouse\\, gazeboing, in which case \\( \\cerulean(lighthouse)=gazeboing \\) and \\( \\cerulean(gazeboing)=gazeboing \\).]\n\nNow suppose \\( marigolds \\) is given and \\( buttercup=\\cerulean\\left(harmonica\\right) \\) for \\( dandelion \\geq 1 \\). If \\( marigolds \\in euphoria \\), then clearly \\( marigolds=tangerine=silhouette=\\cdots \\) and the sequence converges to \\( marigolds \\). If \\( marigolds \\notin euphoria \\), let \\( honeycomb \\) and \\( waterfall \\) be the endpoints of the component \\( partridge \\) of \\( quartzite-euphoria \\) containing \\( marigolds \\), and choose the notation so that\n\\[\npaperclip(honeycomb)<paperclip\\left(marigolds\\right)<paperclip(waterfall) .\n\\]\n(This is possible since \\( paperclip \\) is strictly monotonic on \\( partridge \\).) We have seen that \\( paperclip\\left(tangerine\\right)<paperclip\\left(marigolds\\right) \\) and either \\( tangerine \\in partridge \\) or \\( tangerine \\) is an endpoint of \\( partridge \\), in which case \\( tangerine=honeycomb \\). Repeating this argument, it follows that either the points \\( marigolds, tangerine, silhouette, \\ldots \\) are all in \\( partridge \\) or eventually \\( accordion = honeycomb \\) for some \\( firebrick \\), and then \\( buttercup = honeycomb \\) for all \\( dandelion \\geq firebrick \\). In the latter case, we clearly have \\( buttercup \\rightarrow honeycomb \\). In the former case, \\( marigolds, tangerine, silhouette, \\ldots \\) form a monotonic sequence in \\( partridge \\) since \\( paperclip\\left(marigolds\\right)>paperclip\\left(tangerine\\right)>paperclip\\left(silhouette\\right)>\\cdots \\), so it converges to some point \\( pendulous \\) in \\( \\bar{partridge} \\).\n\nNow \\( cerulean(pendulous)=cerulean\\left(\\lim _{dandelion-\\infty} buttercup\\right)=\\lim _{dandelion-\\infty} \\cerulean\\left(buttercup\\right)=\\lim _{dandelion-\\infty} P_{n+1}=pendulous \\). Thus \\( pendulous \\) is a fixed point of \\( cerulean \\) and \\( pendulous \\in \\bar{partridge} \\). So \\( pendulous=honeycomb \\) or \\( pendulous=waterfall \\). But \\( paperclip(pendulous)=\\lim paperclip\\left(buttercup\\right) \\leq paperclip\\left(marigolds\\right)<paperclip(waterfall) \\), so \\( pendulous=honeycomb \\). Thus we have shown that, in any case, \\( buttercup \\rightarrow honeycomb \\), i.e., if \\( marigolds \\in quartzite-euphoria \\), then \\( buttercup \\) converges to that endpoint of the component of \\( quartzite-euphoria \\) containing \\( marigolds \\) which is closer to \\( asteroid \\). And \\( \\lim buttercup \\) is a fixed point of \\( cerulean \\).\n\nIt is clear that any fixed point \\( cinnamonl \\) of \\( cerulean \\) is the limit of such a sequence: Take \\( marigolds = cinnamonl \\)."
    },
    "descriptive_long_misleading": {
      "map": {
        "P": "emptiness",
        "P_0": "originless",
        "P_1": "termination",
        "P_2": "cessation",
        "P_k": "constantpt",
        "P_n": "fixedspot",
        "P_n-1": "priorvoid",
        "Q": "vastness",
        "R": "nullarea",
        "S": "nothingness",
        "X": "knownspot",
        "Y": "zoneless",
        "n": "immobile",
        "k": "staticvar",
        "C": "flatline",
        "O": "exterior",
        "T": "stagnation",
        "g": "intimacy",
        "F": "movingpts",
        "A": "remotepnt",
        "B": "nearpoint",
        "D": "wholeset"
      },
      "question": "6. Let \\( flatline \\) be a closed convex curve with a continuously turning tangent and let \\( exterior \\) be a point inside \\( flatline \\). With each point \\( emptiness \\) on \\( flatline \\) we associate the point \\( stagnation(emptiness) \\) on \\( flatline \\) which is defined as follows: Draw the tangent to \\( flatline \\) at \\( emptiness \\) and from \\( exterior \\) drop the perpendicular to that tangent. \\( stagnation(emptiness) \\) is then the point at which this perpendicular intersects the curve \\( flatline \\).\n\nStarting now with a point \\( originless \\) on \\( flatline \\), we define points \\( fixedspot \\) by the formula \\( fixedspot = \\) \\( stagnation\\left(priorvoid\\right), immobile \\geq 1 \\). Prove that the points \\( fixedspot \\) approach a limit, and characterize those points which can be limits of sequences \\( fixedspot \\). (You may consider the facts that \\( stagnation \\) is a continuous transformation and that a convex curve lies on one side of each of its tangents as not requiring proofs.)",
      "solution": "Solution. The phrase \"with a continuously turning tangent\" means that \\( flatline \\) is a curve of class \\( C^{1} \\).\n\nLet \\( intimacy(emptiness) \\) be the distance from \\( exterior \\) to a point \\( emptiness \\); this defines a differentiable function on the whole plane except for the point \\( exterior \\). Since \\( flatline \\) does not pass through \\( exterior \\), the restriction of \\( intimacy \\) to \\( flatline \\) is differentiable. It has a critical point at \\( emptiness \\) if and only if the gradient of \\( intimacy \\) at \\( emptiness \\) is normal to \\( flatline \\) at \\( emptiness \\), that is the line \\( \\overleftrightarrow{exterior\\ emptiness} \\) is perpendicular to the tangent to \\( flatline \\) at \\( emptiness \\). This is precisely the condition that \\( stagnation(emptiness)=emptiness \\).\n\nSince \\( stagnation \\) is a continuous map of \\( flatline \\) into itself, the set \\( movingpts \\) of fixed points of \\( stagnation \\) is a closed set, and \\( flatline-movingpts \\), an open set relative to \\( flatline \\), falls into components each of which is an open arc bounded by two members of \\( movingpts \\). The function \\( intimacy \\) is strictly monotonic on any such arc since it has no critical points there.\n\nSuppose \\( emptiness \\) is not a fixed point of \\( stagnation \\). Consider the diagram. \\( \\overleftrightarrow{emptiness\\ vastness} \\) is tangent to the curve \\( flatline \\) at \\( emptiness \\), and \\( \\overparen{exterior\\ vastness} \\) is perpendicular to \\( \\overleftrightarrow{emptiness\\ vastness} \\). Since \\( stagnation(emptiness) \\neq emptiness, emptiness \\neq vastness, so \\( |exterior\\ emptiness|>|exterior\\ vastness| \\).\n\nSince the origin \\( exterior \\) and all of \\( flatline \\) lie on the same side of the tangent \\( \\stackrel{\\leftrightarrow{} }{emptiness\\ vastness} \\), and \\( nullarea=stagnation(emptiness) \\) is on the ray \\( \\overrightarrow{exterior\\ vastness} \\), we have \\( |exterior\\ vastness| \\geq|exterior\\ nullarea| \\). Hence\n\\[\nintimacy(stagnation(emptiness))<intimacy(emptiness) .\n\\]\n\nMoreover, there can be no fixed point \\( nothingness \\) of \\( stagnation \\) on the part of \\( flatline \\) lying in the interior of \\( \\angle emptiness\\ exterior\\ vastness \\). For such a point \\( nothingness \\) would lie in the closed triangular region exterior–emptiness–vastness, and the perpendicular \\( l \\) to \\( exterior\\ nothingness \\) at \\( nothingness \\) would contain a point of the interior of segment \\( exterior\\ emptiness \\), and this point would lie in the interior of \\( flatline \\). But if \\( nothingness=stagnation(nothingness) \\), then \\( l \\) would be tangent to \\( flatline \\) at \\( nothingness \\) and could not contain a point interior to \\( flatline \\). Hence we conclude that the open arc \\( \\overparen{emptiness\\ nullarea} \\) of \\( flatline \\) lies in \\( flatline-movingpts \\) and therefore either \\( nullarea=stagnation(emptiness) \\) is in the same component of \\( flatline-movingpts \\) as \\( emptiness \\) or \\( stagnation(emptiness) \\) is one endpoint of that component and \\( stagnation(emptiness) \\in movingpts \\). [Note that \\( flatline \\) might contain the whole segment \\( emptiness\\ vastness \\), in which case \\( stagnation(emptiness)=vastness \\) and \\( stagnation(vastness)=vastness .]\n\nNow suppose \\( originless \\) is given and \\( fixedspot = stagnation\\left(priorvoid\\right) \\) for \\( immobile \\geq 1 \\). If \\( originless \\in movingpts \\), then clearly \\( originless=termination=cessation=\\cdots \\) and the sequence converges to \\( originless \\). If \\( originless \\notin movingpts \\), let \\( remotepnt \\) and \\( nearpoint \\) be the endpoints of the component \\( wholeset \\) of \\( flatline-movingpts \\) containing \\( originless \\), and choose the notation so that\n\\[\nintimacy(remotepnt)<intimacy\\left(originless\\right)<intimacy(nearpoint) .\n\\]\n(This is possible since \\( intimacy \\) is strictly monotonic on \\( wholeset \\).) We have seen that \\( intimacy\\left(termination\\right)<intimacy\\left(originless\\right) \\) and either \\( termination \\in wholeset \\) or \\( termination \\) is an endpoint of \\( wholeset \\), in which case \\( termination=remotepnt \\). Repeating this argument, it follows that either the points \\( originless, termination, cessation, \\ldots \\) are all in \\( wholeset \\) or eventually \\( constantpt=remotepnt \\) for some \\( staticvar \\), and then \\( fixedspot=remotepnt \\) for all \\( immobile \\geq staticvar \\). In the latter case, we clearly have \\( fixedspot \\rightarrow remotepnt \\). In the former case, \\( originless, termination, cessation, \\ldots \\) is a monotonic sequence in \\( wholeset \\) since \\( intimacy\\left(originless\\right)>intimacy\\left(termination\\right)>intimacy\\left(cessation\\right)>\\cdots \\), so it converges to some point \\( knownspot \\) in \\( \\bar{wholeset} \\).\n\nNow \\( stagnation(knownspot)=stagnation\\left(\\lim _{immobile \\to \\infty} fixedspot\\right)=\\lim _{immobile \\to \\infty} stagnation\\left(fixedspot\\right)=\\lim _{immobile \\to \\infty} fixedspot=knownspot \\). Thus \\( knownspot \\) is a fixed point of \\( stagnation \\) and \\( knownspot \\in \\bar{wholeset} \\). So \\( knownspot=remotepnt \\) or \\( knownspot=nearpoint \\). But \\( intimacy(knownspot)=\\lim intimacy\\left(fixedspot\\right) \\leq intimacy\\left(originless\\right)<intimacy(nearpoint) \\), so \\( knownspot=remotepnt \\). Thus we have shown that, in any case, \\( fixedspot \\rightarrow remotepnt \\), i.e., if \\( originless \\in flatline-movingpts \\), then \\( fixedspot \\) converges to that endpoint of the component of \\( flatline-movingpts \\) containing \\( originless \\) which is closer to \\( exterior \\). And \\( \\lim fixedspot \\) is a fixed point of \\( stagnation \\).\n\nIt is clear that any fixed point \\( zoneless \\) of \\( stagnation \\) is the limit of such a sequence: Take \\( originless=zoneless \\)."
    },
    "garbled_string": {
      "map": {
        "P": "qzxwvtnp",
        "P_0": "hjgrksla",
        "P_1": "bvnxqpoz",
        "P_2": "lskdjfgh",
        "P_k": "vbnmrety",
        "P_n": "asdkfghj",
        "P_n-1": "qwerhjkl",
        "Q": "sdfglopq",
        "R": "plmoknji",
        "S": "xcvbasdf",
        "X": "poiulkjh",
        "Y": "mnbvcxzq",
        "n": "rtghyujk",
        "k": "zxcvbnml",
        "C": "ghytreds",
        "O": "ujmpolkq",
        "T": "fdertyui",
        "g": "bnvcxzas",
        "F": "lkjhgfds",
        "A": "qazwsxed",
        "B": "wsxrfvgt",
        "D": "ikmjnhbg"
      },
      "question": "6. Let \\( ghytreds \\) be a closed convex curve with a continuously turning tangent and let \\( ujmpolkq \\) be a point inside \\( ghytreds \\). With each point \\( qzxwvtnp \\) on \\( ghytreds \\) we associate the point \\( fdertyui(qzxwvtnp) \\) on \\( ghytreds \\) which is defined as follows: Draw the tangent to \\( ghytreds \\) at \\( qzxwvtnp \\) and from \\( ujmpolkq \\) drop the perpendicular to that tangent. \\( fdertyui(qzxwvtnp) \\) is then the point at which this perpendicular intersects the curve \\( ghytreds \\).\n\nStarting now with a point \\( hjgrksla \\) on \\( ghytreds \\), we define points \\( asdkfghj \\) by the formula \\( asdkfghj = fdertyui\\left(qwerhjkl\\right), rtghyujk \\geq 1 \\). Prove that the points \\( asdkfghj \\) approach a limit, and characterize those points which can be limits of sequences \\( asdkfghj \\). (You may consider the facts that \\( fdertyui \\) is a continuous transformation and that a convex curve lies on one side of each of its tangents as not requiring proofs.)",
      "solution": "Solution. The phrase \"with a continuously turning tangent\" means that \\( ghytreds \\) is a curve of class \\( C^{1} \\).\n\nLet \\( bnvcxzas(qzxwvtnp) \\) be the distance from \\( ujmpolkq \\) to a point \\( qzxwvtnp \\); this defines a differentiable function on the whole plane except for the point \\( ujmpolkq \\). Since \\( ghytreds \\) does not pass through \\( ujmpolkq \\), the restriction of \\( bnvcxzas \\) to \\( ghytreds \\) is differentiable. It has a critical point at \\( qzxwvtnp \\) if and only if the gradient of \\( bnvcxzas \\) at \\( qzxwvtnp \\) is normal to \\( ghytreds \\) at \\( qzxwvtnp \\), that is the line \\( \\overleftrightarrow{ujmpolkq qzxwvtnp} \\) is perpendicular to the tangent to \\( ghytreds \\) at \\( qzxwvtnp \\). This is precisely the condition that \\( fdertyui(qzxwvtnp)=qzxwvtnp \\).\n\nSince \\( fdertyui \\) is a continuous map of \\( ghytreds \\) into itself, the set \\( lkjhgfds \\) of fixed points of \\( fdertyui \\) is a closed set, and \\( ghytreds - lkjhgfds \\), an open set relative to \\( ghytreds \\), falls into components each of which is an open arc bounded by two members of \\( lkjhgfds \\). The function \\( bnvcxzas \\) is strictly monotonic on any such arc since it has no critical points there.\n\nSuppose \\( qzxwvtnp \\) is not a fixed point of \\( fdertyui \\). Consider the diagram. \\( \\overleftrightarrow{qzxwvtnp \\, sdfglopq} \\) is tangent to the curve \\( ghytreds \\) at \\( qzxwvtnp \\), and \\( \\overparen{ujmpolkq \\, sdfglopq} \\) is perpendicular to \\( \\overleftrightarrow{qzxwvtnp \\, sdfglopq} \\). Since \\( fdertyui(qzxwvtnp) \\neq qzxwvtnp, qzxwvtnp \\neq sdfglopq \\), we have \\( |ujmpolkq qzxwvtnp| > |ujmpolkq sdfglopq| \\).\n\nSince the origin \\( ujmpolkq \\) and all of \\( ghytreds \\) lie on the same side of the tangent \\( \\stackrel{\\leftrightarrow{sdfglopq}}{qzxwvtnp \\, sdfglopq} \\), and \\( plmoknji = fdertyui(qzxwvtnp) \\) is on the ray \\( \\overrightarrow{ujmpolkq \\, sdfglopq} \\), we have \\( |ujmpolkq sdfglopq| \\geq |ujmpolkq plmoknji| \\). Hence\n\\[\nbnvcxzas(fdertyui(qzxwvtnp)) < bnvcxzas(qzxwvtnp) .\n\\]\n\nMoreover, there can be no fixed point \\( xcvbasdf \\) of \\( fdertyui \\) on the part of \\( ghytreds \\) lying in the interior of \\( \\angle qzxwvtnp \\, ujmpolkq \\, sdfglopq \\). For such a point \\( xcvbasdf \\) would lie in the closed triangular region \\( qzxwvtnp ujmpolkq sdfglopq \\), and the perpendicular \\( l \\) to \\( ujmpolkq xcvbasdf \\) at \\( xcvbasdf \\) would contain a point of the interior of segment \\( ujmpolkq qzxwvtnp \\), and this point would lie in the interior of \\( ghytreds \\). But if \\( xcvbasdf = fdertyui(xcvbasdf) \\), then \\( l \\) would be tangent to \\( ghytreds \\) at \\( xcvbasdf \\) and could not contain a point interior to \\( ghytreds \\). Hence we conclude that the open arc \\( \\overparen{qzxwvtnp \\, plmoknji} \\) of \\( ghytreds \\) lies in \\( ghytreds - lkjhgfds \\), and therefore either \\( plmoknji = fdertyui(qzxwvtnp) \\) is in the same component of \\( ghytreds - lkjhgfds \\) as \\( qzxwvtnp \\) or \\( fdertyui(qzxwvtnp) \\) is one endpoint of that component and \\( fdertyui(qzxwvtnp) \\in lkjhgfds \\). [Note that \\( ghytreds \\) might contain the whole segment \\( qzxwvtnp sdfglopq \\), in which case \\( fdertyui(qzxwvtnp) = sdfglopq \\) and \\( fdertyui(sdfglopq) = sdfglopq \\).]\n\nNow suppose \\( hjgrksla \\) is given and \\( asdkfghj = fdertyui\\left(qwerhjkl\\right) \\) for \\( rtghyujk \\geq 1 \\). If \\( hjgrksla \\in lkjhgfds \\), then clearly \\( hjgrksla = bvnxqpoz = lskdjfgh = \\cdots \\) and the sequence converges to \\( hjgrksla \\). If \\( hjgrksla \\notin lkjhgfds \\), let \\( qazwsxed \\) and \\( wsxrfvgt \\) be the endpoints of the component \\( ikmjnhbg \\) of \\( ghytreds - lkjhgfds \\) containing \\( hjgrksla \\), and choose the notation so that\n\\[\nbnvcxzas(qazwsxed) < bnvcxzas\\left(hjgrksla\\right) < bnvcxzas(wsxrfvgt) .\n\\]\n(This is possible since \\( bnvcxzas \\) is strictly monotonic on \\( ikmjnhbg \\).) We have seen that \\( bnvcxzas\\left(bvnxqpoz\\right) < bnvcxzas\\left(hjgrksla\\right) \\) and either \\( bvnxqpoz \\in ikmjnhbg \\) or \\( bvnxqpoz \\) is an endpoint of \\( ikmjnhbg \\), in which case \\( bvnxqpoz = qazwsxed \\). Repeating this argument, it follows that either the points \\( hjgrksla, bvnxqpoz, lskdjfgh, \\ldots \\) are all in \\( ikmjnhbg \\) or eventually \\( vbnmrety = qazwsxed \\) for some \\( zxcvbnml \\), and then \\( asdkfghj = qazwsxed \\) for all \\( rtghyujk \\geq zxcvbnml \\). In the latter case, we clearly have \\( asdkfghj \\rightarrow qazwsxed \\). In the former case, \\( hjgrksla, bvnxqpoz, lskdjfgh, \\ldots \\) is a monotonic sequence in \\( ikmjnhbg \\) since\n\\( bnvcxzas\\left(hjgrksla\\right) > bnvcxzas\\left(bvnxqpoz\\right) > bnvcxzas\\left(lskdjfgh\\right) > \\cdots \\),\nso it converges to some point \\( poiulkjh \\) in \\( \\bar{ikmjnhbg} \\).\n\nNow\n\\( fdertyui(poiulkjh)=fdertyui\\left(\\lim _{rtghyujk \\to \\infty} asdkfghj\\right)=\\lim _{rtghyujk \\to \\infty} fdertyui\\left(asdkfghj\\right)=\\lim _{rtghyujk \\to \\infty} P_{n+1}=poiulkjh \\).\nThus \\( poiulkjh \\) is a fixed point of \\( fdertyui \\) and \\( poiulkjh \\in \\bar{ikmjnhbg} \\). So \\( poiulkjh = qazwsxed \\) or \\( poiulkjh = wsxrfvgt \\). But\n\\( bnvcxzas(poiulkjh)=\\lim bnvcxzas\\left(asdkfghj\\right) \\leq bnvcxzas\\left(hjgrksla\\right) < bnvcxzas(wsxrfvgt) \\), so \\( poiulkjh = qazwsxed \\). Thus we have shown that, in any case, \\( asdkfghj \\rightarrow qazwsxed \\); i.e., if \\( hjgrksla \\in ghytreds - lkjhgfds \\), then \\( asdkfghj \\) converges to that endpoint of the component of \\( ghytreds - lkjhgfds \\) containing \\( hjgrksla \\) which is closer to \\( ujmpolkq \\). And \\( \\lim asdkfghj \\) is a fixed point of \\( fdertyui \\).\n\nIt is clear that any fixed point \\( mnbvcxzq \\) of \\( fdertyui \\) is the limit of such a sequence: Take \\( hjgrksla = mnbvcxzq \\)."
    },
    "kernel_variant": {
      "question": "Let Γ be a simple, closed and strictly convex C¹-curve in the plane and let I be a point situated in the (bounded) interior of Γ.\n\nFor X ∈ Γ let ℓ_X be the tangent line to Γ at X.  Denote by n_X the line through I that is perpendicular to ℓ_X.  Among the two half–lines that start at I and lie on n_X choose the one that makes an acute angle with the vector IX; call this half–line r_X.  Because Γ is strictly convex and I lies inside Γ, r_X meets Γ in exactly one point different from X.  We denote that point by\n  S(X) ∈ Γ.\n(If the segment IX itself is perpendicular to ℓ_X, then r_X already passes through X and S(X)=X.)  In this way we obtain a continuous map\n  S : Γ → Γ.\n\nStarting with an arbitrary point Q₁ ∈ Γ define the sequence (Q_n) by\n  Q_{n+1}=S(Q_n)  (n ≥ 1).\n\n(a) Prove that the sequence (Q_n) converges.\n\n(b) Describe exactly the set of points L ∈ Γ for which there exists an initial point Q₁ such that  lim_{n→∞} Q_n = L.",
      "solution": "Throughout we write d(X)=|IX| for X∈Γ.\n\n1.  The fixed points.\n\nA point X satisfies S(X)=X if and only if the radius IX is perpendicular to the tangent ℓ_X.  Thus the fixed–point set\n F := { X∈Γ : S(X)=X }\ncoincides with the set of critical points of the restriction d|_Γ.  Because d is continuous on the compact set Γ, F is compact (in fact finite, but this is not needed).\n\n2.  The distance decreases outside F.\n\nFix X∈Γ\\F and put R=S(X).  Let H be the foot of the perpendicular from I to the tangent ℓ_X, so H∈ℓ_X and IH ⟂ ℓ_X.\n\nBecause X∉F, the vector IX is not perpendicular to ℓ_X, hence X≠H.  The triangle I-H-X is right-angled at H and therefore |IX|>|IH|.  On the other hand, R lies on the ray r_X=I H→, hence on the segment IH; indeed R is situated between I and H (otherwise Γ would intersect ℓ_X at a second point, contradicting strict convexity).  Consequently |IR|≤|IH|.  Combining the two inequalities we obtain\n  d(R)=|IR| < |IX| = d(X)  (1)\nfor every X∉F.\n\n3.  The structure of Γ\\F.\n\nBecause S is continuous, F is closed and Γ\\F is the union of its connected components.  Each component is an open arc Δ whose endpoints A,B belong to F.  The function d has no critical point on Δ and is therefore strictly monotone there.  After possibly interchanging the names we may assume\n  d(A) < d(B).  (2)\n\n4.  A key lemma – an orbit cannot jump over a fixed point.\n\nLemma  Let X∈Δ (with Δ as in 3) and R=S(X).  Then either R=A or R∈Δ.\n\nProof.  The two rays IX and IR delimit a convex cone with vertex I.  Denote its interior by C.  Because Γ lies on the side of ℓ_X that contains I, the whole arc of Γ that joins X to R inside that half-plane is contained in C.\n\nSuppose, contrary to the lemma, that the open arc (X,R) contains a fixed point Y∈F different from A.  Then Y lies inside C.  Since IY is perpendicular to the tangent ℓ_Y at Y, the tangent ℓ_Y is perpendicular to a segment that issues from I and meets Y inside the cone C.  Hence ℓ_Y intersects the interior of the segment IX (for otherwise it would be parallel to it).  This point of intersection is strictly inside the convex domain bounded by Γ, whereas ℓ_Y is a supporting line to Γ.  This contradiction shows that no fixed point can occur on (X,R) except possibly A; therefore R=A or R∈Δ. ∎\n\nAs a by-product of the proof we have, whenever X∈Δ\\F,\n  d(A) < d(R) < d(X)  (3)\nby (1) and the strict monotonicity of d on Δ.\n\n5.  Convergence of every orbit.\n\nStart with an arbitrary Q₁∈Γ and define Q_{n+1}=S(Q_n).\n\n• If Q₁∈F the sequence is constant and obviously converges.\n\n• Assume Q₁∉F and let Δ be the component of Γ\\F that contains Q₁; keep the notation of (2).\n\nRelation (1) yields d(Q_{n+1})<d(Q_n) for all n, so the numerical sequence (d(Q_n)) is strictly decreasing and bounded below by d(A); hence it converges to some limit ρ≥d(A).\n\nBy the Lemma, as long as Q_n∉F the next point Q_{n+1} never leaves Δ except possibly by landing exactly at A.  Two cases can occur.\n\n– For some index k we have Q_k∈F.  Necessarily d(Q_k)≤ρ<d(B), hence Q_k=A.  From that moment on the iteration is constant and Q_n=A for every n≥k.\n\n– No iterate is fixed.  Then all Q_n lie in the compact set Δ, and therefore the sequence has an accumulation point X∈\\barΔ.  Continuity of S implies S(X)=X; thus X∈F∩\\barΔ={A,B}.  Because d(X)=ρ<d(B), we must have X=A.  In a Hausdorff space a sequence with exactly one accumulation point converges to it, so Q_n→A.\n\nIn every case the orbit converges, which proves (a).\n\n6.  Description of all possible limits.\n\nSection 5 shows that every orbit converges to a fixed point.  Conversely, each fixed point Y∈F is the limit of the constant sequence started at Q₁=Y.  Hence\n  { limits of all sequences (Q_n) } = F = { X∈Γ : IX ⟂ ℓ_X }.\nThis answers (b).",
      "_meta": {
        "core_steps": [
          "Identify fixed points: OP ⟂ tangent  ⇔  T(P)=P (critical points of distance g).",
          "Show strict descent: if P∉F then g(T(P)) < g(P).",
          "F closed ⇒ C\\F splits into open arcs where g is strictly monotone.",
          "Iterating T keeps points in one arc, gives strictly decreasing g-sequence ⇒ convergence to nearer endpoint (a fixed point).",
          "Any fixed point is attainable as a limit by starting the iteration there."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Reference point inside the curve; any interior point could replace it.",
            "original": "O"
          },
          "slot2": {
            "description": "Specific radial measure; any smooth strictly increasing function of |OP| (e.g. |OP|²) serves the same role.",
            "original": "g(P)=|OP|"
          },
          "slot3": {
            "description": "Indexing convention for the iterative sequence.",
            "original": "P₀, P₁=T(P₀), …"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}