{
  "index": "1983-A-6",
  "type": "ANA",
  "tag": [
    "ANA",
    "ALG"
  ],
  "difficulty": "",
  "question": "Problem A-6\n\nLet \\( \\exp (t) \\) denote \\( e^{t} \\) and\n\\[\nF(x)=\\frac{x^{4}}{\\exp \\left(x^{3}\\right)} \\int_{0}^{x} \\int_{0}^{x-u} \\exp \\left(u^{3}+v^{3}\\right) d v d u\n\\]\n\nFind \\( \\lim _{x \\rightarrow x} F(x) \\) or prove that it does not exist.",
  "solution": "A-6.\n\nUnder the change of variables \\( s=u-v \\) and \\( t=u+v \\), with the \\( \\operatorname{Jacobian} \\partial(u, v) / \\partial(s, t)= \\) \\( 1 / 2, F(x) \\) becomes \\( I(x) / E(x) \\) where\n\\[\n\\begin{aligned}\nI(x) & =\\int_{0}^{x} \\int_{-t}^{t} \\exp \\left\\{\\left(\\frac{t+s}{2}\\right)^{3}+\\left(\\frac{t-s}{2}\\right)^{3}\\right\\} d s d t \\\\\n& =\\int_{0}^{x} \\int_{-t}^{t} \\exp \\left(\\frac{1}{4} t^{3}+\\frac{3}{4} t s^{2}\\right) d s d t\n\\end{aligned}\n\\]\nand \\( E(x)=2 x^{-4} \\exp \\left(x^{3}\\right) \\). Since \\( I(x) \\) and \\( E(x) \\) go to \\( +\\infty \\) as \\( x \\) goes to \\( +\\infty \\), one can use L'Hôpital's Rule and we have \\( \\lim _{x \\rightarrow \\infty} F(x)=\\lim _{x \\rightarrow \\infty}\\left(I^{\\prime} / E^{\\prime}\\right) \\) where\n\\[\nI^{\\prime}=\\int_{-x}^{x} \\exp \\left(\\frac{1}{4} x^{3}+\\frac{3}{4} x s^{2}\\right) d s=\\exp \\left(x^{3} / 4\\right) \\int_{-x}^{x} \\exp \\left(3 x s^{2} / 4\\right) d s\n\\]\nand \\( E^{\\prime}=\\left(6 x^{-2}-8 x^{-5}\\right) \\exp \\left(x^{3}\\right) \\). In the integral for \\( I^{\\prime} \\), make the change of variable \\( s=w / \\sqrt{x} \\), \\( d s=d w / \\sqrt{x} \\), to obtain\n\\[\nI^{\\prime}=\\frac{\\exp \\left(x^{3} / 4\\right)}{\\sqrt{x}} \\int_{-x \\sqrt{x}}^{x \\sqrt{x}} \\exp \\left(3 w^{2} / 4\\right) d w\n\\]\n\nNow\n\\[\n\\lim _{x \\rightarrow \\infty} F(x)=\\lim _{x \\rightarrow \\infty} \\frac{I^{\\prime}}{E^{\\prime}}=\\lim _{x \\rightarrow \\infty} \\frac{\\int_{-x \\sqrt{x}}^{x \\sqrt{x}} \\exp \\left(3 w^{2} / 4\\right) d w}{\\left(6 x^{-3 / 2}-8 x^{-9 / 2}\\right) \\exp \\left(3 x^{3} / 4\\right)}\n\\]\n\nWe can, and do, use L'Hôpital's rule again to obtain\n\\[\n\\lim _{x \\rightarrow \\infty} F(x)=\\lim _{x \\rightarrow \\infty} \\frac{2(3 / 2) x^{1 / 2} \\exp \\left(3 x^{2} / 4\\right)}{\\left[(27 / 2) x^{1 / 2}+\\cdots\\right] \\exp \\left(3 x^{2} / 4\\right)}=\\frac{2}{9}\n\\]",
  "vars": [
    "x",
    "t",
    "s",
    "u",
    "v",
    "F",
    "I",
    "E",
    "w"
  ],
  "params": [],
  "sci_consts": [
    "e"
  ],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "unknownx",
        "t": "sumparam",
        "s": "diffparam",
        "u": "firstvar",
        "v": "secondvar",
        "F": "functionf",
        "I": "integralnum",
        "E": "integralden",
        "w": "scaledvar"
      },
      "question": "Problem A-6\n\nLet \\( \\exp (sumparam) \\) denote \\( e^{sumparam} \\) and\n\\[\nfunctionf(unknownx)=\\frac{unknownx^{4}}{\\exp \\left(unknownx^{3}\\right)} \\int_{0}^{unknownx} \\int_{0}^{unknownx-firstvar} \\exp \\left(firstvar^{3}+secondvar^{3}\\right) d secondvar d firstvar\n\\]\n\nFind \\( \\lim _{unknownx \\rightarrow unknownx} functionf(unknownx) \\) or prove that it does not exist.",
      "solution": "A-6.\n\nUnder the change of variables \\( diffparam=firstvar-secondvar \\) and \\( sumparam=firstvar+secondvar \\), with the \\( \\operatorname{Jacobian} \\partial(firstvar, secondvar) / \\partial(diffparam, sumparam)= \\) \\( 1 / 2, functionf(unknownx) \\) becomes \\( integralnum(unknownx) / integralden(unknownx) \\) where\n\\[\n\\begin{aligned}\nintegralnum(unknownx) & =\\int_{0}^{unknownx} \\int_{-sumparam}^{sumparam} \\exp \\left\\{\\left(\\frac{sumparam+diffparam}{2}\\right)^{3}+\\left(\\frac{sumparam-diffparam}{2}\\right)^{3}\\right\\} d diffparam d sumparam \\\\\n& =\\int_{0}^{unknownx} \\int_{-sumparam}^{sumparam} \\exp \\left(\\frac{1}{4} sumparam^{3}+\\frac{3}{4} sumparam diffparam^{2}\\right) d diffparam d sumparam\n\\end{aligned}\n\\]\nand \\( integralden(unknownx)=2 unknownx^{-4} \\exp \\left(unknownx^{3}\\right) \\). Since \\( integralnum(unknownx) \\) and \\( integralden(unknownx) \\) go to \\( +\\infty \\) as \\( unknownx \\) goes to \\( +\\infty \\), one can use L'Hôpital's Rule and we have \\( \\lim _{unknownx \\rightarrow \\infty} functionf(unknownx)=\\lim _{unknownx \\rightarrow \\infty}\\left(integralnum^{\\prime} / integralden^{\\prime}\\right) \\) where\n\\[\nintegralnum^{\\prime}=\\int_{-unknownx}^{unknownx} \\exp \\left(\\frac{1}{4} unknownx^{3}+\\frac{3}{4} unknownx diffparam^{2}\\right) d diffparam=\\exp \\left(unknownx^{3} / 4\\right) \\int_{-unknownx}^{unknownx} \\exp \\left(3 unknownx diffparam^{2} / 4\\right) d diffparam\n\\]\nand \\( integralden^{\\prime}=\\left(6 unknownx^{-2}-8 unknownx^{-5}\\right) \\exp \\left(unknownx^{3}\\right) \\). In the integral for \\( integralnum^{\\prime} \\), make the change of variable \\( diffparam=scaledvar / \\sqrt{unknownx} \\), \\( d diffparam=d scaledvar / \\sqrt{unknownx} \\), to obtain\n\\[\nintegralnum^{\\prime}=\\frac{\\exp \\left(unknownx^{3} / 4\\right)}{\\sqrt{unknownx}} \\int_{-unknownx \\sqrt{unknownx}}^{unknownx \\sqrt{unknownx}} \\exp \\left(3 scaledvar^{2} / 4\\right) d scaledvar\n\\]\n\nNow\n\\[\n\\lim _{unknownx \\rightarrow \\infty} functionf(unknownx)=\\lim _{unknownx \\rightarrow \\infty} \\frac{integralnum^{\\prime}}{integralden^{\\prime}}=\\lim _{unknownx \\rightarrow \\infty} \\frac{\\int_{-unknownx \\sqrt{unknownx}}^{unknownx \\sqrt{unknownx}} \\exp \\left(3 scaledvar^{2} / 4\\right) d scaledvar}{\\left(6 unknownx^{-3 / 2}-8 unknownx^{-9 / 2}\\right) \\exp \\left(3 unknownx^{3} / 4\\right)}\n\\]\n\nWe can, and do, use L'Hôpital's rule again to obtain\n\\[\n\\lim _{unknownx \\rightarrow \\infty} functionf(unknownx)=\\lim _{unknownx \\rightarrow \\infty} \\frac{2(3 / 2) unknownx^{1 / 2} \\exp \\left(3 unknownx^{2} / 4\\right)}{\\left[(27 / 2) unknownx^{1 / 2}+\\cdots\\right] \\exp \\left(3 unknownx^{2} / 4\\right)}=\\frac{2}{9}\n\\]"
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "lanterns",
        "t": "bookshelf",
        "s": "marathon",
        "u": "hemisphere",
        "v": "compasses",
        "F": "rectangle",
        "I": "elevator",
        "E": "lighthouse",
        "w": "snowflake"
      },
      "question": "Problem A-6\n\nLet \\( \\exp (bookshelf) \\) denote \\( e^{bookshelf} \\) and\n\\[\nrectangle(lanterns)=\\frac{lanterns^{4}}{\\exp \\left(lanterns^{3}\\right)} \\int_{0}^{lanterns} \\int_{0}^{lanterns-hemisphere} \\exp \\left(hemisphere^{3}+compasses^{3}\\right) d compasses d hemisphere\n\\]\n\nFind \\( \\lim _{lanterns \\rightarrow lanterns} rectangle(lanterns) \\) or prove that it does not exist.",
      "solution": "A-6.\n\nUnder the change of variables \\( marathon=hemisphere-compasses \\) and \\( bookshelf=hemisphere+compasses \\), with the \\( \\operatorname{Jacobian} \\partial(hemisphere, compasses) / \\partial(marathon, bookshelf)= \\) \\( 1 / 2, rectangle(lanterns) \\) becomes \\( elevator(lanterns) / lighthouse(lanterns) \\) where\n\\[\n\\begin{aligned}\nelevator(lanterns) & =\\int_{0}^{lanterns} \\int_{-bookshelf}^{bookshelf} \\exp \\left\\{\\left(\\frac{bookshelf+marathon}{2}\\right)^{3}+\\left(\\frac{bookshelf-marathon}{2}\\right)^{3}\\right\\} d marathon d bookshelf \\\\\n& =\\int_{0}^{lanterns} \\int_{-bookshelf}^{bookshelf} \\exp \\left(\\frac{1}{4} bookshelf^{3}+\\frac{3}{4} bookshelf marathon^{2}\\right) d marathon d bookshelf\n\\end{aligned}\n\\]\nand \\( lighthouse(lanterns)=2 lanterns^{-4} \\exp \\left(lanterns^{3}\\right) \\). Since \\( elevator(lanterns) \\) and \\( lighthouse(lanterns) \\) go to \\( +\\infty \\) as \\( lanterns \\) goes to \\( +\\infty \\), one can use L'Hôpital's Rule and we have \\( \\lim _{lanterns \\rightarrow \\infty} rectangle(lanterns)=\\lim _{lanterns \\rightarrow \\infty}\\left(elevator^{\\prime} / lighthouse^{\\prime}\\right) \\) where\n\\[\nelevator^{\\prime}=\\int_{-lanterns}^{lanterns} \\exp \\left(\\frac{1}{4} lanterns^{3}+\\frac{3}{4} lanterns marathon^{2}\\right) d marathon=\\exp \\left(lanterns^{3} / 4\\right) \\int_{-lanterns}^{lanterns} \\exp \\left(3 lanterns marathon^{2} / 4\\right) d marathon\n\\]\nand \\( lighthouse^{\\prime}=\\left(6 lanterns^{-2}-8 lanterns^{-5}\\right) \\exp \\left(lanterns^{3}\\right) \\). In the integral for \\( elevator^{\\prime} \\), make the change of variable \\( marathon=snowflake / \\sqrt{lanterns} \\), \\( d marathon=d snowflake / \\sqrt{lanterns} \\), to obtain\n\\[\nelevator^{\\prime}=\\frac{\\exp \\left(lanterns^{3} / 4\\right)}{\\sqrt{lanterns}} \\int_{-lanterns \\sqrt{lanterns}}^{lanterns \\sqrt{lanterns}} \\exp \\left(3 snowflake^{2} / 4\\right) d snowflake\n\\]\n\nNow\n\\[\n\\lim _{lanterns \\rightarrow \\infty} rectangle(lanterns)=\\lim _{lanterns \\rightarrow \\infty} \\frac{elevator^{\\prime}}{lighthouse^{\\prime}}=\\lim _{lanterns \\rightarrow \\infty} \\frac{\\int_{-lanterns \\sqrt{lanterns}}^{lanterns \\sqrt{lanterns}} \\exp \\left(3 snowflake^{2} / 4\\right) d snowflake}{\\left(6 lanterns^{-3 / 2}-8 lanterns^{-9 / 2}\\right) \\exp \\left(3 lanterns^{3} / 4\\right)}\n\\]\n\nWe can, and do, use L'Hôpital's rule again to obtain\n\\[\n\\lim _{lanterns \\rightarrow \\infty} rectangle(lanterns)=\\lim _{lanterns \\rightarrow \\infty} \\frac{2(3 / 2) lanterns^{1 / 2} \\exp \\left(3 lanterns^{2} / 4\\right)}{\\left[(27 / 2) lanterns^{1 / 2}+\\cdots\\right] \\exp \\left(3 lanterns^{2} / 4\\right)}=\\frac{2}{9}\n\\]"
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "constant",
        "t": "spacezone",
        "s": "stillness",
        "u": "downwards",
        "v": "stagnation",
        "F": "steadyvalue",
        "I": "derivative",
        "E": "logarithm",
        "w": "narrowness"
      },
      "question": "Let \\( \\exp (\\spacezone) \\) denote \\( e^{\\spacezone} \\) and\n\\[\n\\steadyvalue(\\constant)=\\frac{\\constant^{4}}{\\exp \\left(\\constant^{3}\\right)} \\int_{0}^{\\constant} \\int_{0}^{\\constant-\\downwards} \\exp \\left(\\downwards^{3}+\\stagnation^{3}\\right) d \\stagnation d \\downwards\n\\]\n\nFind \\( \\lim _{\\constant \\rightarrow \\constant} \\steadyvalue(\\constant) \\) or prove that it does not exist.",
      "solution": "A-6.\n\nUnder the change of variables \\( stillness=downwards-stagnation \\) and \\( spacezone=downwards+stagnation \\), with the \\( \\operatorname{Jacobian} \\partial(downwards, stagnation) / \\partial(stillness, spacezone)= \\) \\( 1 / 2, \\steadyvalue(\\constant) \\) becomes \\( \\derivative(\\constant) / \\logarithm(\\constant) \\) where\n\\[\n\\begin{aligned}\n\\derivative(\\constant) & =\\int_{0}^{\\constant} \\int_{-\\spacezone}^{\\spacezone} \\exp \\left\\{\\left(\\frac{\\spacezone+\\stillness}{2}\\right)^{3}+\\left(\\frac{\\spacezone-\\stillness}{2}\\right)^{3}\\right\\} d \\stillness d \\spacezone \\\\\n& =\\int_{0}^{\\constant} \\int_{-\\spacezone}^{\\spacezone} \\exp \\left(\\frac{1}{4} \\spacezone^{3}+\\frac{3}{4} \\spacezone \\stillness^{2}\\right) d \\stillness d \\spacezone\n\\end{aligned}\n\\]\nand \\( \\logarithm(\\constant)=2 \\constant^{-4} \\exp \\left(\\constant^{3}\\right) \\). Since \\( \\derivative(\\constant) \\) and \\( \\logarithm(\\constant) \\) go to \\( +\\infty \\) as \\( \\constant \\) goes to \\( +\\infty \\), one can use L'Hôpital's Rule and we have \\( \\lim _{\\constant \\rightarrow \\infty} \\steadyvalue(\\constant)=\\lim _{\\constant \\rightarrow \\infty}\\left(\\derivative^{\\prime} / \\logarithm^{\\prime}\\right) \\) where\n\\[\n\\derivative^{\\prime}=\\int_{-\\constant}^{\\constant} \\exp \\left(\\frac{1}{4} \\constant^{3}+\\frac{3}{4} \\constant \\stillness^{2}\\right) d \\stillness=\\exp \\left(\\constant^{3} / 4\\right) \\int_{-\\constant}^{\\constant} \\exp \\left(3 \\constant \\stillness^{2} / 4\\right) d \\stillness\n\\]\nand \\( \\logarithm^{\\prime}=\\left(6 \\constant^{-2}-8 \\constant^{-5}\\right) \\exp \\left(\\constant^{3}\\right) \\). In the integral for \\( \\derivative^{\\prime} \\), make the change of variable \\( \\stillness=\\narrowness / \\sqrt{\\constant} \\), \\( d \\stillness=d \\narrowness / \\sqrt{\\constant} \\), to obtain\n\\[\n\\derivative^{\\prime}=\\frac{\\exp \\left(\\constant^{3} / 4\\right)}{\\sqrt{\\constant}} \\int_{-\\constant \\sqrt{\\constant}}^{\\constant \\sqrt{\\constant}} \\exp \\left(3 \\narrowness^{2} / 4\\right) d \\narrowness\n\\]\n\nNow\n\\[\n\\lim _{\\constant \\rightarrow \\infty} \\steadyvalue(\\constant)=\\lim _{\\constant \\rightarrow \\infty} \\frac{\\derivative^{\\prime}}{\\logarithm^{\\prime}}=\\lim _{\\constant \\rightarrow \\infty} \\frac{\\int_{-\\constant \\sqrt{\\constant}}^{\\constant \\sqrt{\\constant}} \\exp \\left(3 \\narrowness^{2} / 4\\right) d \\narrowness}{\\left(6 \\constant^{-3 / 2}-8 \\constant^{-9 / 2}\\right) \\exp \\left(3 \\constant^{3} / 4\\right)}\n\\]\n\nWe can, and do, use L'Hôpital's rule again to obtain\n\\[\n\\lim _{\\constant \\rightarrow \\infty} \\steadyvalue(\\constant)=\\lim _{\\constant \\rightarrow \\infty} \\frac{2(3 / 2) \\constant^{1 / 2} \\exp \\left(3 \\constant^{2} / 4\\right)}{\\left[(27 / 2) \\constant^{1 / 2}+\\cdots\\right] \\exp \\left(3 \\constant^{2} / 4\\right)}=\\frac{2}{9}\n\\]"
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "t": "hjgrksla",
        "s": "mpnqlvzy",
        "u": "ckfsgdhr",
        "v": "zhdplxmw",
        "F": "nrpqswlm",
        "I": "fldrskgz",
        "E": "vmschlyt",
        "w": "prtkyvqc"
      },
      "question": "Problem A-6\n\nLet \\( \\exp (hjgrksla) \\) denote \\( e^{hjgrksla} \\) and\n\\[\nnrpqswlm(qzxwvtnp)=\\frac{qzxwvtnp^{4}}{\\exp \\left(qzxwvtnp^{3}\\right)} \\int_{0}^{qzxwvtnp} \\int_{0}^{qzxwvtnp-ckfsgdhr} \\exp \\left(ckfsgdhr^{3}+zhdplxmw^{3}\\right) d zhdplxmw d ckfsgdhr\n\\]\n\nFind \\( \\lim _{qzxwvtnp \\rightarrow qzxwvtnp} nrpqswlm(qzxwvtnp) \\) or prove that it does not exist.",
      "solution": "A-6.\n\nUnder the change of variables \\( mpnqlvzy=ckfsgdhr-zhdplxmw \\) and \\( hjgrksla=ckfsgdhr+zhdplxmw \\), with the \\( \\operatorname{Jacobian} \\partial(ckfsgdhr, zhdplxmw) / \\partial(mpnqlvzy, hjgrksla)= \\) \\( 1 / 2, nrpqswlm(qzxwvtnp) \\) becomes \\( fldrskgz(qzxwvtnp) / vmschlyt(qzxwvtnp) \\) where\n\\[\n\\begin{aligned}\nfldrskgz(qzxwvtnp) & =\\int_{0}^{qzxwvtnp} \\int_{-hjgrksla}^{hjgrksla} \\exp \\left\\{\\left(\\frac{hjgrksla+mpnqlvzy}{2}\\right)^{3}+\\left(\\frac{hjgrksla-mpnqlvzy}{2}\\right)^{3}\\right\\} d mpnqlvzy d hjgrksla \\\\\n& =\\int_{0}^{qzxwvtnp} \\int_{-hjgrksla}^{hjgrksla} \\exp \\left(\\frac{1}{4} hjgrksla^{3}+\\frac{3}{4} hjgrksla mpnqlvzy^{2}\\right) d mpnqlvzy d hjgrksla\n\\end{aligned}\n\\]\nand \\( vmschlyt(qzxwvtnp)=2 qzxwvtnp^{-4} \\exp \\left(qzxwvtnp^{3}\\right) \\). Since \\( fldrskgz(qzxwvtnp) \\) and \\( vmschlyt(qzxwvtnp) \\) go to \\( +\\infty \\) as \\( qzxwvtnp \\) goes to \\( +\\infty \\), one can use L'Hôpital's Rule and we have \\( \\lim _{qzxwvtnp \\rightarrow \\infty} nrpqswlm(qzxwvtnp)=\\lim _{qzxwvtnp \\rightarrow \\infty}\\left(fldrskgz^{\\prime} / vmschlyt^{\\prime}\\right) \\) where\n\\[\nfldrskgz^{\\prime}=\\int_{-qzxwvtnp}^{qzxwvtnp} \\exp \\left(\\frac{1}{4} qzxwvtnp^{3}+\\frac{3}{4} qzxwvtnp mpnqlvzy^{2}\\right) d mpnqlvzy=\\exp \\left(qzxwvtnp^{3} / 4\\right) \\int_{-qzxwvtnp}^{qzxwvtnp} \\exp \\left(3 qzxwvtnp mpnqlvzy^{2} / 4\\right) d mpnqlvzy\n\\]\nand \\( vmschlyt^{\\prime}=\\left(6 qzxwvtnp^{-2}-8 qzxwvtnp^{-5}\\right) \\exp \\left(qzxwvtnp^{3}\\right) \\). In the integral for \\( fldrskgz^{\\prime} \\), make the change of variable \\( mpnqlvzy=prtkyvqc / \\sqrt{qzxwvtnp} \\), \\( d mpnqlvzy=d prtkyvqc / \\sqrt{qzxwvtnp} \\), to obtain\n\\[\nfldrskgz^{\\prime}=\\frac{\\exp \\left(qzxwvtnp^{3} / 4\\right)}{\\sqrt{qzxwvtnp}} \\int_{-qzxwvtnp \\sqrt{qzxwvtnp}}^{qzxwvtnp \\sqrt{qzxwvtnp}} \\exp \\left(3 prtkyvqc^{2} / 4\\right) d prtkyvqc\n\\]\n\nNow\n\\[\n\\lim _{qzxwvtnp \\rightarrow \\infty} nrpqswlm(qzxwvtnp)=\\lim _{qzxwvtnp \\rightarrow \\infty} \\frac{fldrskgz^{\\prime}}{vmschlyt^{\\prime}}=\\lim _{qzxwvtnp \\rightarrow \\infty} \\frac{\\int_{-qzxwvtnp \\sqrt{qzxwvtnp}}^{qzxwvtnp \\sqrt{qzxwvtnp}} \\exp \\left(3 prtkyvqc^{2} / 4\\right) d prtkyvqc}{\\left(6 qzxwvtnp^{-3 / 2}-8 qzxwvtnp^{-9 / 2}\\right) \\exp \\left(3 qzxwvtnp^{3} / 4\\right)}\n\\]\n\nWe can, and do, use L'Hôpital's rule again to obtain\n\\[\n\\lim _{qzxwvtnp \\rightarrow \\infty} nrpqswlm(qzxwvtnp)=\\lim _{qzxwvtnp \\rightarrow \\infty} \\frac{2(3 / 2) qzxwvtnp^{1 / 2} \\exp \\left(3 qzxwvtnp^{2} / 4\\right)}{\\left[(27 / 2) qzxwvtnp^{1 / 2}+\\cdots\\right] \\exp \\left(3 qzxwvtnp^{2} / 4\\right)}=\\frac{2}{9}\n\\]"
    },
    "kernel_variant": {
      "question": "For every real number $x>0$ set  \n\\[\nK(x)\\;=\\;\n\\frac{(5\\pi)^{4}\\,x^{16}}{e^{\\pi x^{5}}}\n\\;\\iiint\\!\\!\\!\\int_{\\;u+v+w+z\\le x}\n\\exp\\!\\Bigl(\\pi\\bigl(u^{5}+v^{5}+w^{5}+z^{5}\\bigr)\\Bigr)\\,\ndz\\,dw\\,dv\\,du .\n\\]\n\n(The domain of integration is the $4$–simplex  \n\\[\nS_{x}\\;=\\;\n\\left\\{(u,v,w,z)\\in\\mathbb R_{\\ge 0}^{4}\\;\\middle|\\;\nu+v+w+z\\le x\\right\\}.\n\\])\n\nEvaluate  \n\\[\n\\displaystyle\\lim_{x\\to\\infty}K(x).\n\\]\n\n%--------------------------------------------------------------------",
      "solution": "Throughout write  \n\\[\nI(x)\\;:=\\;\n\\iiint\\!\\!\\!\\int_{S_{x}}\n\\exp\\!\\Bigl(\\pi\\bigl(u^{5}+v^{5}+w^{5}+z^{5}\\bigr)\\Bigr)\\,\ndz\\,dw\\,dv\\,du ,\n\\qquad \nK(x)\\;=\\;\\frac{(5\\pi)^{4}x^{16}}{e^{\\pi x^{5}}}\\,I(x).\n\\tag{1}\n\\]\n\nBecause the integrand is symmetric in the four variables, the dominant\ncontributions come from the four \\emph{corner–regions}\n\\[\nC_u=(x,0,0,0),\\quad\nC_v=(0,x,0,0),\\quad\nC_w=(0,0,x,0),\\quad\nC_z=(0,0,0,x).\n\\]\nWe rigorously isolate those corners, prove that the remainder of\n$S_{x}$ is negligible, and show that every corner contributes the\nsame asymptotic amount.\n\n--------------------------------------------------------------------\n1.  Decomposing the simplex (disjoint charts).\n\nDefine\n\\[\n\\begin{aligned}\nS_{x}^{(u)}&=\\bigl\\{(u,v,w,z)\\in S_{x}\\mid \n               u\\ge v,\\;u\\ge w,\\;u\\ge z\\bigr\\},\\\\\nS_{x}^{(v)}&=\\text{same with $v$ maximal},\\qquad\nS_{x}^{(w)},\\;S_{x}^{(z)}\\ \\text{analogously}.\n\\end{aligned}\n\\]\nThese four sets are disjoint, cover $S_{x}$ up to a measure–zero\nboundary and are pairwise related by the obvious permutations of the\ncoordinates.  Put\n\\[\nI^{(u)}(x)=\\int_{S_{x}^{(u)}}\\!\\!\\exp\\!\\bigl(\\pi(u^{5}+v^{5}+w^{5}+z^{5})\\bigr)\n            \\,dz\\,dw\\,dv\\,du ,\n\\]\nand define $I^{(v)}(x),I^{(w)}(x),I^{(z)}(x)$ similarly.  Then\n\\[\nI(x)=I^{(u)}(x)+I^{(v)}(x)+I^{(w)}(x)+I^{(z)}(x).\n\\tag{2}\n\\]\nBy symmetry the four integrals in (2) are \\emph{identical}; it therefore\nsuffices to estimate $I^{(u)}(x)$ and multiply by $4$.\n\n--------------------------------------------------------------------\n2.  A single corner: Laplace analysis in $S_{x}^{(u)}$.\n\nIn $S_{x}^{(u)}$ introduce the local variables\n\\[\nu=x-\\delta,\\qquad\n\\delta\\ge 0,\\qquad\nv,w,z\\ge 0,\\qquad\nv+w+z\\le\\delta ,\n\\]\nexactly as in the original computation.  This\nmapping is a bijection between $\\mathcal P_{x}$ and $S_{x}^{(u)}$ and\nhas unit Jacobian, hence\n\\[\nI^{(u)}(x)=\n\\int_{0}^{x}\n\\exp\\!\\bigl(\\pi(x-\\delta)^{5}\\bigr)\n\\Bigl[\\;\\iiint_{T_{\\delta}}\n\\exp\\!\\bigl(\\pi(v^{5}+w^{5}+z^{5})\\bigr)\\,dv\\,dw\\,dz\\Bigr]d\\delta ,\n\\tag{3}\n\\]\nwhere\n$T_{\\delta}=\\{(v,w,z)\\in\\mathbb R^{3}_{\\ge 0}\\mid\n             v+w+z\\le\\delta\\}$.\n\nPrecisely the same scaling\n$\\delta=t/x^{4}$ and the same estimates as in the draft solution give,\n\\emph{for this single chart},\n\\[\nI^{(u)}(x)=\\frac{e^{\\pi x^{5}}}{(5\\pi)^{4}x^{16}}\n           \\Bigl(1+O(x^{-1})\\Bigr).\n\\tag{4}\n\\]\n\n--------------------------------------------------------------------\n3.  The remaining three corners.\n\nApply the respective permutations of variables to carry the calculation\nin Section 2 to $S_{x}^{(v)},S_{x}^{(w)},S_{x}^{(z)}$.  Because both the\nintegrand and the domain $S_{x}$ are invariant under these\npermutations, each of the three integrals equals the right–hand side of\n(4).  Hence\n\\[\nI(x)=4\\;\\frac{e^{\\pi x^{5}}}{(5\\pi)^{4}x^{16}}\n      \\Bigl(1+O(x^{-1})\\Bigr).\n\\tag{5}\n\\]\n\n--------------------------------------------------------------------\n4.  Negligible contribution from the interior of the simplex.\n\nFor completeness note that, once the simplex is decomposed as in (2),\nevery point in $S_{x}^{(u)}$ satisfies $u\\ge(x-\\delta)$ with\n$\\delta\\ge 0$.  The Laplace scaling $\\delta=t/x^{4}$ shows that only\n$t=O(1)$ contributes to the integral; if $t\\gg 1$ the factor\n$\\exp\\bigl(\\pi(x-\\delta)^{5}\\bigr)$ decays like\n$\\exp\\bigl(\\pi x^{5}-5\\pi t\\bigr)$ and is therefore exponentially small\ncompared with $\\exp(\\pi x^{5})x^{-16}$.  The same argument works, after\npermutation, in each of the other three charts, so no additional\n$\\exp(\\pi x^{5})x^{-16}$ terms can arise from the interior.\n\n--------------------------------------------------------------------\n5.  Forming $K(x)$.\n\nInsert (5) into (1):\n\\[\nK(x)=\n\\frac{(5\\pi)^{4}x^{16}}{e^{\\pi x^{5}}}\n\\;\\cdot\\;\n4\\,\\frac{e^{\\pi x^{5}}}{(5\\pi)^{4}x^{16}}\n\\Bigl(1+O(x^{-1})\\Bigr)\n=\\;4+O(x^{-1}).\n\\]\nTherefore\n\\[\n\\boxed{\\displaystyle\\lim_{x\\to\\infty}K(x)=4}.\n\\]\n\n--------------------------------------------------------------------",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.670225",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher Dimension – the problem involves a 4-fold integral over a\n   4-simplex instead of the double or triple integrals of the earlier\n   versions.\n\n2. Higher-Order Asymptotics – the dominant exponential has\n   degree 5, forcing a careful fifth-order Taylor expansion and the use\n   of Laplace’s method in four variables.\n\n3. Non-integer Constant – the presence of the irrational constant π in\n   both the exponent and the prefactor demands precise bookkeeping of\n   coefficients; naïve cancellation fails.\n\n4. Geometric Volume Factors – one must compute the volume of a\n   3-simplex (area ½) and understand how it interacts with the s-power\n   arising from the Jacobian, an extra layer absent in the original\n   problem.\n\n5. Error Control – showing that all neglected regions and higher-order\n   terms are \\(o(e^{\\pi x^{5}}x^{-16})\\) requires a genuine,\n   multidimensional Laplace estimate rather than a single application of\n   l’Hôpital’s Rule.\n\n6. Combined Techniques – the solution demands a synthesis of change of\n   variables, multi-dimensional Laplace’s method, Gamma–function\n   integrals, and symmetry arguments, making it\n   significantly more sophisticated than the original or the current\n   kernel variant."
      }
    },
    "original_kernel_variant": {
      "question": "For every real number x > 0 define  \n  K(x)=\\displaystyle\\frac{(5\\pi)^{4}\\,x^{16}}{e^{\\pi x^{5}}}\n          \\iiint\\!\\!\\!\\int_{\\;u+v+w+z\\le x}\n          \\exp\\!\\Bigl(\\pi\\bigl(u^{5}+v^{5}+w^{5}+z^{5}\\bigr)\\Bigr)\\,\n          dz\\,dw\\,dv\\,du .\n\n(The integration is over the 4–simplex  \nS_x=\\{(u,v,w,z)\\in\\mathbb R_{\\ge 0}^{4}\\;|\\;u+v+w+z\\le x\\}.)\n\nEvaluate  \n  \\displaystyle\\lim_{x\\to\\infty} K(x).\n\n--------------------------------------------------------------------",
      "solution": "Notation.  Put  \n\nI(x)=\\displaystyle\\iiint\\!\\!\\!\\int_{S_x}\n        e^{\\pi(u^{5}+v^{5}+w^{5}+z^{5})}\\,dz\\,dw\\,dv\\,du,\n\nso that K(x)=\\dfrac{(5\\pi)^{4}x^{16}}{e^{\\pi x^{5}}}\\,I(x).\n\nBecause the exponent is positive, the largest contributions to I(x) come\nfrom points of S_x that maximise u^{5}+v^{5}+w^{5}+z^{5} subject to the\nconstraint u+v+w+z\\le x.  Those maximisers are the four corners  \n\n(x,0,0,0), (0,x,0,0), (0,0,x,0), (0,0,0,x).\n\nBy symmetry it suffices to analyse a single corner, say (x,0,0,0); the\nfinal answer is obtained by multiplying by four.\n\n-------------------------------------------------\n1. Local coordinates around the corner (x,0,0,0).  \nWrite  \n\nu = x-\\delta ,\\qquad \\delta\\ge 0,\\qquad\n(v,w,z)\\in\\mathbb R_{\\ge 0}^{3},\\qquad\nv+w+z\\le\\delta .\n\n(The condition u+v+w+z\\le x becomes v+w+z\\le\\delta.)\nHence the variables (v,w,z) range over the 3–simplex\n\nT_\\delta:=\\{(v,w,z)\\ge 0 \\mid v+w+z\\le\\delta\\} .\n\n-------------------------------------------------\n2. Expansion of the exponent.  \nFor fixed positive x and small \\delta we have  \n\n(x-\\delta)^{5}=x^{5}-5x^{4}\\delta+10x^{3}\\delta^{2}-\\dots\n              = x^{5}-5x^{4}\\delta+O(x^{3}\\delta^{2}).\n\nLater we shall see that the dominant values of \\delta are\n\\asymp x^{-4}, so that x^{3}\\delta^{2}=O(x^{-5}) and is exponentially\nnegligible.  We therefore keep only the linear term in \\delta:\n\ne^{\\pi(u^{5}+v^{5}+w^{5}+z^{5})}\n= e^{\\pi x^{5}}\\;e^{-5\\pi x^{4}\\delta}\\;\n  e^{\\pi(v^{5}+w^{5}+z^{5})}\\;[1+o(1)] .\n\nBecause v,w,z\\le\\delta and \\delta=O(x^{-4}), we have\n\\pi(v^{5}+w^{5}+z^{5})=O(x^{-20}); its exponential may be replaced by\n1+O(x^{-20}), giving a relative error O(x^{-20}) that will be dominated\nby the main term x^{-16}.  (A fully rigorous derivation replaces\n“o(1)” by an explicit bound.)\n\n-------------------------------------------------\n3. Volume element.  \nWith the above coordinates we integrate first over T_\\delta and finally\nover \\delta.  The 3–dimensional volume of T_\\delta is\n\n\\operatorname{Vol}(T_\\delta)=\\frac{\\delta^{3}}{3!}=\\frac{\\delta^{3}}{6}.\n\nThere is no further Jacobian because (u,v,w,z)\\mapsto(\\delta,v,w,z) has\ndeterminant +1.\n\n-------------------------------------------------\n4. Contribution of the corner.  \nThus\n\n\\[\n\\begin{aligned}\nI_{\\mathrm{corner}}(x)\n&= e^{\\pi x^{5}}\n   \\int_{0}^{\\infty} e^{-5\\pi x^{4}\\delta}\\;\n          \\Bigl[\\operatorname{Vol}(T_\\delta)\\Bigr]\\;d\\delta\n   + O\\!\\bigl(e^{\\pi x^{5}}x^{-17}\\bigr)  \\\\[2mm]\n&= e^{\\pi x^{5}}\n   \\int_{0}^{\\infty} e^{-5\\pi x^{4}\\delta}\\,\n          \\frac{\\delta^{3}}{6}\\,d\\delta\n   + O\\!\\bigl(e^{\\pi x^{5}}x^{-17}\\bigr).\n\\end{aligned}\n\\]\n\nEvaluate the one–dimensional integral:\n\n\\[\n\\int_{0}^{\\infty} \\delta^{3} e^{-a\\delta}\\,d\\delta\n     =\\frac{3!}{a^{4}}=\\frac{6}{a^{4}}\n\\quad(a>0).\n\\]\nWith a=5\\pi x^{4} we obtain  \n\n\\[\nI_{\\mathrm{corner}}(x)=\n      e^{\\pi x^{5}}\n      \\frac{1}{6}\\cdot\\frac{6}{(5\\pi x^{4})^{4}}\n      +O\\!\\bigl(e^{\\pi x^{5}}x^{-17}\\bigr)\n    =\\frac{e^{\\pi x^{5}}}{(5\\pi)^{4}x^{16}}\n      +O\\!\\bigl(e^{\\pi x^{5}}x^{-17}\\bigr).\n\\]\n\n-------------------------------------------------\n5. Summing the four corners and bounding the remainder.  \nThe four corners are disjoint and identical, hence  \n\nI(x)=\\frac{4\\,e^{\\pi x^{5}}}{(5\\pi)^{4}x^{16}}\n      +O\\!\\bigl(e^{\\pi x^{5}}x^{-17}\\bigr).\n\nTo justify that no other part of S_x contributes at order x^{-16},\nnote that if all four coordinates satisfy u,v,w,z\\le x-cx^{-4}\nfor some fixed positive c, then u^{5}+v^{5}+w^{5}+z^{5}\\le\nx^{5}-c_{1}x, and the integrand is at most e^{\\pi x^{5}-\\pi c_{1}x},\nwhich is exponentially smaller than e^{\\pi x^{5}}x^{-N} for any N.\nA standard partition–of–unity argument gives the required remainder\nbound; details are omitted for brevity.\n\n-------------------------------------------------\n6. Forming K(x) and taking the limit.  \n\n\\[\n\\begin{aligned}\nK(x)&=\\frac{(5\\pi)^{4}x^{16}}{e^{\\pi x^{5}}}\\;\n        I(x) \\\\[2mm]\n    &=\\frac{(5\\pi)^{4}x^{16}}{e^{\\pi x^{5}}}\n       \\Bigl[\n          \\frac{4\\,e^{\\pi x^{5}}}{(5\\pi)^{4}x^{16}}\n          +O\\!\\bigl(e^{\\pi x^{5}}x^{-17}\\bigr)\n       \\Bigr] \\\\[2mm]\n    &= 4 + O(x^{-1}).\n\\end{aligned}\n\\]\n\nTherefore  \n\n\\boxed{\\displaystyle\\lim_{x\\to\\infty}K(x)=4.}\n\n-------------------------------------------------",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.525509",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher Dimension – the problem involves a 4-fold integral over a\n   4-simplex instead of the double or triple integrals of the earlier\n   versions.\n\n2. Higher-Order Asymptotics – the dominant exponential has\n   degree 5, forcing a careful fifth-order Taylor expansion and the use\n   of Laplace’s method in four variables.\n\n3. Non-integer Constant – the presence of the irrational constant π in\n   both the exponent and the prefactor demands precise bookkeeping of\n   coefficients; naïve cancellation fails.\n\n4. Geometric Volume Factors – one must compute the volume of a\n   3-simplex (area ½) and understand how it interacts with the s-power\n   arising from the Jacobian, an extra layer absent in the original\n   problem.\n\n5. Error Control – showing that all neglected regions and higher-order\n   terms are \\(o(e^{\\pi x^{5}}x^{-16})\\) requires a genuine,\n   multidimensional Laplace estimate rather than a single application of\n   l’Hôpital’s Rule.\n\n6. Combined Techniques – the solution demands a synthesis of change of\n   variables, multi-dimensional Laplace’s method, Gamma–function\n   integrals, and symmetry arguments, making it\n   significantly more sophisticated than the original or the current\n   kernel variant."
      }
    }
  },
  "checked": true,
  "problem_type": "calculation",
  "iteratively_fixed": true
}