{
  "index": "1997-A-3",
  "type": "ANA",
  "tag": [
    "ANA",
    "ALG"
  ],
  "difficulty": "",
  "question": "Evaluate\n\\begin{gather*}\n\\int_0^\\infty \\left(x-\\frac{x^3}{2}+\\frac{x^5}{2\\cdot\n4}-\\frac{x^7}{2\\cdot 4\\cdot 6}+\\cdots\\right) \\\\\n\\left(1+\\frac{x^2}{2^2}+\n\\frac{x^4}{2^2\\cdot 4^2}+\\frac{x^6}{2^2\\cdot 4^2 \\cdot 6^2}+\\cdots\\right)\\,dx.\n\\end{gather*}",
  "solution": "Note that the series on the left is simply $x \\exp (-x^2/2)$.  By\nintegration by parts,\n\\[\n\\int_0^\\infty x^{2n+1} e^{-x^2/2} dx =\n2n \\int_0^\\infty x^{2n-1} e^{-x^2/2} dx\n\\]\nand so by induction,\n\\[\n\\int_0^\\infty x^{2n+1} e^{-x^2/2} dx =\n2 \\times 4 \\times \\cdots \\times 2n.\n\\]\nThus the desired\nintegral is simply\n\\[\n\\sum_{n=0}^\\infty \\frac{1}{2^n n!} = \\sqrt{e}.\n\\]",
  "vars": [
    "x"
  ],
  "params": [
    "n"
  ],
  "sci_consts": [
    "e"
  ],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "variable",
        "n": "indexer"
      },
      "question": "Evaluate\n\\begin{gather*}\n\\int_0^\\infty \\left(variable-\\frac{variable^3}{2}+\\frac{variable^5}{2\\cdot\n4}-\\frac{variable^7}{2\\cdot 4\\cdot 6}+\\cdots\\right) \\\\\n\\left(1+\\frac{variable^2}{2^2}+\\frac{variable^4}{2^2\\cdot 4^2}+\\frac{variable^6}{2^2\\cdot 4^2 \\cdot 6^2}+\\cdots\\right)\\,dvariable.\n\\end{gather*}",
      "solution": "Note that the series on the left is simply $variable \\exp (-variable^2/2)$.  By\nintegration by parts,\n\\[\n\\int_0^\\infty variable^{2indexer+1} e^{-variable^2/2} dvariable =\n2indexer \\int_0^\\infty variable^{2indexer-1} e^{-variable^2/2} dvariable\n\\]\nand so by induction,\n\\[\n\\int_0^\\infty variable^{2indexer+1} e^{-variable^2/2} dvariable =\n2 \\times 4 \\times \\cdots \\times 2indexer.\n\\]\nThus the desired\nintegral is simply\n\\[\n\\sum_{indexer=0}^\\infty \\frac{1}{2^{indexer} indexer!} = \\sqrt{e}.\n\\]"
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "caterpillar",
        "n": "marshmallow"
      },
      "question": "Evaluate\n\\begin{gather*}\n\\int_0^\\infty \\left(caterpillar-\\frac{caterpillar^3}{2}+\\frac{caterpillar^5}{2\\cdot\n4}-\\frac{caterpillar^7}{2\\cdot 4\\cdot 6}+\\cdots\\right) \\\\\n\\left(1+\\frac{caterpillar^2}{2^2}+\\frac{caterpillar^4}{2^2\\cdot 4^2}+\\frac{caterpillar^6}{2^2\\cdot 4^2 \\cdot 6^2}+\\cdots\\right)\\,dcaterpillar.\n\\end{gather*}",
      "solution": "Note that the series on the left is simply $caterpillar \\exp (-caterpillar^2/2)$.  By\nintegration by parts,\n\\[\n\\int_0^\\infty caterpillar^{2marshmallow+1} e^{-caterpillar^2/2} dcaterpillar =\n2marshmallow \\int_0^\\infty caterpillar^{2marshmallow-1} e^{-caterpillar^2/2} dcaterpillar\n\\]\nand so by induction,\n\\[\n\\int_0^\\infty caterpillar^{2marshmallow+1} e^{-caterpillar^2/2} dcaterpillar =\n2 \\times 4 \\times \\cdots \\times 2marshmallow.\n\\]\nThus the desired\nintegral is simply\n\\[\n\\sum_{marshmallow=0}^\\infty \\frac{1}{2^{marshmallow} marshmallow!} = \\sqrt{e}.\n\\]"
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "constantval",
        "n": "fixedindex"
      },
      "question": "\\begin{gather*}\n\\int_0^\\infty \\left(constantval-\\frac{constantval^3}{2}+\\frac{constantval^5}{2\\cdot\n4}-\\frac{constantval^7}{2\\cdot 4\\cdot 6}+\\cdots\\right) \\\\\n\\left(1+\\frac{constantval^2}{2^2}+\\frac{constantval^4}{2^2\\cdot 4^2}+\\frac{constantval^6}{2^2\\cdot 4^2 \\cdot 6^2}+\\cdots\\right)\\,dconstantval.\n\\end{gather*}",
      "solution": "Note that the series on the left is simply $constantval \\exp (-constantval^2/2)$.  By\nintegration by parts,\n\\[\n\\int_0^\\infty constantval^{2fixedindex+1} e^{-constantval^2/2} dconstantval =\n2fixedindex \\int_0^\\infty constantval^{2fixedindex-1} e^{-constantval^2/2} dconstantval\n\\]\nand so by induction,\n\\[\n\\int_0^\\infty constantval^{2fixedindex+1} e^{-constantval^2/2} dconstantval =\n2 \\times 4 \\times \\cdots \\times 2fixedindex.\n\\]\nThus the desired\nintegral is simply\n\\[\n\\sum_{fixedindex=0}^\\infty \\frac{1}{2^{fixedindex} fixedindex!} = \\sqrt{e}.\n\\]"
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "n": "hjgrksla"
      },
      "question": "Evaluate\n\\begin{gather*}\n\\int_0^\\infty \\left(qzxwvtnp-\\frac{qzxwvtnp^3}{2}+\\frac{qzxwvtnp^5}{2\\cdot\n4}-\\frac{qzxwvtnp^7}{2\\cdot 4\\cdot 6}+\\cdots\\right) \\\\\n\\left(1+\\frac{qzxwvtnp^2}{2^2}+\\frac{qzxwvtnp^4}{2^2\\cdot 4^2}+\\frac{qzxwvtnp^6}{2^2\\cdot 4^2 \\cdot 6^2}+\\cdots\\right)\\,dqzxwvtnp.\n\\end{gather*}",
      "solution": "Note that the series on the left is simply $qzxwvtnp \\exp (-qzxwvtnp^2/2)$.  By\nintegration by parts,\n\\[\n\\int_0^\\infty qzxwvtnp^{2hjgrksla+1} e^{-qzxwvtnp^2/2} dqzxwvtnp =\n2hjgrksla \\int_0^\\infty qzxwvtnp^{2hjgrksla-1} e^{-qzxwvtnp^2/2} dqzxwvtnp\n\\]\nand so by induction,\n\\[\n\\int_0^\\infty qzxwvtnp^{2hjgrksla+1} e^{-qzxwvtnp^2/2} dqzxwvtnp =\n2 \\times 4 \\times \\cdots \\times 2hjgrksla.\n\\]\nThus the desired\nintegral is simply\n\\[\n\\sum_{hjgrksla=0}^\\infty \\frac{1}{2^{hjgrksla} hjgrksla!} = \\sqrt{e}.\n\\]"
    },
    "kernel_variant": {
      "question": "Evaluate the double integral over the first quadrant  \n\\[\nI=\\iint_{0}^{\\infty}\n\\Bigl(x-\\frac{x^{3}}{4}+\\frac{x^{5}}{4\\cdot 8}-\\frac{x^{7}}{4\\cdot 8\\cdot 12}+\\cdots\\Bigr)\n\\Bigl(y-\\frac{y^{3}}{4}+\\frac{y^{5}}{4\\cdot 8}-\\frac{y^{7}}{4\\cdot 8\\cdot 12}+\\cdots\\Bigr)\n\\Bigl(1+\\frac{x^{2}+y^{2}}{4^{2}}+\\frac{(x^{2}+y^{2})^{2}}{(4\\!\\cdot\\!8)^{2}}+\\frac{(x^{2}+y^{2})^{3}}{(4\\!\\cdot\\!8\\!\\cdot\\!12)^{2}}+\\cdots\\Bigr)\\,dx\\,dy.\n\\]\n\n(The first two brackets alternate in sign; the coefficient of $x^{2n+1}$ or $y^{2n+1}$ is $(-1)^n/(4\\!\\cdot\\!8\\!\\cdots\\!4n)$, while in the third bracket the coefficient of $(x^{2}+y^{2})^{n}$ is $1/\\bigl[(4\\!\\cdot\\!8\\!\\cdots\\!4n)^{2}\\bigr]$.)",
      "solution": "Step 1.  Convert each series to a closed–form expression.  \n\nFor $n\\ge0$,  \n\\[\n4\\cdot 8\\cdots 4n = 4^{n}(1\\cdot2\\cdots n)=4^{n}n!.\n\\]\n\nHence  \n\\[\n\\begin{aligned}\nA(x)&:=x-\\frac{x^{3}}{4}+\\frac{x^{5}}{4\\cdot8}-\\frac{x^{7}}{4\\cdot8\\cdot12}+\\cdots\n     =\\sum_{n=0}^{\\infty}\\frac{(-1)^n\\,x^{2n+1}}{4^{n}n!}\n     =x\\,e^{-x^{2}/4},\\\\[4pt]\nB(y)&:=y-\\frac{y^{3}}{4}+\\frac{y^{5}}{4\\cdot8}-\\cdots\n     =y\\,e^{-y^{2}/4}.\n\\end{aligned}\n\\]\n\nFor the third bracket set $r=\\sqrt{x^{2}+y^{2}}$.  Using the same product,\n\\[\nC(r):=\\sum_{n=0}^{\\infty}\\frac{(x^{2}+y^{2})^{n}}{(4^{n}n!)^{2}}\n     =\\sum_{n=0}^{\\infty}\\frac{r^{2n}}{4^{2n}(n!)^{2}}\n     ={}_0F_{1}\\!\\bigl(;1;\\tfrac{r^{2}}{16}\\bigr)\n     =I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr),\n\\]\nbecause $_0F_{1}(\\,;1;z)=I_{0}(2\\sqrt z)$.\n\nThus the integrand becomes\n\\[\nA(x)B(y)C(r)=x\\,y\\,e^{-(x^{2}+y^{2})/4}\\,I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr).\n\\]\n\nStep 2.  Pass to polar coordinates in the first quadrant.  \nPut $x=r\\cos\\theta$, $y=r\\sin\\theta$ with $\\theta\\in(0,\\pi/2)$ and $r\\in(0,\\infty)$; then $dx\\,dy=r\\,dr\\,d\\theta$ and\n\\[\nx\\,y=r^{2}\\cos\\theta\\sin\\theta,\\qquad r=\\sqrt{x^{2}+y^{2}}.\n\\]\nHence\n\\[\nI=\\int_{0}^{\\pi/2}\\!\\!\\int_{0}^{\\infty}\nr^{2}\\cos\\theta\\sin\\theta\\,e^{-r^{2}/4}\\,I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr)\\,r\\,dr\\,d\\theta\n      =\\Bigl(\\int_{0}^{\\pi/2}\\cos\\theta\\sin\\theta\\,d\\theta\\Bigr)\n       \\int_{0}^{\\infty}r^{3}e^{-r^{2}/4}I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr)\\,dr.\n\\]\n\nBecause $\\int_{0}^{\\pi/2}\\cos\\theta\\sin\\theta\\,d\\theta=\\frac12$,\n\\[\nI=\\frac12\\int_{0}^{\\infty}r^{3}e^{-r^{2}/4}I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr)\\,dr.\n\\]\n\nStep 3.  Isolate a standard one–dimensional integral.  \nLet $r=2t$; then $dr=2\\,dt$ and\n\\[\nr^{3}e^{-r^{2}/4}I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr)\\,dr\n      =(2t)^{3}e^{-t^{2}}I_{0}(t)\\,(2\\,dt)=16\\,t^{3}e^{-t^{2}}I_{0}(t)\\,dt.\n\\]\nThus\n\\[\nI=8\\int_{0}^{\\infty}t^{3}e^{-t^{2}}I_{0}(t)\\,dt.\n\\]\n\nDenote\n\\[\nK=\\int_{0}^{\\infty}t^{3}e^{-t^{2}}I_{0}(t)\\,dt,\n\\qquad\\text{so that}\\qquad I=8K.\n\\]\n\nStep 4.  Evaluate $K$ through a parameter–differentiation trick.  \n\nA classical integral (proved by Laplace transform or differentiation under the integral sign) states\n\\[\nF(a,b)=\\int_{0}^{\\infty}t\\,e^{-b t^{2}}I_{0}(a t)\\,dt=\\frac{1}{2b}\\exp\\!\\Bigl(\\frac{a^{2}}{4b}\\Bigr)\n\\qquad(b>0).\n\\]\n\nDifferentiating w.r.t.\\ $b$ gives  \n\\[\n\\frac{\\partial F}{\\partial b}\n      =-\\int_{0}^{\\infty}t^{3}e^{-b t^{2}}I_{0}(a t)\\,dt\n      =-\\frac{e^{a^{2}/(4b)}}{2b^{2}}\\Bigl(1+\\frac{a^{2}}{4b}\\Bigr).\n\\]\n\nSet $a=1,\\;b=1$ to match $K$:\n\\[\nK=\\int_{0}^{\\infty}t^{3}e^{-t^{2}}I_{0}(t)\\,dt\n   =-\\Bigl.\\frac{\\partial F}{\\partial b}\\Bigr|_{a=1,b=1}\n   =e^{1/4}\\Bigl(\\frac12+\\frac18\\Bigr)=\\frac{5}{8}e^{1/4}.\n\\]\n\nStep 5.  Assemble the result.  \nRecall $I=8K$, therefore\n\\[\nI=8\\cdot\\frac{5}{8}e^{1/4}=5\\,e^{1/4}.\n\\]\n\nHence\n\\[\n\\boxed{\\,I=5e^{1/4}\\,}.\n\\]",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.751281",
        "was_fixed": false,
        "difficulty_analysis": "1.  Higher dimensions: the problem passes from a single integral to a two–dimensional integral that must be handled in polar coordinates.  \n2.  Multiple interacting series: three infinite series interact, two depending on different variables and the third on their quadratic combination.  \n3.  Sophisticated structures: the series sum to exponential functions and to the modified Bessel function $I_{0}$ through the confluent hypergeometric representation; recognising and manipulating these special functions is required.  \n4.  Deeper techniques: evaluation needs (i) conversion to Bessel form, (ii) a non-central Gaussian integral in polar coordinates, (iii) a special integral $\\int t e^{-bt^{2}}I_{0}(at)\\,dt$ and its parameter differentiation to generate the $t^{3}$ moment, and (iv) careful tracking of geometric factors.  \n5.  More steps: each transformation (series → closed form → polar coordinates → substitution → parameter differentiation) adds layers, making the route to the final constant $5e^{1/4}$ considerably longer and conceptually richer than in the original single–series, one–dimensional kernel problem."
      }
    },
    "original_kernel_variant": {
      "question": "Evaluate the double integral over the first quadrant  \n\\[\nI=\\iint_{0}^{\\infty}\n\\Bigl(x-\\frac{x^{3}}{4}+\\frac{x^{5}}{4\\cdot 8}-\\frac{x^{7}}{4\\cdot 8\\cdot 12}+\\cdots\\Bigr)\n\\Bigl(y-\\frac{y^{3}}{4}+\\frac{y^{5}}{4\\cdot 8}-\\frac{y^{7}}{4\\cdot 8\\cdot 12}+\\cdots\\Bigr)\n\\Bigl(1+\\frac{x^{2}+y^{2}}{4^{2}}+\\frac{(x^{2}+y^{2})^{2}}{(4\\!\\cdot\\!8)^{2}}+\\frac{(x^{2}+y^{2})^{3}}{(4\\!\\cdot\\!8\\!\\cdot\\!12)^{2}}+\\cdots\\Bigr)\\,dx\\,dy.\n\\]\n\n(The first two brackets alternate in sign; the coefficient of $x^{2n+1}$ or $y^{2n+1}$ is $(-1)^n/(4\\!\\cdot\\!8\\!\\cdots\\!4n)$, while in the third bracket the coefficient of $(x^{2}+y^{2})^{n}$ is $1/\\bigl[(4\\!\\cdot\\!8\\!\\cdots\\!4n)^{2}\\bigr]$.)",
      "solution": "Step 1.  Convert each series to a closed–form expression.  \n\nFor $n\\ge0$,  \n\\[\n4\\cdot 8\\cdots 4n = 4^{n}(1\\cdot2\\cdots n)=4^{n}n!.\n\\]\n\nHence  \n\\[\n\\begin{aligned}\nA(x)&:=x-\\frac{x^{3}}{4}+\\frac{x^{5}}{4\\cdot8}-\\frac{x^{7}}{4\\cdot8\\cdot12}+\\cdots\n     =\\sum_{n=0}^{\\infty}\\frac{(-1)^n\\,x^{2n+1}}{4^{n}n!}\n     =x\\,e^{-x^{2}/4},\\\\[4pt]\nB(y)&:=y-\\frac{y^{3}}{4}+\\frac{y^{5}}{4\\cdot8}-\\cdots\n     =y\\,e^{-y^{2}/4}.\n\\end{aligned}\n\\]\n\nFor the third bracket set $r=\\sqrt{x^{2}+y^{2}}$.  Using the same product,\n\\[\nC(r):=\\sum_{n=0}^{\\infty}\\frac{(x^{2}+y^{2})^{n}}{(4^{n}n!)^{2}}\n     =\\sum_{n=0}^{\\infty}\\frac{r^{2n}}{4^{2n}(n!)^{2}}\n     ={}_0F_{1}\\!\\bigl(;1;\\tfrac{r^{2}}{16}\\bigr)\n     =I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr),\n\\]\nbecause $_0F_{1}(\\,;1;z)=I_{0}(2\\sqrt z)$.\n\nThus the integrand becomes\n\\[\nA(x)B(y)C(r)=x\\,y\\,e^{-(x^{2}+y^{2})/4}\\,I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr).\n\\]\n\nStep 2.  Pass to polar coordinates in the first quadrant.  \nPut $x=r\\cos\\theta$, $y=r\\sin\\theta$ with $\\theta\\in(0,\\pi/2)$ and $r\\in(0,\\infty)$; then $dx\\,dy=r\\,dr\\,d\\theta$ and\n\\[\nx\\,y=r^{2}\\cos\\theta\\sin\\theta,\\qquad r=\\sqrt{x^{2}+y^{2}}.\n\\]\nHence\n\\[\nI=\\int_{0}^{\\pi/2}\\!\\!\\int_{0}^{\\infty}\nr^{2}\\cos\\theta\\sin\\theta\\,e^{-r^{2}/4}\\,I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr)\\,r\\,dr\\,d\\theta\n      =\\Bigl(\\int_{0}^{\\pi/2}\\cos\\theta\\sin\\theta\\,d\\theta\\Bigr)\n       \\int_{0}^{\\infty}r^{3}e^{-r^{2}/4}I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr)\\,dr.\n\\]\n\nBecause $\\int_{0}^{\\pi/2}\\cos\\theta\\sin\\theta\\,d\\theta=\\frac12$,\n\\[\nI=\\frac12\\int_{0}^{\\infty}r^{3}e^{-r^{2}/4}I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr)\\,dr.\n\\]\n\nStep 3.  Isolate a standard one–dimensional integral.  \nLet $r=2t$; then $dr=2\\,dt$ and\n\\[\nr^{3}e^{-r^{2}/4}I_{0}\\!\\Bigl(\\frac{r}{2}\\Bigr)\\,dr\n      =(2t)^{3}e^{-t^{2}}I_{0}(t)\\,(2\\,dt)=16\\,t^{3}e^{-t^{2}}I_{0}(t)\\,dt.\n\\]\nThus\n\\[\nI=8\\int_{0}^{\\infty}t^{3}e^{-t^{2}}I_{0}(t)\\,dt.\n\\]\n\nDenote\n\\[\nK=\\int_{0}^{\\infty}t^{3}e^{-t^{2}}I_{0}(t)\\,dt,\n\\qquad\\text{so that}\\qquad I=8K.\n\\]\n\nStep 4.  Evaluate $K$ through a parameter–differentiation trick.  \n\nA classical integral (proved by Laplace transform or differentiation under the integral sign) states\n\\[\nF(a,b)=\\int_{0}^{\\infty}t\\,e^{-b t^{2}}I_{0}(a t)\\,dt=\\frac{1}{2b}\\exp\\!\\Bigl(\\frac{a^{2}}{4b}\\Bigr)\n\\qquad(b>0).\n\\]\n\nDifferentiating w.r.t.\\ $b$ gives  \n\\[\n\\frac{\\partial F}{\\partial b}\n      =-\\int_{0}^{\\infty}t^{3}e^{-b t^{2}}I_{0}(a t)\\,dt\n      =-\\frac{e^{a^{2}/(4b)}}{2b^{2}}\\Bigl(1+\\frac{a^{2}}{4b}\\Bigr).\n\\]\n\nSet $a=1,\\;b=1$ to match $K$:\n\\[\nK=\\int_{0}^{\\infty}t^{3}e^{-t^{2}}I_{0}(t)\\,dt\n   =-\\Bigl.\\frac{\\partial F}{\\partial b}\\Bigr|_{a=1,b=1}\n   =e^{1/4}\\Bigl(\\frac12+\\frac18\\Bigr)=\\frac{5}{8}e^{1/4}.\n\\]\n\nStep 5.  Assemble the result.  \nRecall $I=8K$, therefore\n\\[\nI=8\\cdot\\frac{5}{8}e^{1/4}=5\\,e^{1/4}.\n\\]\n\nHence\n\\[\n\\boxed{\\,I=5e^{1/4}\\,}.\n\\]",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.579606",
        "was_fixed": false,
        "difficulty_analysis": "1.  Higher dimensions: the problem passes from a single integral to a two–dimensional integral that must be handled in polar coordinates.  \n2.  Multiple interacting series: three infinite series interact, two depending on different variables and the third on their quadratic combination.  \n3.  Sophisticated structures: the series sum to exponential functions and to the modified Bessel function $I_{0}$ through the confluent hypergeometric representation; recognising and manipulating these special functions is required.  \n4.  Deeper techniques: evaluation needs (i) conversion to Bessel form, (ii) a non-central Gaussian integral in polar coordinates, (iii) a special integral $\\int t e^{-bt^{2}}I_{0}(at)\\,dt$ and its parameter differentiation to generate the $t^{3}$ moment, and (iv) careful tracking of geometric factors.  \n5.  More steps: each transformation (series → closed form → polar coordinates → substitution → parameter differentiation) adds layers, making the route to the final constant $5e^{1/4}$ considerably longer and conceptually richer than in the original single–series, one–dimensional kernel problem."
      }
    }
  },
  "checked": true,
  "problem_type": "calculation"
}