{
  "index": "2010-A-6",
  "type": "ANA",
  "tag": [
    "ANA"
  ],
  "difficulty": "",
  "question": "Let $f:[0,\\infty)\\to \\mathbb{R}$ be a strictly decreasing continuous function\nsuch that $\\lim_{x\\to\\infty} f(x) = 0$. Prove that\n$\\int_0^\\infty \\frac{f(x)-f(x+1)}{f(x)}\\,dx$ diverges.",
  "solution": "\\textbf{First solution.}\nNote that the hypotheses on $f$ imply that $f(x) > 0$ for all $x \\in [0, +\\infty)$,\nso the integrand is a continuous function of $f$ and the integral makes sense. Rewrite the integral as\n\\[\n\\int_0^\\infty \\left(1 - \\frac{f(x+1)}{f(x)} \\right)\\,dx,\n\\]\nand suppose by way of contradiction that it converges to a finite limit $L$.\nFor $n \\geq 0$, define the Lebesgue measurable set\n\\[\nI_n = \\{x \\in [0,1]: 1 - \\frac{f(x+n+1)}{f(x+n)} \\leq 1/2 \\}.\n\\]\nThen $L \\geq \\sum_{n=0}^\\infty \\frac{1}{2} (1 - \\mu(I_n))$,\nso the latter sum converges.\nIn particular, there exists a nonnegative integer $N$ for which $\\sum_{n=N}^\\infty (1 - \\mu(I_n)) < 1$;\nthe intersection\n\\[\nI = \\bigcup_{n=N}^\\infty I_n = [0,1] - \\bigcap_{n=N}^\\infty ([0,1] - I_n)\n\\]\nthen has positive Lebesgue measure.\n\nBy Taylor's theorem with remainder, for $t \\in [0,1/2]$,\n\\begin{align*}\n-\\log (1-t) &\\leq t + \\frac{t^2}{2} \\sup_{t \\in [0,1/2]} \\left\\{\\frac{1}{(1-t)^2}\n\\right\\} \\\\\n&= t + 2 t^2 \\leq 2t.\n\\end{align*}\nFor each nonnegative integer $n \\geq N$, we then have\n\\begin{align*}\nL &\\geq \\int_N^{n} \\left(1 - \\frac{f(x+1)}{f(x)} \\right)\\,dx \\\\\n&= \\sum_{i=N}^{n-1} \\int_0^1 \\left( 1 - \\frac{f(x+i+1)}{f(x+i)}\\right)\\,dx \\\\\n&\\geq \\sum_{i=N}^{n-1} \\int_I \\left( 1 - \\frac{f(x+i+1)}{f(x+i)}\\right)\\,dx \\\\\n&\\geq \\frac{1}{2} \\sum_{i=N}^{n-1} \\int_I \\log \\frac{f(x+i)}{f(x+i+1)}\\,dx \\\\\n&= \\frac{1}{2} \\int_I \\left( \\sum_{i=N}^{n-1} \\log \\frac{f(x+i)}{f(x+i+1)}\\right) \\,dx \\\\\n&= \\frac{1}{2} \\int_I \\log \\frac{f(x+N)}{f(x+n)} \\,dx.\n\\end{align*}\nFor each $x \\in I$, $\\log f(x+N)/f(x+n)$ is a strictly increasing unbounded function of $n$.\nBy the monotone convergence theorem, the integral $\\int_I \\log (f(x+N)/f(x+n)) \\,dx$ grows without bound\nas $n \\to +\\infty$, a contradiction. Thus the original integral diverges, as desired.\n\n\\textbf{Remark.}\nThis solution is motivated by the commonly-used fact that an infinite product\n$(1 + x_1)(1 + x_2) \\cdots$ converges absolutely if and only if the sum\n$x_1 + x_2 + \\cdots$ converges absolutely. The additional measure-theoretic argument at the beginning is needed\nbecause one cannot bound $-\\log(1-t)$ by a fixed multiple of $t$ uniformly for all $t \\in [0,1)$.\n\nGreg Martin suggests a variant solution that avoids use of Lebesgue measure.\nNote first that if $f(y) > 2f(y+1)$, then either $f(y) > \\sqrt{2} f(y+1/2)$ or $f(y+1/2) > \\sqrt{2} f(y+1)$,\nand in either case we deduce that\n\\[\n\\int_{y-1/2}^{y+1/2} \\frac{f(x)-f(x+1)}{f(x)}\\,dx > \\frac{1}{2} \\left(1 - \\frac{1}{\\sqrt{2}} \\right)  > \\frac{1}{7}.\n\\]\nIf there exist arbitrarily large values of $y$ for which $f(y) > 2f(y+1)$, we deduce that\nthe original integral is greater than any multiple of $1/7$, and so diverges. Otherwise,\nfor $x$ large we may argue that\n\\[\n\\frac{f(x)-f(x+1)}{f(x)} > \\frac{3}{5} \\log \\frac{f(x)}{f(x+1)}\n\\]\nas in the above solution, and again get divergence using a telescoping sum.\n\n\\textbf{Second solution.}\n(Communicated by Paul Allen.)\nLet $b>a$ be nonnegative integers. Then\n\\begin{align*}\n\\int_a^b \\frac{f(x)-f(x+1)}{f(x)}dx &=\n\\sum_{k=a}^{b-1} \\int_0^1 \\frac{f(x+k)-f(x+k+1)}{f(x+k)}dx \\\\\n&= \\int_0^1 \\sum_{k=a}^{b-1} \\frac{f(x+k)-f(x+k+1)}{f(x+k)}dx \\\\\n&\\geq \\int_0^1 \\sum_{k=a}^{b-1} \\frac{f(x+k)-f(x+k+1)}{f(x+a)}dx \\\\\n&= \\int_0^1 \\frac{f(x+a)-f(x+b)}{f(x+a)} dx.\n\\end{align*}\nNow since $f(x)\\rightarrow 0$, given $a$, we can choose an integer $l(a)>a$ for which $f(l(a)) < f(a+1)/2$; then $\\frac{f(x+a)-f(x+l(a))}{f(x+a)} \\geq 1 - \\frac{f(l(a))}{f(a+1)} > 1/2$ for all $x\\in [0,1]$. Thus if we define a sequence of integers $a_n$ by $a_0=0$, $a_{n+1}=l(a_n)$, then\n\\begin{align*}\n\\int_0^\\infty \\frac{f(x)-f(x+1)}{f(x)} dx &=\n\\sum_{n=0}^\\infty \\int_{a_n}^{a_{n+1}} \\frac{f(x)-f(x+1)}{f(x)} dx \\\\\n&> \\sum_{n=0}^\\infty \\int_0^1 (1/2) dx,\n\\end{align*}\nand the final sum clearly diverges.\n\n\\textbf{Third solution.}\n(By Joshua Rosenberg, communicated by Catalin Zara.)\nIf the original integral converges, then\non one hand the integrand $(f(x)-f(x+1))/f(x) = 1 - f(x+1)/f(x)$\ncannot tend to 1 as $x \\to \\infty$.\nOn the other hand, for any $a \\geq 0$,\n\\begin{align*}\n0 &< \\frac{f(a+1)}{f(a)} \\\\\n&< \\frac{1}{f(a)} \\int_a^{a+1} f(x)\\,dx \\\\\n&= \\frac{1}{f(a)} \\int_a^\\infty (f(x) - f(x+1))\\,dx \\\\\n&\\leq \\int_a^\\infty \\frac{f(x) - f(x+1)}{f(x)}\\,dx,\n\\end{align*}\nand the last expression tends to 0 as $a \\to \\infty$.\nHence by the squeeze theorem, $f(a+1)/f(a) \\to 0$ as $a \\to \\infty$, a contradiction.",
  "vars": [
    "x"
  ],
  "params": [
    "f",
    "n",
    "L",
    "I_n",
    "i",
    "y",
    "a",
    "b",
    "k",
    "l",
    "N",
    "I",
    "t",
    "\\\\mu"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "position",
        "f": "decfunc",
        "n": "indexn",
        "L": "finlimit",
        "I_n": "setslice",
        "i": "indexi",
        "y": "pointy",
        "a": "starta",
        "b": "endpoint",
        "k": "indexk",
        "l": "indexl",
        "N": "boundn",
        "I": "setbig",
        "t": "smallt",
        "\\mu": "measure"
      },
      "question": "Let $decfunc:[0,\\infty)\\to \\mathbb{R}$ be a strictly decreasing continuous function such that $\\lim_{position\\to\\infty} decfunc(position) = 0$. Prove that $\\int_0^\\infty \\frac{decfunc(position)-decfunc(position+1)}{decfunc(position)}\\,d position$ diverges.",
      "solution": "\\textbf{First solution.}\nNote that the hypotheses on $decfunc$ imply that $decfunc(position) > 0$ for all $position \\in [0, +\\infty)$, so the integrand is a continuous function of $decfunc$ and the integral makes sense. Rewrite the integral as\n\\[\n\\int_0^\\infty \\left(1 - \\frac{decfunc(position+1)}{decfunc(position)} \\right)\\,d position,\n\\]\nand suppose by way of contradiction that it converges to a finite limit $finlimit$.\nFor $indexn \\geq 0$, define the Lebesgue measurable set\n\\[\nsetslice = \\{position \\in [0,1]: 1 - \\frac{decfunc(position+indexn+1)}{decfunc(position+indexn)} \\leq 1/2 \\}.\n\\]\nThen $finlimit \\geq \\sum_{indexn=0}^\\infty \\frac{1}{2} (1 - measure(setslice))$, so the latter sum converges.\nIn particular, there exists a nonnegative integer $boundn$ for which $\\sum_{indexn=boundn}^\\infty (1 - measure(setslice)) < 1$; the intersection\n\\[\nsetbig = \\bigcup_{indexn=boundn}^\\infty setslice = [0,1] - \\bigcap_{indexn=boundn}^\\infty ([0,1] - setslice)\n\\]\nthen has positive Lebesgue measure.\n\nBy Taylor's theorem with remainder, for $smallt \\in [0,1/2]$,\n\\begin{align*}\n-\\log (1-smallt) &\\leq smallt + \\frac{smallt^2}{2} \\sup_{smallt \\in [0,1/2]} \\left\\{\\frac{1}{(1-smallt)^2}\n\\right\\} \\\\\n&= smallt + 2 smallt^2 \\leq 2 smallt.\n\\end{align*}\nFor each nonnegative integer $indexn \\geq boundn$, we then have\n\\begin{align*}\nfinlimit &\\geq \\int_{boundn}^{indexn} \\left(1 - \\frac{decfunc(position+1)}{decfunc(position)} \\right)\\,d position \\\\\n&= \\sum_{indexi=boundn}^{indexn-1} \\int_0^1 \\left( 1 - \\frac{decfunc(position+indexi+1)}{decfunc(position+indexi)}\\right)\\,d position \\\\\n&\\geq \\sum_{indexi=boundn}^{indexn-1} \\int_{setbig} \\left( 1 - \\frac{decfunc(position+indexi+1)}{decfunc(position+indexi)}\\right)\\,d position \\\\\n&\\geq \\frac{1}{2} \\sum_{indexi=boundn}^{indexn-1} \\int_{setbig} \\log \\frac{decfunc(position+indexi)}{decfunc(position+indexi+1)}\\,d position \\\\\n&= \\frac{1}{2} \\int_{setbig} \\left( \\sum_{indexi=boundn}^{indexn-1} \\log \\frac{decfunc(position+indexi)}{decfunc(position+indexi+1)}\\right) \\,d position \\\\\n&= \\frac{1}{2} \\int_{setbig} \\log \\frac{decfunc(position+boundn)}{decfunc(position+indexn)} \\,d position.\n\\end{align*}\nFor each $position \\in setbig$, $\\log \\decfunc(position+boundn)/decfunc(position+indexn)$ is a strictly increasing unbounded function of $indexn$.\nBy the monotone convergence theorem, the integral $\\int_{setbig} \\log (decfunc(position+boundn)/decfunc(position+indexn)) \\,d position$ grows without bound as $indexn \\to +\\infty$, a contradiction. Thus the original integral diverges, as desired.\n\n\\textbf{Remark.}\nThis solution is motivated by the commonly-used fact that an infinite product $(1 + position_1)(1 + position_2) \\cdots$ converges absolutely if and only if the sum position_1 + position_2 + \\cdots converges absolutely. The additional measure-theoretic argument at the beginning is needed because one cannot bound $-\\log(1-smallt)$ by a fixed multiple of $smallt$ uniformly for all $smallt \\in [0,1)$.\n\nGreg Martin suggests a variant solution that avoids use of Lebesgue measure. Note first that if $decfunc(pointy) > 2decfunc(pointy+1)$, then either $decfunc(pointy) > \\sqrt{2} decfunc(pointy+1/2)$ or $decfunc(pointy+1/2) > \\sqrt{2} decfunc(pointy+1)$, and in either case we deduce that\n\\[\n\\int_{pointy-1/2}^{pointy+1/2} \\frac{decfunc(position)-decfunc(position+1)}{decfunc(position)}\\,d position > \\frac{1}{2} \\left(1 - \\frac{1}{\\sqrt{2}} \\right)  > \\frac{1}{7}.\n\\]\nIf there exist arbitrarily large values of $pointy$ for which $decfunc(pointy) > 2decfunc(pointy+1)$, we deduce that the original integral is greater than any multiple of $1/7$, and so diverges. Otherwise, for $position$ large we may argue that\n\\[\n\\frac{decfunc(position)-decfunc(position+1)}{decfunc(position)} > \\frac{3}{5} \\log \\frac{decfunc(position)}{decfunc(position+1)}\n\\]\nas in the above solution, and again get divergence using a telescoping sum.\n\n\\textbf{Second solution.}\n(Communicated by Paul Allen.)\nLet $endpoint>starta$ be nonnegative integers. Then\n\\begin{align*}\n\\int_{starta}^{endpoint} \\frac{decfunc(position)-decfunc(position+1)}{decfunc(position)}d position &=\n\\sum_{indexk=starta}^{endpoint-1} \\int_0^1 \\frac{decfunc(position+indexk)-decfunc(position+indexk+1)}{decfunc(position+indexk)}d position \\\\\n&= \\int_0^1 \\sum_{indexk=starta}^{endpoint-1} \\frac{decfunc(position+indexk)-decfunc(position+indexk+1)}{decfunc(position+indexk)}d position \\\\\n&\\geq \\int_0^1 \\sum_{indexk=starta}^{endpoint-1} \\frac{decfunc(position+indexk)-decfunc(position+indexk+1)}{decfunc(position+starta)}d position \\\\\n&= \\int_0^1 \\frac{decfunc(position+starta)-decfunc(position+endpoint)}{decfunc(position+starta)} d position.\n\\end{align*}\nNow since $decfunc(position)\\rightarrow 0$, given $starta$, we can choose an integer $indexl(starta)>starta$ for which $decfunc(indexl(starta)) < decfunc(starta+1)/2$; then $\\frac{decfunc(position+starta)-decfunc(position+indexl(starta))}{decfunc(position+starta)} \\geq 1 - \\frac{decfunc(indexl(starta))}{decfunc(starta+1)} > 1/2$ for all $position\\in [0,1]$. Thus if we define a sequence of integers $starta_{indexn}$ by $starta_0=0$, $starta_{indexn+1}=indexl(starta_{indexn})$, then\n\\begin{align*}\n\\int_0^\\infty \\frac{decfunc(position)-decfunc(position+1)}{decfunc(position)} d position &=\n\\sum_{indexn=0}^\\infty \\int_{starta_{indexn}}^{starta_{indexn+1}} \\frac{decfunc(position)-decfunc(position+1)}{decfunc(position)} d position \\\\\n&> \\sum_{indexn=0}^\\infty \\int_0^1 (1/2) d position,\n\\end{align*}\nand the final sum clearly diverges.\n\n\\textbf{Third solution.}\n(By Joshua Rosenberg, communicated by Catalin Zara.)\nIf the original integral converges, then on one hand the integrand $(decfunc(position)-decfunc(position+1))/decfunc(position) = 1 - decfunc(position+1)/decfunc(position)$ cannot tend to 1 as $position \\to \\infty$.\nOn the other hand, for any $starta \\geq 0$,\n\\begin{align*}\n0 &< \\frac{decfunc(starta+1)}{decfunc(starta)} \\\\\n&< \\frac{1}{decfunc(starta)} \\int_{starta}^{starta+1} decfunc(position)\\,d position \\\\\n&= \\frac{1}{decfunc(starta)} \\int_{starta}^\\infty (decfunc(position) - decfunc(position+1))\\,d position \\\\\n&\\leq \\int_{starta}^\\infty \\frac{decfunc(position) - decfunc(position+1)}{decfunc(position)}\\,d position,\n\\end{align*}\nand the last expression tends to 0 as $starta \\to \\infty$.\nHence by the squeeze theorem, $decfunc(starta+1)/decfunc(starta) \\to 0$ as $starta \\to \\infty$, a contradiction."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "gingerale",
        "f": "cinnamon",
        "n": "marshmall",
        "L": "butterscotch",
        "I_n": "crocodile",
        "i": "turnpike",
        "y": "waterside",
        "a": "blackbird",
        "b": "timeshare",
        "k": "paperclip",
        "l": "peppermint",
        "N": "shipwreck",
        "I": "rockslide",
        "t": "windchime",
        "\\\\mu": "sunflower"
      },
      "question": "Let $cinnamon:[0,\\infty)\\to \\mathbb{R}$ be a strictly decreasing continuous function\nsuch that $\\lim_{gingerale\\to\\infty} cinnamon(gingerale) = 0$. Prove that\n$\\int_0^\\infty \\frac{cinnamon(gingerale)-cinnamon(gingerale+1)}{cinnamon(gingerale)}\\,d gingerale$ diverges.",
      "solution": "\\textbf{First solution.}\nNote that the hypotheses on $cinnamon$ imply that $cinnamon(gingerale) > 0$ for all $gingerale \\in [0, +\\infty)$,\nso the integrand is a continuous function of $cinnamon$ and the integral makes sense. Rewrite the integral as\n\\[\n\\int_0^\\infty \\left(1 - \\frac{cinnamon(gingerale+1)}{cinnamon(gingerale)} \\right)\\,d gingerale,\n\\]\nand suppose by way of contradiction that it converges to a finite limit $butterscotch$.\nFor $marshmall \\geq 0$, define the Lebesgue measurable set\n\\[\ncrocodile = \\{gingerale \\in [0,1]: 1 - \\frac{cinnamon(gingerale+marshmall+1)}{cinnamon(gingerale+marshmall)} \\leq 1/2 \\}.\n\\]\nThen $butterscotch \\geq \\sum_{marshmall=0}^\\infty \\frac{1}{2} (1 - sunflower(crocodile))$,\nso the latter sum converges.\nIn particular, there exists a nonnegative integer $shipwreck$ for which $\\sum_{marshmall=shipwreck}^\\infty (1 - sunflower(crocodile)) < 1$;\nthe intersection\n\\[\nrockslide = \\bigcup_{marshmall=shipwreck}^\\infty crocodile = [0,1] - \\bigcap_{marshmall=shipwreck}^\\infty ([0,1] - crocodile)\n\\]\nthen has positive Lebesgue measure.\n\nBy Taylor's theorem with remainder, for $windchime \\in [0,1/2]$,\n\\begin{align*}\n-\\log (1-windchime) &\\leq windchime + \\frac{windchime^2}{2} \\sup_{windchime\\in [0,1/2]} \\left\\{\\frac{1}{(1-windchime)^2}\\right\\} \\\\\n&= windchime + 2 windchime^2 \\leq 2windchime.\n\\end{align*}\nFor each nonnegative integer $marshmall \\geq shipwreck$, we then have\n\\begin{align*}\nbutterscotch &\\geq \\int_{shipwreck}^{marshmall} \\left(1 - \\frac{cinnamon(gingerale+1)}{cinnamon(gingerale)} \\right)\\,d gingerale \\\\\n&= \\sum_{turnpike=shipwreck}^{marshmall-1} \\int_0^1 \\left( 1 - \\frac{cinnamon(gingerale+turnpike+1)}{cinnamon(gingerale+turnpike)}\\right)\\,d gingerale \\\\\n&\\geq \\sum_{turnpike=shipwreck}^{marshmall-1} \\int_{rockslide} \\left( 1 - \\frac{cinnamon(gingerale+turnpike+1)}{cinnamon(gingerale+turnpike)}\\right)\\,d gingerale \\\\\n&\\geq \\frac{1}{2} \\sum_{turnpike=shipwreck}^{marshmall-1} \\int_{rockslide} \\log \\frac{cinnamon(gingerale+turnpike)}{cinnamon(gingerale+turnpike+1)}\\,d gingerale \\\\\n&= \\frac{1}{2} \\int_{rockslide} \\left( \\sum_{turnpike=shipwreck}^{marshmall-1} \\log \\frac{cinnamon(gingerale+turnpike)}{cinnamon(gingerale+turnpike+1)}\\right) \\,d gingerale \\\\\n&= \\frac{1}{2} \\int_{rockslide} \\log \\frac{cinnamon(gingerale+shipwreck)}{cinnamon(gingerale+marshmall)} \\,d gingerale.\n\\end{align*}\nFor each $gingerale \\in rockslide$, $\\log cinnamon(gingerale+shipwreck)/cinnamon(gingerale+marshmall)$ is a strictly increasing unbounded function of $marshmall$.\nBy the monotone convergence theorem, the integral $\\int_{rockslide} \\log (cinnamon(gingerale+shipwreck)/cinnamon(gingerale+marshmall)) \\,d gingerale$ grows without bound\nas $marshmall \\to +\\infty$, a contradiction. Thus the original integral diverges, as desired.\n\n\\textbf{Remark.}\nThis solution is motivated by the commonly-used fact that an infinite product\n$(1 + x_1)(1 + x_2) \\cdots$ converges absolutely if and only if the sum\n$x_1 + x_2 + \\cdots$ converges absolutely. The additional measure-theoretic argument at the beginning is needed\nbecause one cannot bound $-\\log(1-windchime)$ by a fixed multiple of $windchime$ uniformly for all $windchime \\in [0,1)$.\n\nGreg Martin suggests a variant solution that avoids use of Lebesgue measure.\nNote first that if $cinnamon(waterside) > 2cinnamon(waterside+1)$, then either $cinnamon(waterside) > \\sqrt{2} \\,cinnamon(waterside+1/2)$ or $cinnamon(waterside+1/2) > \\sqrt{2}\\,cinnamon(waterside+1)$,\nand in either case we deduce that\n\\[\n\\int_{waterside-1/2}^{waterside+1/2} \\frac{cinnamon(gingerale)-cinnamon(gingerale+1)}{cinnamon(gingerale)}\\,d gingerale > \\frac{1}{2} \\left(1 - \\frac{1}{\\sqrt{2}} \\right)  > \\frac{1}{7}.\n\\]\nIf there exist arbitrarily large values of $waterside$ for which $cinnamon(waterside) > 2cinnamon(waterside+1)$, we deduce that\nthe original integral is greater than any multiple of $1/7$, and so diverges. Otherwise,\nfor $gingerale$ large we may argue that\n\\[\n\\frac{cinnamon(gingerale)-cinnamon(gingerale+1)}{cinnamon(gingerale)} > \\frac{3}{5} \\log \\frac{cinnamon(gingerale)}{cinnamon(gingerale+1)}\n\\]\nas in the above solution, and again get divergence using a telescoping sum.\n\n\\textbf{Second solution.}\n(Communicated by Paul Allen.)\nLet $timeshare>blackbird$ be nonnegative integers. Then\n\\begin{align*}\n\\int_{blackbird}^{timeshare} \\frac{cinnamon(gingerale)-cinnamon(gingerale+1)}{cinnamon(gingerale)}d gingerale &=\n\\sum_{paperclip=blackbird}^{timeshare-1} \\int_0^1 \\frac{cinnamon(gingerale+paperclip)-cinnamon(gingerale+paperclip+1)}{cinnamon(gingerale+paperclip)}d gingerale \\\\\n&= \\int_0^1 \\sum_{paperclip=blackbird}^{timeshare-1} \\frac{cinnamon(gingerale+paperclip)-cinnamon(gingerale+paperclip+1)}{cinnamon(gingerale+paperclip)}d gingerale \\\\\n&\\geq \\int_0^1 \\sum_{paperclip=blackbird}^{timeshare-1} \\frac{cinnamon(gingerale+paperclip)-cinnamon(gingerale+paperclip+1)}{cinnamon(gingerale+blackbird)}d gingerale \\\\\n&= \\int_0^1 \\frac{cinnamon(gingerale+blackbird)-cinnamon(gingerale+timeshare)}{cinnamon(gingerale+blackbird)} d gingerale.\n\\end{align*}\nNow since $cinnamon(gingerale)\\rightarrow 0$, given $blackbird$, we can choose an integer $peppermint(blackbird)>blackbird$ for which $cinnamon(peppermint(blackbird)) < cinnamon(blackbird+1)/2$; then $\\frac{cinnamon(gingerale+blackbird)-cinnamon(gingerale+peppermint(blackbird))}{cinnamon(gingerale+blackbird)} \\geq 1 - \\frac{cinnamon(peppermint(blackbird))}{cinnamon(blackbird+1)} > 1/2$ for all $gingerale\\in [0,1]$. Thus if we define a sequence of integers $blackbird_{marshmall}$ by $blackbird_0=0$, $blackbird_{marshmall+1}=peppermint(blackbird_{marshmall})$, then\n\\begin{align*}\n\\int_0^\\infty \\frac{cinnamon(gingerale)-cinnamon(gingerale+1)}{cinnamon(gingerale)} d gingerale &=\n\\sum_{marshmall=0}^\\infty \\int_{blackbird_{marshmall}}^{blackbird_{marshmall+1}} \\frac{cinnamon(gingerale)-cinnamon(gingerale+1)}{cinnamon(gingerale)} d gingerale \\\\\n&> \\sum_{marshmall=0}^\\infty \\int_0^1 (1/2) d gingerale,\n\\end{align*}\nand the final sum clearly diverges.\n\n\\textbf{Third solution.}\n(By Joshua Rosenberg, communicated by Catalin Zara.)\nIf the original integral converges, then\non one hand the integrand $(cinnamon(gingerale)-cinnamon(gingerale+1))/cinnamon(gingerale) = 1 - cinnamon(gingerale+1)/cinnamon(gingerale)$\ncannot tend to 1 as $gingerale \\to \\infty$.\nOn the other hand, for any $blackbird \\geq 0$,\n\\begin{align*}\n0 &< \\frac{cinnamon(blackbird+1)}{cinnamon(blackbird)} \\\\\n&< \\frac{1}{cinnamon(blackbird)} \\int_{blackbird}^{blackbird+1} cinnamon(gingerale)\\,d gingerale \\\\\n&= \\frac{1}{cinnamon(blackbird)} \\int_{blackbird}^\\infty (cinnamon(gingerale) - cinnamon(gingerale+1))\\,d gingerale \\\\\n&\\leq \\int_{blackbird}^\\infty \\frac{cinnamon(gingerale) - cinnamon(gingerale+1)}{cinnamon(gingerale)}\\,d gingerale,\n\\end{align*}\nand the last expression tends to 0 as $blackbird \\to \\infty$.\nHence by the squeeze theorem, $cinnamon(blackbird+1)/cinnamon(blackbird) \\to 0$ as $blackbird \\to \\infty$, a contradiction."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "knownvalue",
        "f": "malfunction",
        "n": "continuousvar",
        "L": "unbounded",
        "I_n": "fullrange",
        "i": "bigindex",
        "y": "fixedpoint",
        "a": "ceilingval",
        "b": "groundval",
        "k": "outerloop",
        "l": "shrinker",
        "N": "minimalset",
        "I": "emptyset",
        "t": "massivevar",
        "\\\\mu": "nullmass"
      },
      "question": "Let $malfunction:[0,\\infty)\\to \\mathbb{R}$ be a strictly decreasing continuous function such that $\\lim_{knownvalue\\to\\infty} malfunction(knownvalue) = 0$. Prove that\n$\\int_0^\\infty \\frac{malfunction(knownvalue)-malfunction(knownvalue+1)}{malfunction(knownvalue)}\\,dknownvalue$ diverges.",
      "solution": "\\textbf{First solution.}\nNote that the hypotheses on $malfunction$ imply that $malfunction(knownvalue) > 0$ for all $knownvalue \\in [0, +\\infty)$, so the integrand is a continuous function of $malfunction$ and the integral makes sense. Rewrite the integral as\n\\[\n\\int_0^\\infty \\left(1 - \\frac{malfunction(knownvalue+1)}{malfunction(knownvalue)} \\right)\\,dknownvalue,\n\\]\nand suppose by way of contradiction that it converges to a finite limit $unbounded$.\nFor $continuousvar \\geq 0$, define the Lebesgue measurable set\n\\[\nfullrange = \\{knownvalue \\in [0,1]: 1 - \\frac{malfunction(knownvalue+continuousvar+1)}{malfunction(knownvalue+continuousvar)} \\leq 1/2 \\}.\n\\]\nThen $unbounded \\geq \\sum_{continuousvar=0}^\\infty \\frac{1}{2} (1 - nullmass(fullrange))$, so the latter sum converges.\nIn particular, there exists a nonnegative integer $minimalset$ for which $\\sum_{continuousvar=minimalset}^\\infty (1 - nullmass(fullrange)) < 1$; the intersection\n\\[\nemptyset = \\bigcup_{continuousvar=minimalset}^\\infty fullrange = [0,1] - \\bigcap_{continuousvar=minimalset}^\\infty ([0,1] - fullrange)\n\\]\nthen has positive Lebesgue measure.\n\nBy Taylor's theorem with remainder, for $massivevar \\in [0,1/2]$,\n\\begin{align*}\n-\\log (1-massivevar) &\\leq massivevar + \\frac{massivevar^2}{2} \\sup_{massivevar \\in [0,1/2]} \\left\\{\\frac{1}{(1-massivevar)^2}\\right\\} \\\\\n&= massivevar + 2 massivevar^2 \\leq 2massivevar.\n\\end{align*}\nFor each nonnegative integer $continuousvar \\geq minimalset$, we then have\n\\begin{align*}\nunbounded &\\geq \\int_{minimalset}^{continuousvar} \\left(1 - \\frac{malfunction(knownvalue+1)}{malfunction(knownvalue)} \\right)\\,dknownvalue \\\\\n&= \\sum_{bigindex=minimalset}^{continuousvar-1} \\int_0^1 \\left( 1 - \\frac{malfunction(knownvalue+bigindex+1)}{malfunction(knownvalue+bigindex)}\\right)\\,dknownvalue \\\\\n&\\geq \\sum_{bigindex=minimalset}^{continuousvar-1} \\int_{emptyset} \\left( 1 - \\frac{malfunction(knownvalue+bigindex+1)}{malfunction(knownvalue+bigindex)}\\right)\\,dknownvalue \\\\\n&\\geq \\frac{1}{2} \\sum_{bigindex=minimalset}^{continuousvar-1} \\int_{emptyset} \\log \\frac{malfunction(knownvalue+bigindex)}{malfunction(knownvalue+bigindex+1)}\\,dknownvalue \\\\\n&= \\frac{1}{2} \\int_{emptyset} \\left( \\sum_{bigindex=minimalset}^{continuousvar-1} \\log \\frac{malfunction(knownvalue+bigindex)}{malfunction(knownvalue+bigindex+1)}\\right) \\,dknownvalue \\\\\n&= \\frac{1}{2} \\int_{emptyset} \\log \\frac{malfunction(knownvalue+minimalset)}{malfunction(knownvalue+continuousvar)} \\,dknownvalue.\n\\end{align*}\nFor each $knownvalue \\in emptyset$, $\\log \\frac{malfunction(knownvalue+minimalset)}{malfunction(knownvalue+continuousvar)}$ is a strictly increasing unbounded function of $continuousvar$. By the monotone convergence theorem, the integral $\\int_{emptyset} \\log \\frac{malfunction(knownvalue+minimalset)}{malfunction(knownvalue+continuousvar)} \\,dknownvalue$ grows without bound as $continuousvar \\to +\\infty$, a contradiction. Thus the original integral diverges, as desired.\n\n\\textbf{Remark.}\nThis solution is motivated by the commonly-used fact that an infinite product $(1 + knownvalue_1)(1 + knownvalue_2) \\cdots$ converges absolutely if and only if the sum $knownvalue_1 + knownvalue_2 + \\cdots$ converges absolutely. The additional measure-theoretic argument at the beginning is needed because one cannot bound $-\\log(1-massivevar)$ by a fixed multiple of $massivevar$ uniformly for all $massivevar \\in [0,1)$.\n\nGreg Martin suggests a variant solution that avoids use of Lebesgue measure. Note first that if $malfunction(fixedpoint) > 2malfunction(fixedpoint+1)$, then either $malfunction(fixedpoint) > \\sqrt{2}\\,malfunction(fixedpoint+1/2)$ or $malfunction(fixedpoint+1/2) > \\sqrt{2}\\,malfunction(fixedpoint+1)$, and in either case we deduce that\n\\[\n\\int_{fixedpoint-1/2}^{fixedpoint+1/2} \\frac{malfunction(knownvalue)-malfunction(knownvalue+1)}{malfunction(knownvalue)}\\,dknownvalue > \\frac{1}{2} \\left(1 - \\frac{1}{\\sqrt{2}} \\right)  > \\frac{1}{7}.\n\\]\nIf there exist arbitrarily large values of $fixedpoint$ for which $malfunction(fixedpoint) > 2malfunction(fixedpoint+1)$, we deduce that the original integral is greater than any multiple of $1/7$, and so diverges. Otherwise, for $knownvalue$ large we may argue that\n\\[\n\\frac{malfunction(knownvalue)-malfunction(knownvalue+1)}{malfunction(knownvalue)} > \\frac{3}{5} \\log \\frac{malfunction(knownvalue)}{malfunction(knownvalue+1)}\n\\]\nas in the above solution, and again get divergence using a telescoping sum.\n\n\\textbf{Second solution.}\n(Communicated by Paul Allen.)\nLet $groundval>ceilingval$ be nonnegative integers. Then\n\\begin{align*}\n\\int_{ceilingval}^{groundval} \\frac{malfunction(knownvalue)-malfunction(knownvalue+1)}{malfunction(knownvalue)}dknownvalue &=\n\\sum_{outerloop=ceilingval}^{groundval-1} \\int_0^1 \\frac{malfunction(knownvalue+outerloop)-malfunction(knownvalue+outerloop+1)}{malfunction(knownvalue+outerloop)}dknownvalue \\\\\n&= \\int_0^1 \\sum_{outerloop=ceilingval}^{groundval-1} \\frac{malfunction(knownvalue+outerloop)-malfunction(knownvalue+outerloop+1)}{malfunction(knownvalue+outerloop)}dknownvalue \\\\\n&\\geq \\int_0^1 \\sum_{outerloop=ceilingval}^{groundval-1} \\frac{malfunction(knownvalue+outerloop)-malfunction(knownvalue+outerloop+1)}{malfunction(knownvalue+ceilingval)}dknownvalue \\\\\n&= \\int_0^1 \\frac{malfunction(knownvalue+ceilingval)-malfunction(knownvalue+groundval)}{malfunction(knownvalue+ceilingval)} dknownvalue.\n\\end{align*}\nNow since $malfunction(knownvalue)\\rightarrow 0$, given $ceilingval$, we can choose an integer $shrinker(ceilingval)>ceilingval$ for which $malfunction(shrinker(ceilingval)) < malfunction(ceilingval+1)/2$; then $\\frac{malfunction(knownvalue+ceilingval)-malfunction(knownvalue+shrinker(ceilingval))}{malfunction(knownvalue+ceilingval)} \\geq 1 - \\frac{malfunction(shrinker(ceilingval))}{malfunction(ceilingval+1)} > 1/2$ for all $knownvalue\\in [0,1]$. Thus if we define a sequence of integers $ceilingval_{continuousvar}$ by $ceilingval_0=0$, $ceilingval_{continuousvar+1}=shrinker(ceilingval_{continuousvar})$, then\n\\begin{align*}\n\\int_0^\\infty \\frac{malfunction(knownvalue)-malfunction(knownvalue+1)}{malfunction(knownvalue)} dknownvalue &=\n\\sum_{continuousvar=0}^\\infty \\int_{ceilingval_{continuousvar}}^{ceilingval_{continuousvar+1}} \\frac{malfunction(knownvalue)-malfunction(knownvalue+1)}{malfunction(knownvalue)} dknownvalue \\\\\n&> \\sum_{continuousvar=0}^\\infty \\int_0^1 (1/2) dknownvalue,\n\\end{align*}\nand the final sum clearly diverges.\n\n\\textbf{Third solution.}\n(By Joshua Rosenberg, communicated by Catalin Zara.)\nIf the original integral converges, then on one hand the integrand $(malfunction(knownvalue)-malfunction(knownvalue+1))/malfunction(knownvalue) = 1 - malfunction(knownvalue+1)/malfunction(knownvalue)$ cannot tend to $1$ as $knownvalue \\to \\infty$.\nOn the other hand, for any $ceilingval \\geq 0$,\\begin{align*}\n0 &< \\frac{malfunction(ceilingval+1)}{malfunction(ceilingval)} \\\\\n&< \\frac{1}{malfunction(ceilingval)} \\int_{ceilingval}^{ceilingval+1} malfunction(knownvalue)\\,dknownvalue \\\\\n&= \\frac{1}{malfunction(ceilingval)} \\int_{ceilingval}^\\infty (malfunction(knownvalue) - malfunction(knownvalue+1))\\,dknownvalue \\\\\n&\\leq \\int_{ceilingval}^\\infty \\frac{malfunction(knownvalue) - malfunction(knownvalue+1)}{malfunction(knownvalue)}\\,dknownvalue,\n\\end{align*}\nand the last expression tends to $0$ as $ceilingval \\to \\infty$. Hence by the squeeze theorem, $malfunction(ceilingval+1)/malfunction(ceilingval) \\to 0$ as $ceilingval \\to \\infty$, a contradiction."
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "f": "hjgrksla",
        "n": "ptzmskch",
        "L": "mbvcxzlk",
        "I_n": "tyghbnjm",
        "i": "asdfghjk",
        "y": "plmoknij",
        "a": "ujmnhytr",
        "b": "yhnujmik",
        "k": "ikmjunhy",
        "l": "okmijnuh",
        "N": "bnvctrpl",
        "I": "edcvfrtg",
        "t": "rfvgtbhy",
        "\\mu": "qazxswed"
      },
      "question": "Let $hjgrksla:[0,\\infty)\\to \\mathbb{R}$ be a strictly decreasing continuous function\nsuch that $\\lim_{qzxwvtnp\\to\\infty} hjgrksla(qzxwvtnp) = 0$. Prove that\n$\\int_0^\\infty \\frac{hjgrksla(qzxwvtnp)-hjgrksla(qzxwvtnp+1)}{hjgrksla(qzxwvtnp)}\\,dqzxwvtnp$ diverges.",
      "solution": "\\textbf{First solution.}\nNote that the hypotheses on $hjgrksla$ imply that $hjgrksla(qzxwvtnp) > 0$ for all $qzxwvtnp \\in [0, +\\infty)$,\nso the integrand is a continuous function of $hjgrksla$ and the integral makes sense. Rewrite the integral as\n\\[\n\\int_0^\\infty \\left(1 - \\frac{hjgrksla(qzxwvtnp+1)}{hjgrksla(qzxwvtnp)} \\right)\\,dqzxwvtnp,\n\\]\nand suppose by way of contradiction that it converges to a finite limit $mbvcxzlk$.\nFor $ptzmskch \\ge 0$, define the Lebesgue measurable set\n\\[\ntyghbnjm = \\{qzxwvtnp \\in [0,1]: 1 - \\frac{hjgrksla(qzxwvtnp+ptzmskch+1)}{hjgrksla(qzxwvtnp+ptzmskch)} \\le 1/2 \\}.\n\\]\nThen $mbvcxzlk \\ge \\sum_{ptzmskch=0}^\\infty \\frac{1}{2} (1 - \\qazxswed(tyghbnjm))$,\nso the latter sum converges.\nIn particular, there exists a nonnegative integer $bnvctrpl$ for which $\\sum_{ptzmskch=bnvctrpl}^\\infty (1 - \\qazxswed(tyghbnjm)) < 1$;\nthe intersection\n\\[\nedcvfrtg = \\bigcup_{ptzmskch=bnvctrpl}^\\infty tyghbnjm = [0,1] - \\bigcap_{ptzmskch=bnvctrpl}^\\infty ([0,1] - tyghbnjm)\n\\]\nthen has positive Lebesgue measure.\n\nBy Taylor's theorem with remainder, for $rfvgtbhy \\in [0,1/2]$,\n\\begin{align*}\n-\\log (1-rfvgtbhy) &\\le rfvgtbhy + \\frac{rfvgtbhy^2}{2} \\sup_{rfvgtbhy \\in [0,1/2]} \\left\\{\\frac{1}{(1-rfvgtbhy)^2}\n\\right\\} \\\\\n&= rfvgtbhy + 2 rfvgtbhy^2 \\le 2rfvgtbhy.\n\\end{align*}\nFor each nonnegative integer $ptzmskch \\ge bnvctrpl$, we then have\n\\begin{align*}\nmbvcxzlk &\\ge \\int_{bnvctrpl}^{ptzmskch} \\left(1 - \\frac{hjgrksla(qzxwvtnp+1)}{hjgrksla(qzxwvtnp)} \\right)\\,dqzxwvtnp \\\\\n&= \\sum_{asdfghjk=bnvctrpl}^{ptzmskch-1} \\int_0^1 \\left( 1 - \\frac{hjgrksla(qzxwvtnp+asdfghjk+1)}{hjgrksla(qzxwvtnp+asdfghjk)}\\right)\\,dqzxwvtnp \\\\\n&\\ge \\sum_{asdfghjk=bnvctrpl}^{ptzmskch-1} \\int_{edcvfrtg} \\left( 1 - \\frac{hjgrksla(qzxwvtnp+asdfghjk+1)}{hjgrksla(qzxwvtnp+asdfghjk)}\\right)\\,dqzxwvtnp \\\\\n&\\ge \\frac{1}{2} \\sum_{asdfghjk=bnvctrpl}^{ptzmskch-1} \\int_{edcvfrtg} \\log \\frac{hjgrksla(qzxwvtnp+asdfghjk)}{hjgrksla(qzxwvtnp+asdfghjk+1)}\\,dqzxwvtnp \\\\\n&= \\frac{1}{2} \\int_{edcvfrtg} \\left( \\sum_{asdfghjk=bnvctrpl}^{ptzmskch-1} \\log \\frac{hjgrksla(qzxwvtnp+asdfghjk)}{hjgrksla(qzxwvtnp+asdfghjk+1)}\\right) \\,dqzxwvtnp \\\\\n&= \\frac{1}{2} \\int_{edcvfrtg} \\log \\frac{hjgrksla(qzxwvtnp+bnvctrpl)}{hjgrksla(qzxwvtnp+ptzmskch)} \\,dqzxwvtnp.\n\\end{align*}\nFor each $qzxwvtnp \\in edcvfrtg$, $\\log hjgrksla(qzxwvtnp+bnvctrpl)/hjgrksla(qzxwvtnp+ptzmskch)$ is a strictly increasing unbounded function of $ptzmskch$.\nBy the monotone convergence theorem, the integral $\\int_{edcvfrtg} \\log (hjgrksla(qzxwvtnp+bnvctrpl)/hjgrksla(qzxwvtnp+ptzmskch)) \\,dqzxwvtnp$ grows without bound\nas $ptzmskch \\to +\\infty$, a contradiction. Thus the original integral diverges, as desired.\n\n\\textbf{Remark.}\nThis solution is motivated by the commonly-used fact that an infinite product\n$(1 + qzxwvtnp_1)(1 + qzxwvtnp_2) \\cdots$ converges absolutely if and only if the sum\n$qzxwvtnp_1 + qzxwvtnp_2 + \\cdots$ converges absolutely. The additional measure-theoretic argument at the beginning is needed\nbecause one cannot bound $-\\log(1-rfvgtbhy)$ by a fixed multiple of $rfvgtbhy$ uniformly for all $rfvgtbhy \\in [0,1)$.\n\nGreg Martin suggests a variant solution that avoids use of Lebesgue measure.\nNote first that if $hjgrksla(plmoknij) > 2hjgrksla(plmoknij+1)$, then either $hjgrksla(plmoknij) > \\sqrt{2} hjgrksla(plmoknij+1/2)$ or $hjgrksla(plmoknij+1/2) > \\sqrt{2} hjgrksla(plmoknij+1)$,\nand in either case we deduce that\n\\[\n\\int_{plmoknij-1/2}^{plmoknij+1/2} \\frac{hjgrksla(qzxwvtnp)-hjgrksla(qzxwvtnp+1)}{hjgrksla(qzxwvtnp)}\\,dqzxwvtnp > \\frac{1}{2} \\left(1 - \\frac{1}{\\sqrt{2}} \\right)  > \\frac{1}{7}.\n\\]\nIf there exist arbitrarily large values of $plmoknij$ for which $hjgrksla(plmoknij) > 2hjgrksla(plmoknij+1)$, we deduce that\nthe original integral is greater than any multiple of $1/7$, and so diverges. Otherwise,\nfor $qzxwvtnp$ large we may argue that\n\\[\n\\frac{hjgrksla(qzxwvtnp)-hjgrksla(qzxwvtnp+1)}{hjgrksla(qzxwvtnp)} > \\frac{3}{5} \\log \\frac{hjgrksla(qzxwvtnp)}{hjgrksla(qzxwvtnp+1)}\n\\]\nas in the above solution, and again get divergence using a telescoping sum.\n\n\\textbf{Second solution.}\n(Communicated by Paul Allen.)\nLet $yhnujmik>ujmnhytr$ be nonnegative integers. Then\n\\begin{align*}\n\\int_{ujmnhytr}^{yhnujmik} \\frac{hjgrksla(qzxwvtnp)-hjgrksla(qzxwvtnp+1)}{hjgrksla(qzxwvtnp)}dqzxwvtnp &=\n\\sum_{ikmjunhy=ujmnhytr}^{yhnujmik-1} \\int_0^1 \\frac{hjgrksla(qzxwvtnp+ikmjunhy)-hjgrksla(qzxwvtnp+ikmjunhy+1)}{hjgrksla(qzxwvtnp+ikmjunhy)}dqzxwvtnp \\\\\n&= \\int_0^1 \\sum_{ikmjunhy=ujmnhytr}^{yhnujmik-1} \\frac{hjgrksla(qzxwvtnp+ikmjunhy)-hjgrksla(qzxwvtnp+ikmjunhy+1)}{hjgrksla(qzxwvtnp+ikmjunhy)}dqzxwvtnp \\\\\n&\\geq \\int_0^1 \\sum_{ikmjunhy=ujmnhytr}^{yhnujmik-1} \\frac{hjgrksla(qzxwvtnp+ikmjunhy)-hjgrksla(qzxwvtnp+ikmjunhy+1)}{hjgrksla(qzxwvtnp+ujmnhytr)}dqzxwvtnp \\\\\n&= \\int_0^1 \\frac{hjgrksla(qzxwvtnp+ujmnhytr)-hjgrksla(qzxwvtnp+yhnujmik)}{hjgrksla(qzxwvtnp+ujmnhytr)} dqzxwvtnp.\n\\end{align*}\nNow since $hjgrksla(qzxwvtnp)\\rightarrow 0$, given $ujmnhytr$, we can choose an integer $okmijnuh(ujmnhytr)>ujmnhytr$ for which $hjgrksla(okmijnuh(ujmnhytr)) < hjgrksla(ujmnhytr+1)/2$; then $\\frac{hjgrksla(qzxwvtnp+ujmnhytr)-hjgrksla(qzxwvtnp+okmijnuh(ujmnhytr))}{hjgrksla(qzxwvtnp+ujmnhytr)} \\geq 1 - \\frac{hjgrksla(okmijnuh(ujmnhytr))}{hjgrksla(ujmnhytr+1)} > 1/2$ for all $qzxwvtnp\\in [0,1]$. Thus if we define a sequence of integers $ujmnhytr_{ptzmskch}$ by $ujmnhytr_0=0$, $ujmnhytr_{ptzmskch+1}=okmijnuh(ujmnhytr_{ptzmskch})$, then\n\\begin{align*}\n\\int_0^\\infty \\frac{hjgrksla(qzxwvtnp)-hjgrksla(qzxwvtnp+1)}{hjgrksla(qzxwvtnp)} dqzxwvtnp &=\n\\sum_{ptzmskch=0}^\\infty \\int_{ujmnhytr_{ptzmskch}}^{ujmnhytr_{ptzmskch+1}} \\frac{hjgrksla(qzxwvtnp)-hjgrksla(qzxwvtnp+1)}{hjgrksla(qzxwvtnp)} dqzxwvtnp \\\\\n&> \\sum_{ptzmskch=0}^\\infty \\int_0^1 (1/2) dqzxwvtnp,\n\\end{align*}\nand the final sum clearly diverges.\n\n\\textbf{Third solution.}\n(By Joshua Rosenberg, communicated by Catalin Zara.)\nIf the original integral converges, then\non one hand the integrand $(hjgrksla(qzxwvtnp)-hjgrksla(qzxwvtnp+1))/hjgrksla(qzxwvtnp) = 1 - hjgrksla(qzxwvtnp+1)/hjgrksla(qzxwvtnp)$\ncannot tend to 1 as $qzxwvtnp \\to \\infty$.\nOn the other hand, for any $ujmnhytr \\ge 0$,\n\\begin{align*}\n0 &< \\frac{hjgrksla(ujmnhytr+1)}{hjgrksla(ujmnhytr)} \\\\\n&< \\frac{1}{hjgrksla(ujmnhytr)} \\int_{ujmnhytr}^{ujmnhytr+1} hjgrksla(qzxwvtnp)\\,dqzxwvtnp \\\\\n&= \\frac{1}{hjgrksla(ujmnhytr)} \\int_{ujmnhytr}^\\infty (hjgrksla(qzxwvtnp) - hjgrksla(qzxwvtnp+1))\\,dqzxwvtnp \\\\\n&\\le \\int_{ujmnhytr}^\\infty \\frac{hjgrksla(qzxwvtnp) - hjgrksla(qzxwvtnp+1)}{hjgrksla(qzxwvtnp)}\\,dqzxwvtnp,\n\\end{align*}\nand the last expression tends to 0 as $ujmnhytr \\to \\infty$.\nHence by the squeeze theorem, $hjgrksla(ujmnhytr+1)/hjgrksla(ujmnhytr) \\to 0$ as $ujmnhytr \\to \\infty$, a contradiction."
    },
    "kernel_variant": {
      "question": "Let $f:[0,\n\\infty)\\to\\mathbb R$ be a strictly decreasing, continuous function with\n\\displaystyle \\lim_{x\\to\\infty}f(x)=0.  Prove that the improper integral\n\\[\n\\int_{0}^{\\infty} \\frac{f(x)-f(x+2)}{f(x)}\\,dx\n\\]\ndiverges.",
      "solution": "Assume, for contradiction, that\nL := \\int_{0}^{\\infty}\\Bigl(1-\\frac{f(x+2)}{f(x)}\\Bigr)\\,dx < \\infty.\n\n1.  Split into blocks of length 2.  Write x=y+2n with y\\in[0,2], n=0,1,2,….  Then\n   L = \\sum_{n=0}^\\infty \\int_{0}^{2}\\Bigl(1-\\frac{f(y+2n+2)}{f(y+2n)}\\Bigr)\\,dy.\n\n2.  Find where the drop is small.  Fix τ=1/3 and define for each n≥0\n   Iₙ=\\{y\\in[0,2]:1-\\frac{f(y+2n+2)}{f(y+2n)}\\le τ\\}.\nOn the complement [0,2]\\Iₙ the integrand ≥τ, so\n   L ≥ \\sum_{n=0}^\\infty τ\\,(2-μ(Iₙ)).\nSince L<∞, the series ∑(2−μ(Iₙ)) converges.  Hence for some N we have ∑_{k=N}^∞(2−μ(Iₖ))<2, and therefore\n   μ\\Bigl(\\bigcap_{k=N}^\\infty Iₖ\\Bigr)=2-μ\\Bigl(\\bigcup_{k=N}^\\infty Iₖ^c\\Bigr)\\ge2-\\sum_{k=N}^\\infty(2-μ(Iₖ))>0.\nLet I=⋂_{k=N}^∞Iₖ, so μ(I)>0.\n\n3.  A logarithmic bound.  For 0≤t≤τ=1/3 one shows by the Mean Value Theorem that\n   −\\log(1−t)≤3t.\nSetting t=1−f(y+2n+2)/f(y+2n) gives\n   1−\\frac{f(y+2n+2)}{f(y+2n)} \\ge \\tfrac13\\bigl(-\\log(\\tfrac{f(y+2n+2)}{f(y+2n)})\\bigr)\n                               = \\tfrac13\\log\\frac{f(y+2n)}{f(y+2n+2)}.\n\n4.  Summing over k=n,…,m−1 on I and using Tonelli’s theorem (all integrands nonnegative) yields\n   L\\ge \\sum_{k=n}^{m-1}\\int_I\\Bigl(1-\\frac{f(y+2k+2)}{f(y+2k)}\\Bigr)dy\n     \\ge \\tfrac13\\int_I \\sum_{k=n}^{m-1}\\log\\frac{f(y+2k)}{f(y+2k+2)}\\,dy\n     =\\tfrac13\\int_I\\log\\frac{f(y+2n)}{f(y+2m)}\\,dy.\n\n5.  Let m→∞.  For each fixed y∈I we have f(y+2m)→0, so log(f(y+2n)/f(y+2m))→+∞.  By the Monotone Convergence Theorem,\n   \\int_I\\log\\frac{f(y+2n)}{f(y+2m)}\\,dy\\to+\\infty,\ncontradicting L<∞.  Therefore the original integral diverges.",
      "_meta": {
        "core_steps": [
          "Assume the integral converges and split it into a series of unit‐length pieces.",
          "Relate each piece 1−f(x+1)/f(x) to −log(f(x+1)/f(x)) via the inequality −log(1−t)≤C·t for small t.",
          "Choose a positive‐measure subset of the unit interval where the ratio f(x+1)/f(x) is uniformly bounded away from 1.",
          "Sum (or integrate) the logarithmic estimates over successive intervals to obtain a telescoping log f(x) term that grows without bound (monotone convergence/telescoping product argument).",
          "Contradict the assumed convergence; hence the original integral diverges."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Size of the forward shift in the integrand, i.e. the +1 in f(x+1). Any fixed positive constant would work.",
            "original": 1
          },
          "slot2": {
            "description": "Threshold τ used to declare the ratio ‘small’ (here 1−f(x+1)/f(x)≤τ with τ=1/2). Any τ in (0,1) suffices.",
            "original": 0.5
          },
          "slot3": {
            "description": "Length of the base interval [0,1] used for partitioning and measure arguments. Any fixed positive length L could replace 1.",
            "original": 1
          },
          "slot4": {
            "description": "Constant C in the bound −log(1−t)≤C·t (here C=2). Any constant exceeding the true supremum on the chosen t‐range works.",
            "original": 2
          },
          "slot5": {
            "description": "Upper limit on t for which the logarithmic inequality is invoked (here t≤1/2). Any number in (0,1) would do.",
            "original": 0.5
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}