{
  "index": "1966-B-3",
  "type": "ANA",
  "tag": [
    "ANA",
    "NT"
  ],
  "difficulty": "",
  "question": "B-3. Show that if the series\n\\[\n\\sum_{n=1}^{\\infty} \\frac{1}{p_{n}}\n\\]\nis convergent, where \\( p_{1}, p_{2}, p_{3}, \\cdots, p_{n}, \\cdots \\) are positive real numbers, then the series\n\\[\n\\sum_{n=1}^{\\infty} \\frac{n^{2}}{\\left(p_{1}+p_{2}+\\cdots+p_{n}\\right)^{2}} p_{n}\n\\]\nis also convergent.",
  "solution": "B-3 Set \\( q_{n}=p_{1}+p_{2}+\\cdots+p_{n}\\left(q_{0}=0\\right) \\). We are led to estimate \\( S_{N} \\) \\( =\\sum_{x=1}^{N}\\left(n / q_{n}\\right)^{2}\\left(q_{n}-q_{n-1}\\right) \\) in terms of \\( T=\\sum_{n=1}^{\\infty} 1 / p_{n} \\). Note that\n\\[\n\\begin{aligned}\nS_{N} & \\leqq \\frac{1}{p_{1}}+\\sum_{n=2}^{N} \\frac{n^{2}}{q_{n} q_{n-1}}\\left(q_{n}-q_{n-1}\\right)=\\frac{1}{p_{1}}+\\sum_{n=2}^{N} \\frac{n^{2}}{q_{n-1}}-\\sum_{n=2}^{N} \\frac{n^{2}}{q_{n}} \\\\\n& =\\frac{1}{p_{1}}+\\sum_{n=1}^{N-1} \\frac{(n+1)^{2}}{q_{n}}-\\sum_{n=2}^{N} \\frac{n^{2}}{q_{n}} \\leqq \\frac{5}{p_{1}}+2 \\sum_{n=2}^{N} \\frac{n}{q_{n}}+\\sum_{n=2}^{N} \\frac{1}{q_{n}} .\n\\end{aligned}\n\\]\n\nBy Schwarz's inequality,\n\\[\n\\left(\\sum_{n=2}^{N} \\frac{n}{q_{n}}\\right)^{2} \\leqq \\sum_{n=2}^{N} \\frac{n^{2}}{q_{n}^{2}} p_{n} \\sum_{n=1}^{\\infty} \\frac{1}{p_{n}}\n\\]\nand thus\n\\[\nS_{N} \\leqq \\frac{5}{p_{1}}+2 \\sqrt{S_{N} T}+T\n\\]\n\nThis quadratic inequality implies that \\( \\sqrt{S_{N}} \\leqq \\sqrt{T}+\\sqrt{2 T+5 / p_{1}} \\).",
  "vars": [
    "n",
    "x",
    "N"
  ],
  "params": [
    "p_n",
    "p_1",
    "p_2",
    "p_3",
    "q_n",
    "q_n-1",
    "q_0",
    "S_N",
    "T"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "n": "indexvar",
        "x": "summand",
        "N": "uppernn",
        "p_n": "seqelem",
        "p_1": "firstelem",
        "p_2": "secondelem",
        "p_3": "thirdelem",
        "q_n": "partialsum",
        "q_n-1": "previouspartial",
        "q_0": "zeropartial",
        "S_N": "partialseries",
        "T": "totalsum"
      },
      "question": "B-3. Show that if the series\n\\[\n\\sum_{indexvar=1}^{\\infty} \\frac{1}{seqelem}\n\\]\nis convergent, where \\( firstelem, secondelem, thirdelem, \\cdots, seqelem, \\cdots \\) are positive real numbers, then the series\n\\[\n\\sum_{indexvar=1}^{\\infty} \\frac{indexvar^{2}}{\\left(firstelem+secondelem+\\cdots+seqelem\\right)^{2}} seqelem\n\\]\nis also convergent.",
      "solution": "B-3 Set \\( partialsum=firstelem+secondelem+\\cdots+seqelem\\left(zeropartial=0\\right) \\). We are led to estimate \\( partialseries \\) \\( =\\sum_{summand=1}^{uppernn}\\left(indexvar / partialsum\\right)^{2}\\left(partialsum-previouspartial\\right) \\) in terms of \\( totalsum=\\sum_{indexvar=1}^{\\infty} 1 / seqelem \\). Note that\n\\[\n\\begin{aligned}\npartialseries & \\leqq \\frac{1}{firstelem}+\\sum_{indexvar=2}^{uppernn} \\frac{indexvar^{2}}{partialsum\\,previouspartial}\\left(partialsum-previouspartial\\right)=\\frac{1}{firstelem}+\\sum_{indexvar=2}^{uppernn} \\frac{indexvar^{2}}{previouspartial}-\\sum_{indexvar=2}^{uppernn} \\frac{indexvar^{2}}{partialsum} \\\\\n& =\\frac{1}{firstelem}+\\sum_{indexvar=1}^{uppernn-1} \\frac{(indexvar+1)^{2}}{partialsum}-\\sum_{indexvar=2}^{uppernn} \\frac{indexvar^{2}}{partialsum} \\leqq \\frac{5}{firstelem}+2 \\sum_{indexvar=2}^{uppernn} \\frac{indexvar}{partialsum}+\\sum_{indexvar=2}^{uppernn} \\frac{1}{partialsum} .\n\\end{aligned}\n\\]\n\nBy Schwarz's inequality,\n\\[\n\\left(\\sum_{indexvar=2}^{uppernn} \\frac{indexvar}{partialsum}\\right)^{2} \\leqq \\sum_{indexvar=2}^{uppernn} \\frac{indexvar^{2}}{partialsum^{2}} seqelem \\sum_{indexvar=1}^{\\infty} \\frac{1}{seqelem}\n\\]\nand thus\n\\[\npartialseries \\leqq \\frac{5}{firstelem}+2 \\sqrt{partialseries\\, totalsum}+totalsum\n\\]\n\nThis quadratic inequality implies that \\( \\sqrt{partialseries} \\leqq \\sqrt{totalsum}+\\sqrt{2\\, totalsum+5 / firstelem} \\)."
    },
    "descriptive_long_confusing": {
      "map": {
        "n": "pineapple",
        "x": "tangerine",
        "N": "toothbrush",
        "p_n": "marigolds",
        "p_1": "jellyfish",
        "p_2": "blackbird",
        "p_3": "rainstorm",
        "q_n": "snowflake",
        "q_n-1": "lemonade",
        "q_0": "treasurer",
        "S_N": "horseshoe",
        "T": "whisperer"
      },
      "question": "B-3. Show that if the series\n\\[\n\\sum_{pineapple=1}^{\\infty} \\frac{1}{marigolds}\n\\]\nis convergent, where \\( jellyfish, blackbird, rainstorm, \\cdots, marigolds, \\cdots \\) are positive real numbers, then the series\n\\[\n\\sum_{pineapple=1}^{\\infty} \\frac{pineapple^{2}}{\\left(jellyfish+blackbird+\\cdots+marigolds\\right)^{2}} marigolds\n\\]\nis also convergent.",
      "solution": "B-3 Set \\( snowflake=jellyfish+blackbird+\\cdots+marigolds\\left(treasurer=0\\right) \\). We are led to estimate \\( horseshoe \\) \\( =\\sum_{tangerine=1}^{toothbrush}\\left(pineapple / snowflake\\right)^{2}\\left(snowflake-lemonade\\right) \\) in terms of \\( whisperer=\\sum_{pineapple=1}^{\\infty} 1 / marigolds \\). Note that\n\\[\n\\begin{aligned}\nhorseshoe & \\leqq \\frac{1}{jellyfish}+\\sum_{pineapple=2}^{toothbrush} \\frac{pineapple^{2}}{snowflake \\, lemonade}\\left(snowflake-lemonade\\right)=\\frac{1}{jellyfish}+\\sum_{pineapple=2}^{toothbrush} \\frac{pineapple^{2}}{lemonade}-\\sum_{pineapple=2}^{toothbrush} \\frac{pineapple^{2}}{snowflake} \\\\\n& =\\frac{1}{jellyfish}+\\sum_{pineapple=1}^{toothbrush-1} \\frac{(pineapple+1)^{2}}{snowflake}-\\sum_{pineapple=2}^{toothbrush} \\frac{pineapple^{2}}{snowflake} \\leqq \\frac{5}{jellyfish}+2 \\sum_{pineapple=2}^{toothbrush} \\frac{pineapple}{snowflake}+\\sum_{pineapple=2}^{toothbrush} \\frac{1}{snowflake} .\n\\end{aligned}\n\\]\n\nBy Schwarz's inequality,\n\\[\n\\left(\\sum_{pineapple=2}^{toothbrush} \\frac{pineapple}{snowflake}\\right)^{2} \\leqq \\sum_{pineapple=2}^{toothbrush} \\frac{pineapple^{2}}{snowflake^{2}} marigolds \\sum_{pineapple=1}^{\\infty} \\frac{1}{marigolds}\n\\]\nand thus\n\\[\nhorseshoe \\leqq \\frac{5}{jellyfish}+2 \\sqrt{horseshoe \\, whisperer}+whisperer\n\\]\n\nThis quadratic inequality implies that \\( \\sqrt{horseshoe} \\leqq \\sqrt{whisperer}+\\sqrt{2 \\, whisperer+5 / jellyfish} \\)."
    },
    "descriptive_long_misleading": {
      "map": {
        "n": "finishline",
        "x": "stillness",
        "N": "originpoint",
        "p_n": "negativeseq",
        "p_1": "negseqfirst",
        "p_2": "negseqsecond",
        "p_3": "negseqthird",
        "q_n": "differencevar",
        "q_n-1": "differenceprev",
        "q_0": "differencestart",
        "S_N": "gapaggregate",
        "T": "singlevalue"
      },
      "question": "B-3. Show that if the series\n\\[\n\\sum_{finishline=1}^{\\infty} \\frac{1}{negativeseq_{finishline}}\n\\]\nis convergent, where \\( negseqfirst, negseqsecond, negseqthird, \\cdots, negativeseq_{finishline}, \\cdots \\) are positive real numbers, then the series\n\\[\n\\sum_{finishline=1}^{\\infty} \\frac{finishline^{2}}{\\left(negseqfirst+negseqsecond+\\cdots+negativeseq_{finishline}\\right)^{2}} \\, negativeseq_{finishline}\n\\]\nis also convergent.",
      "solution": "B-3 Set \\( differencevar_{finishline}=negseqfirst+negseqsecond+\\cdots+negativeseq_{finishline}\\left(differencestart=0\\right) \\). We are led to estimate \\( gapaggregate \\) \\( =\\sum_{stillness=1}^{originpoint}\\left(finishline / differencevar_{finishline}\\right)^{2}\\left(differencevar_{finishline}-differenceprev\\right) \\) in terms of \\( singlevalue=\\sum_{finishline=1}^{\\infty} 1 / negativeseq_{finishline} \\). Note that\n\\[\n\\begin{aligned}\ngapaggregate & \\leqq \\frac{1}{negseqfirst}+\\sum_{finishline=2}^{originpoint} \\frac{finishline^{2}}{differencevar_{finishline}\\,differenceprev}\\left(differencevar_{finishline}-differenceprev\\right)=\\frac{1}{negseqfirst}+\\sum_{finishline=2}^{originpoint} \\frac{finishline^{2}}{differenceprev}-\\sum_{finishline=2}^{originpoint} \\frac{finishline^{2}}{differencevar_{finishline}} \\\\\n& =\\frac{1}{negseqfirst}+\\sum_{finishline=1}^{originpoint-1} \\frac{(finishline+1)^{2}}{differencevar_{finishline}}-\\sum_{finishline=2}^{originpoint} \\frac{finishline^{2}}{differencevar_{finishline}} \\leqq \\frac{5}{negseqfirst}+2 \\sum_{finishline=2}^{originpoint} \\frac{finishline}{differencevar_{finishline}}+\\sum_{finishline=2}^{originpoint} \\frac{1}{differencevar_{finishline}} .\n\\end{aligned}\n\\]\n\nBy Schwarz's inequality,\n\\[\n\\left(\\sum_{finishline=2}^{originpoint} \\frac{finishline}{differencevar_{finishline}}\\right)^{2} \\leqq \\sum_{finishline=2}^{originpoint} \\frac{finishline^{2}}{differencevar_{finishline}^{2}} \\, negativeseq_{finishline} \\, \\sum_{finishline=1}^{\\infty} \\frac{1}{negativeseq_{finishline}}\n\\]\nand thus\n\\[\ngapaggregate \\leqq \\frac{5}{negseqfirst}+2 \\sqrt{gapaggregate \\, singlevalue}+singlevalue\n\\]\n\nThis quadratic inequality implies that \\( \\sqrt{gapaggregate} \\leqq \\sqrt{singlevalue}+\\sqrt{2 \\, singlevalue+5 / negseqfirst} \\)."
    },
    "garbled_string": {
      "map": {
        "n": "vzxlrkqa",
        "x": "mfdjwhzs",
        "N": "tbqphkro",
        "p_n": "epqtrmno",
        "p_1": "uavdkhzc",
        "p_2": "jrosqnel",
        "p_3": "abqlisef",
        "q_n": "ulfwymte",
        "q_n-1": "wtbrmlas",
        "q_0": "gksajfzi",
        "S_N": "bnkohuds",
        "T": "rgtdycwe"
      },
      "question": "B-3. Show that if the series\n\\[\n\\sum_{vzxlrkqa=1}^{\\infty} \\frac{1}{epqtrmno}\n\\]\nis convergent, where \\( uavdkhzc, jrosqnel, abqlisef, \\cdots, epqtrmno, \\cdots \\) are positive real numbers, then the series\n\\[\n\\sum_{vzxlrkqa=1}^{\\infty} \\frac{vzxlrkqa^{2}}{\\left(uavdkhzc+jrosqnel+\\cdots+epqtrmno\\right)^{2}} epqtrmno\n\\]\nis also convergent.",
      "solution": "B-3 Set \\( ulfwymte=uavdkhzc+jrosqnel+\\cdots+epqtrmno\\left(gksajfzi=0\\right) \\). We are led to estimate \\( bnkohuds \\) \\( =\\sum_{mfdjwhzs=1}^{tbqphkro}\\left(vzxlrkqa / ulfwymte\\right)^{2}\\left(ulfwymte-wtbrmlas\\right) \\) in terms of \\( rgtdycwe=\\sum_{vzxlrkqa=1}^{\\infty} 1 / epqtrmno \\). Note that\n\\[\n\\begin{aligned}\nbnkohuds & \\leqq \\frac{1}{uavdkhzc}+\\sum_{vzxlrkqa=2}^{tbqphkro} \\frac{vzxlrkqa^{2}}{ulfwymte wtbrmlas}\\left(ulfwymte-wtbrmlas\\right)=\\frac{1}{uavdkhzc}+\\sum_{vzxlrkqa=2}^{tbqphkro} \\frac{vzxlrkqa^{2}}{wtbrmlas}-\\sum_{vzxlrkqa=2}^{tbqphkro} \\frac{vzxlrkqa^{2}}{ulfwymte} \\\\\n& =\\frac{1}{uavdkhzc}+\\sum_{vzxlrkqa=1}^{tbqphkro-1} \\frac{(vzxlrkqa+1)^{2}}{ulfwymte}-\\sum_{vzxlrkqa=2}^{tbqphkro} \\frac{vzxlrkqa^{2}}{ulfwymte} \\leqq \\frac{5}{uavdkhzc}+2 \\sum_{vzxlrkqa=2}^{tbqphkro} \\frac{vzxlrkqa}{ulfwymte}+\\sum_{vzxlrkqa=2}^{tbqphkro} \\frac{1}{ulfwymte} .\n\\end{aligned}\n\\]\n\nBy Schwarz's inequality,\n\\[\n\\left(\\sum_{vzxlrkqa=2}^{tbqphkro} \\frac{vzxlrkqa}{ulfwymte}\\right)^{2} \\leqq \\sum_{vzxlrkqa=2}^{tbqphkro} \\frac{vzxlrkqa^{2}}{ulfwymte^{2}} epqtrmno \\sum_{vzxlrkqa=1}^{\\infty} \\frac{1}{epqtrmno}\n\\]\nand thus\n\\[\nbnkohuds \\leqq \\frac{5}{uavdkhzc}+2 \\sqrt{bnkohuds\\, rgtdycwe}+rgtdycwe\n\\]\n\nThis quadratic inequality implies that \\( \\sqrt{bnkohuds} \\leqq \\sqrt{rgtdycwe}+\\sqrt{2 rgtdycwe+5 / uavdkhzc} \\)."
    },
    "kernel_variant": {
      "question": "Let $(p_n)_{n\\ge 1}$ be a sequence of positive real numbers such that\n\\[\\sum_{n=1}^{\\infty}\\frac1{p_n}<\\infty.\\]\nDenote the partial sums by $q_n=p_1+p_2+\\dots+p_n\\;(q_0:=0)$.  Prove that the series\n\\[\\sum_{n=1}^{\\infty}\\frac{n(n+1)}{q_n^{\\,2}}\\,p_n\\]\nconverges.",
      "solution": "Let p₁,p₂,… be positive with ∑ₙ1/pₙ=T<∞, and set q₀=0, qₙ=p₁+⋯+pₙ.  We must show the partial sums\n  S_N=∑_{n=1}^N n(n+1)·pₙ/qₙ²\nstay bounded as N→∞.\n\n1.  For n=1,\n  1·2·p₁/q₁²=2p₁/p₁²=2/p₁.\n\n2.  For n≥2, qₙ≥q_{n-1}>0 implies 1/qₙ² ≤1/(qₙ q_{n-1}).  Hence\n  n(n+1)pₙ/qₙ²=n(n+1)(qₙ−q_{n-1})/qₙ²\n    ≤n(n+1)(qₙ−q_{n-1})/(qₙ q_{n-1})\n    =n(n+1)(1/q_{n-1}−1/qₙ).\nThus\n  S_N ≤2/p₁ + ∑_{n=2}^N n(n+1)(1/q_{n-1}−1/qₙ).\n\n3.  Telescope the sum for n≥2.  Writing\n  ∑_{n=2}^N n(n+1)/q_{n-1}−∑_{n=2}^N n(n+1)/qₙ\nand shifting indices in the first sum shows it equals\n  6/p₁ + ∑_{k=2}^{N-1}2(k+1)/q_k − N(N+1)/q_N.\nDropping the nonpositive last term gives\n  ∑_{n=2}^N n(n+1)(1/q_{n-1}−1/qₙ)\n    ≤6/p₁ +2∑_{k=2}^{N-1}(k+1)/q_k\n    =6/p₁ +2∑_{k=2}^{N-1}k/q_k +2∑_{k=2}^{N-1}1/q_k.\n\n4.  Hence\n  S_N ≤2/p₁ +6/p₁ +2∑_{k=2}^{N-1}k/q_k +2∑_{k=2}^{N-1}1/q_k\n       =8/p₁ +2∑_{k=2}^{N-1}k/q_k +2∑_{k=2}^{N-1}1/q_k.\nSince q_k≥p_k,  ∑1/q_k ≤∑1/p_k=T, so\n  S_N ≤8/p₁ +2∑_{k=2}^{N-1}k/q_k +2T.\n\n5.  By Cauchy–Schwarz with a_k=k√pₖ/qₖ, b_k=1/√pₖ,\n  (∑k/q_k)²=(∑a_k b_k)²≤(∑a_k²)(∑b_k²)\n    ≤(∑k²pₖ/qₖ²)(∑1/pₖ)≤S_N·T.\nThus A=∑_{k=2}^{N-1}k/q_k ≤√(T·S_N).\n\n6.  Therefore\n  S_N ≤8/p₁ +2√(T·S_N)+2T,\nso setting x=√S_N gives\n  x²−2√T·x−(8/p₁+2T)≤0\n⇔x≤√T+√(3T+8/p₁).\nThus S_N remains bounded as N→∞, and ∑n(n+1)pₙ/qₙ² converges.",
      "_meta": {
        "core_steps": [
          "Introduce partial sums q_n = p_1 + … + p_n and rewrite S_N as Σ (n/q_n)^2 (q_n − q_{n−1}).",
          "Use the inequality (q_n − q_{n−1})/q_{n−1} ≤ 1 to obtain a telescoping bound that reduces S_N to a linear combination of Σ n/q_n, Σ 1/q_n, and a finite constant.",
          "Apply Cauchy–Schwarz to bound Σ n/q_n by √(S_N · T), where T = Σ 1/p_n is finite.",
          "Combine the bounds to get a quadratic inequality S_N ≤ C_0 + 2√(S_N T) + T.",
          "Solve the quadratic inequality to show S_N is bounded, hence the target series converges."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Exact finite constant that comes from isolating the first few terms in the telescoping estimate (written as 5/p_1 in the solution). Any other fixed constant produced by treating finitely many initial indices separately would work.",
            "original": "5"
          },
          "slot2": {
            "description": "The point at which the sum is split off before applying the inequality (the solution starts telescoping from n = 2). Any fixed starting index k ≥ 2 would leave the argument intact after adjusting the finite constant.",
            "original": "n = 2"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}