{
  "index": "1949-A-3",
  "type": "ANA",
  "tag": [
    "ANA",
    "COMB"
  ],
  "difficulty": "",
  "question": "3. Assume that the complex numbers \\( a_{1}, a_{2}, \\ldots, a_{n}, \\ldots \\) are all different from zero, and that \\( \\left|a_{r}-a_{s}\\right|>1 \\) for \\( r \\neq s \\). Show that the series\n\\[\n\\sum_{n=1}^{\\infty} \\frac{1}{a_{n}{ }^{3}}\n\\]\nconverges.",
  "solution": "Solution. Let \\( S_{k}=\\left\\{n: k<\\left|a_{n}\\right| \\leq k+1\\right\\} \\) for \\( k=0,1,2, \\ldots \\). The discs \\( \\left|z-a_{n}\\right| \\leq \\frac{1}{2} \\) are all disjoint by hypothesis, and for \\( n \\in S_{k} \\) these discs all lie in the annulus\n\\[\n\\left\\{z: k-\\frac{1}{2} \\leq|z| \\leq k+\\frac{3}{2}\\right\\},\n\\]\n(a disc if \\( k=0 \\) ). Let the cardinality of the set \\( S_{k} \\) be denoted by \\( \\left|S_{k}\\right| \\). Then adding areas gives\n\\[\n\\left|S_{k}\\right| \\frac{\\pi}{4} \\leq \\pi\\left[\\left(k+\\frac{3}{2}\\right)^{2}-\\left(k-\\frac{1}{2}\\right)^{2}\\right]=2 \\pi(2 k+1)\n\\]\nso that \\( \\left|S_{k}\\right| \\leq 8(2 k+1) \\) for \\( k>0 \\). A separate calculation shows that \\( \\left|S_{0}\\right| \\leq 9 \\).\nThen\n\\[\n\\sum_{n \\in S_{k}} \\frac{1}{\\left|a_{n}\\right|^{3}} \\leq \\frac{\\left|S_{k}\\right|}{k^{3}} \\leq \\frac{8(2 k+1)}{k^{3}} \\leq \\frac{24}{k^{2}}\n\\]\nfor \\( k \\geq 1 \\) because \\( 2 k+1 \\leq 3 k \\). Since \\( S_{0} \\) is finite,\n\\[\n\\sum_{n \\in S_{0}} \\frac{1}{\\left|a_{n}\\right|^{3}}\n\\]\nis finite.\nHence we have\n\\[\n\\sum_{n=1}^{\\infty} \\frac{1}{\\left|a_{n}\\right|^{3}}=\\sum_{k=0}^{\\infty} \\sum_{n \\in S_{k}} \\frac{1}{\\left|a_{n}\\right|^{3}} \\leq \\sum_{n \\in S_{0}} \\frac{1}{\\left|a_{n}\\right|^{3}}+\\sum_{k=1}^{\\infty} \\frac{24}{k^{2}}<\\infty .\n\\]\n\nThe rearrangement of the sum in the first step is permissible since the terms are all positive. Thus the original series converges absolutely.",
  "vars": [
    "k",
    "n",
    "r",
    "s",
    "z",
    "S_k",
    "S_0"
  ],
  "params": [
    "a_n",
    "a_r",
    "a_s"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "k": "indexkappa",
        "n": "indexnu",
        "r": "indexrho",
        "s": "indexsigma",
        "z": "complexzeta",
        "S_k": "setkappa",
        "S_0": "setzero",
        "a_n": "coefficientnu",
        "a_r": "coefficientrho",
        "a_s": "coefficientsigma"
      },
      "question": "3. Assume that the complex numbers \\( a_{1}, a_{2}, \\ldots, coefficientnu, \\ldots \\) are all different from zero, and that \\( \\left|coefficientrho-coefficientsigma\\right|>1 \\) for \\( indexrho \\neq indexsigma \\). Show that the series\n\\[\n\\sum_{indexnu=1}^{\\infty} \\frac{1}{coefficientnu^{3}}\n\\]\nconverges.",
      "solution": "Solution. Let \\( setkappa=\\left\\{indexnu: indexkappa<\\left|coefficientnu\\right| \\leq indexkappa+1\\right\\} \\) for \\( indexkappa=0,1,2, \\ldots \\). The discs \\( \\left|complexzeta-coefficientnu\\right| \\leq \\frac{1}{2} \\) are all disjoint by hypothesis, and for \\( indexnu \\in setkappa \\) these discs all lie in the annulus\n\\[\n\\left\\{complexzeta: indexkappa-\\frac{1}{2} \\leq|complexzeta| \\leq indexkappa+\\frac{3}{2}\\right\\},\n\\]\n(a disc if \\( indexkappa=0 \\) ). Let the cardinality of the set \\( setkappa \\) be denoted by \\( \\left|setkappa\\right| \\). Then adding areas gives\n\\[\n\\left|setkappa\\right| \\frac{\\pi}{4} \\leq \\pi\\left[\\left(indexkappa+\\frac{3}{2}\\right)^{2}-\\left(indexkappa-\\frac{1}{2}\\right)^{2}\\right]=2 \\pi(2 indexkappa+1)\n\\]\nso that \\( \\left|setkappa\\right| \\leq 8(2 indexkappa+1) \\) for \\( indexkappa>0 \\). A separate calculation shows that \\( \\left|setzero\\right| \\leq 9 \\).\nThen\n\\[\n\\sum_{indexnu \\in setkappa} \\frac{1}{\\left|coefficientnu\\right|^{3}} \\leq \\frac{\\left|setkappa\\right|}{indexkappa^{3}} \\leq \\frac{8(2 indexkappa+1)}{indexkappa^{3}} \\leq \\frac{24}{indexkappa^{2}}\n\\]\nfor \\( indexkappa \\geq 1 \\) because \\( 2 indexkappa+1 \\leq 3 indexkappa \\). Since \\( setzero \\) is finite,\n\\[\n\\sum_{indexnu \\in setzero} \\frac{1}{\\left|coefficientnu\\right|^{3}}\n\\]\nis finite.\nHence we have\n\\[\n\\sum_{indexnu=1}^{\\infty} \\frac{1}{\\left|coefficientnu\\right|^{3}}=\\sum_{indexkappa=0}^{\\infty} \\sum_{indexnu \\in setkappa} \\frac{1}{\\left|coefficientnu\\right|^{3}} \\leq \\sum_{indexnu \\in setzero} \\frac{1}{\\left|coefficientnu\\right|^{3}}+\\sum_{indexkappa=1}^{\\infty} \\frac{24}{indexkappa^{2}}<\\infty .\n\\]\n\nThe rearrangement of the sum in the first step is permissible since the terms are all positive. Thus the original series converges absolutely."
    },
    "descriptive_long_confusing": {
      "map": {
        "k": "kingfisher",
        "n": "nightshade",
        "r": "rhinestone",
        "s": "salamander",
        "z": "zeppelin",
        "S_k": "sandcastle",
        "S_0": "strawberry",
        "a_n": "anchorage",
        "a_r": "astrolabe",
        "a_s": "albatross"
      },
      "question": "3. Assume that the complex numbers \\( a_{1}, a_{2}, \\ldots, anchorage, \\ldots \\) are all different from zero, and that \\( \\left|astrolabe-albatross\\right|>1 \\) for \\( rhinestone \\neq salamander \\). Show that the series\n\\[\n\\sum_{nightshade=1}^{\\infty} \\frac{1}{anchorage^{3}}\n\\]\nconverges.",
      "solution": "Solution. Let \\( sandcastle=\\left\\{nightshade: kingfisher<\\left|anchorage\\right| \\leq kingfisher+1\\right\\} \\) for \\( kingfisher=0,1,2, \\ldots \\). The discs \\( \\left|zeppelin-anchorage\\right| \\leq \\frac{1}{2} \\) are all disjoint by hypothesis, and for \\( nightshade \\in sandcastle \\) these discs all lie in the annulus\n\\[\n\\left\\{zeppelin: kingfisher-\\tfrac{1}{2} \\leq|zeppelin| \\leq kingfisher+\\tfrac{3}{2}\\right\\},\n\\]\n(a disc if \\( kingfisher=0 \\) ). Let the cardinality of the set \\( sandcastle \\) be denoted by \\( |sandcastle| \\). Then adding areas gives\n\\[\n|sandcastle| \\tfrac{\\pi}{4} \\leq \\pi\\big[\\big(kingfisher+\\tfrac{3}{2}\\big)^{2}-\\big(kingfisher-\\tfrac{1}{2}\\big)^{2}\\big]=2\\pi(2\\,kingfisher+1)\n\\]\nso that \\( |sandcastle| \\leq 8(2\\,kingfisher+1) \\) for \\( kingfisher>0 \\). A separate calculation shows that \\( |strawberry| \\leq 9 \\).\n\nThen\n\\[\n\\sum_{nightshade \\in sandcastle} \\frac{1}{|anchorage|^{3}} \\leq \\frac{|sandcastle|}{kingfisher^{3}} \\leq \\frac{8(2\\,kingfisher+1)}{kingfisher^{3}} \\leq \\frac{24}{kingfisher^{2}}\n\\]\nfor \\( kingfisher \\geq 1 \\) because \\( 2\\,kingfisher+1 \\leq 3\\,kingfisher \\). Since \\( strawberry \\) is finite,\n\\[\n\\sum_{nightshade \\in strawberry} \\frac{1}{|anchorage|^{3}}\n\\]\nis finite.\n\nHence we have\n\\[\n\\sum_{nightshade=1}^{\\infty} \\frac{1}{|anchorage|^{3}}=\\sum_{kingfisher=0}^{\\infty} \\sum_{nightshade \\in sandcastle} \\frac{1}{|anchorage|^{3}} \\leq \\sum_{nightshade \\in strawberry} \\frac{1}{|anchorage|^{3}}+\\sum_{kingfisher=1}^{\\infty} \\frac{24}{kingfisher^{2}}<\\infty .\n\\]\n\nThe rearrangement of the sum in the first step is permissible since the terms are all positive. Thus the original series converges absolutely."
    },
    "descriptive_long_misleading": {
      "map": {
        "k": "constant",
        "n": "totality",
        "r": "columnar",
        "s": "immutable",
        "z": "realaxis",
        "S_k": "singleton",
        "S_0": "solitary",
        "a_n": "terminus",
        "a_r": "termstart",
        "a_s": "termfinal"
      },
      "question": "3. Assume that the complex numbers \\( a_{1}, a_{2}, \\ldots, terminus, \\ldots \\) are all different from zero, and that \\( \\left|termstart-termfinal\\right|>1 \\) for \\( columnar \\neq immutable \\). Show that the series\n\\[\n\\sum_{totality=1}^{\\infty} \\frac{1}{terminus{ }^{3}}\n\\]\nconverges.",
      "solution": "Solution. Let \\( singleton=\\left\\{totality: constant<\\left|terminus\\right| \\leq constant+1\\right\\} \\) for \\( constant=0,1,2, \\ldots \\). The discs \\( \\left|realaxis-terminus\\right| \\leq \\frac{1}{2} \\) are all disjoint by hypothesis, and for \\( totality \\in singleton \\) these discs all lie in the annulus\n\\[\n\\left\\{realaxis: constant-\\frac{1}{2} \\leq|realaxis| \\leq constant+\\frac{3}{2}\\right\\},\n\\]\n(a disc if \\( constant=0 \\) ). Let the cardinality of the set \\( singleton \\) be denoted by \\( \\left|singleton\\right| \\). Then adding areas gives\n\\[\n\\left|singleton\\right| \\frac{\\pi}{4} \\leq \\pi\\left[\\left(constant+\\frac{3}{2}\\right)^{2}-\\left(constant-\\frac{1}{2}\\right)^{2}\\right]=2 \\pi(2 constant+1)\n\\]\nso that \\( \\left|singleton\\right| \\leq 8(2 constant+1) \\) for \\( constant>0 \\). A separate calculation shows that \\( \\left|solitary\\right| \\leq 9 \\).\nThen\n\\[\n\\sum_{totality \\in singleton} \\frac{1}{\\left|terminus\\right|^{3}} \\leq \\frac{\\left|singleton\\right|}{constant^{3}} \\leq \\frac{8(2 constant+1)}{constant^{3}} \\leq \\frac{24}{constant^{2}}\n\\]\nfor \\( constant \\geq 1 \\) because \\( 2 constant+1 \\leq 3 constant \\). Since \\( solitary \\) is finite,\n\\[\n\\sum_{totality \\in solitary} \\frac{1}{\\left|terminus\\right|^{3}}\n\\]\nis finite.\nHence we have\n\\[\n\\sum_{totality=1}^{\\infty} \\frac{1}{\\left|terminus\\right|^{3}}=\\sum_{constant=0}^{\\infty} \\sum_{totality \\in singleton} \\frac{1}{\\left|terminus\\right|^{3}} \\leq \\sum_{totality \\in solitary} \\frac{1}{\\left|terminus\\right|^{3}}+\\sum_{constant=1}^{\\infty} \\frac{24}{constant^{2}}<\\infty .\n\\]\n\nThe rearrangement of the sum in the first step is permissible since the terms are all positive. Thus the original series converges absolutely."
    },
    "garbled_string": {
      "map": {
        "k": "zxqvbnmpl",
        "n": "rfgthjklq",
        "r": "plmoknijb",
        "s": "qazwsxedc",
        "z": "ujmnhytre",
        "S_k": "lpkjihgfs",
        "S_0": "rewqasdfg",
        "a_n": "bnmlkjhgf",
        "a_r": "vfrtgbhyn",
        "a_s": "ctfvgbhyu"
      },
      "question": "3. Assume that the complex numbers \\( a_{1}, a_{2}, \\ldots, bnmlkjhgf, \\ldots \\) are all different from zero, and that \\( \\left|vfrtgbhyn-ctfvgbhyu\\right|>1 \\) for \\( plmoknijb \\neq qazwsxedc \\). Show that the series\n\\[\n\\sum_{rfgthjklq=1}^{\\infty} \\frac{1}{bnmlkjhgf{ }^{3}}\n\\]\nconverges.",
      "solution": "Solution. Let \\( lpkjihgfs=\\left\\{rfgthjklq: zxqvbnmpl<\\left|bnmlkjhgf\\right| \\leq zxqvbnmpl+1\\right\\} \\) for \\( zxqvbnmpl=0,1,2, \\ldots \\). The discs \\( \\left|ujmnhytre-bnmlkjhgf\\right| \\leq \\frac{1}{2} \\) are all disjoint by hypothesis, and for \\( rfgthjklq \\in lpkjihgfs \\) these discs all lie in the annulus\n\\[\n\\left\\{ujmnhytre: zxqvbnmpl-\\frac{1}{2} \\leq|ujmnhytre| \\leq zxqvbnmpl+\\frac{3}{2}\\right\\},\n\\]\n(a disc if \\( zxqvbnmpl=0 \\) ). Let the cardinality of the set \\( lpkjihgfs \\) be denoted by \\( \\left|lpkjihgfs\\right| \\). Then adding areas gives\n\\[\n\\left|lpkjihgfs\\right| \\frac{\\pi}{4} \\leq \\pi\\left[\\left(zxqvbnmpl+\\frac{3}{2}\\right)^{2}-\\left(zxqvbnmpl-\\frac{1}{2}\\right)^{2}\\right]=2 \\pi(2 zxqvbnmpl+1)\n\\]\nso that \\( \\left|lpkjihgfs\\right| \\leq 8(2 zxqvbnmpl+1) \\) for \\( zxqvbnmpl>0 \\). A separate calculation shows that \\( \\left|rewqasdfg\\right| \\leq 9 \\).\nThen\n\\[\n\\sum_{rfgthjklq \\in lpkjihgfs} \\frac{1}{\\left|bnmlkjhgf\\right|^{3}} \\leq \\frac{\\left|lpkjihgfs\\right|}{zxqvbnmpl^{3}} \\leq \\frac{8(2 zxqvbnmpl+1)}{zxqvbnmpl^{3}} \\leq \\frac{24}{zxqvbnmpl^{2}}\n\\]\nfor \\( zxqvbnmpl \\geq 1 \\) because \\( 2 zxqvbnmpl+1 \\leq 3 zxqvbnmpl \\). Since \\( rewqasdfg \\) is finite,\n\\[\n\\sum_{rfgthjklq \\in rewqasdfg} \\frac{1}{\\left|bnmlkjhgf\\right|^{3}}\n\\]\nis finite.\nHence we have\n\\[\n\\sum_{rfgthjklq=1}^{\\infty} \\frac{1}{\\left|bnmlkjhgf\\right|^{3}}=\\sum_{zxqvbnmpl=0}^{\\infty} \\sum_{rfgthjklq \\in lpkjihgfs} \\frac{1}{\\left|bnmlkjhgf\\right|^{3}} \\leq \\sum_{rfgthjklq \\in rewqasdfg} \\frac{1}{\\left|bnmlkjhgf\\right|^{3}}+\\sum_{zxqvbnmpl=1}^{\\infty} \\frac{24}{zxqvbnmpl^{2}}<\\infty .\n\\]\n\nThe rearrangement of the sum in the first step is permissible since the terms are all positive. Thus the original series converges absolutely."
    },
    "kernel_variant": {
      "question": "Let $\\,(c_n)_{n\\ge 1}$ be a sequence of pair-wise distinct, non–zero complex numbers that satisfies the variable–gap condition  \n\\[\n\\tag{\\ast}\\label{gap}\n|c_r-c_s|>\\bigl(1+|c_r|\\;|c_s|\\bigr)^{-1/2}\\qquad(r\\neq s).\n\\]\n\n(a) Prove that for every real number $\\varepsilon>0$ the series  \n\\[\n\\sum_{n=1}^{\\infty}\\frac{1}{|c_n|^{\\,4+\\varepsilon}}\n\\]\nconverges absolutely.\n\n(b) Show that the exponent $4$ is optimal by constructing an explicit sequence $\\,(d_n)_{n\\ge 1}$ that still satisfies \\eqref{gap} but for which  \n\\[\n\\sum_{n=1}^{\\infty}\\frac{1}{|d_n|^{\\,4}}\n\\]\ndiverges.",
      "solution": "Write, for every integer $k\\ge 0$,  \n\\[\nS_k:=\\bigl\\{n:\\,k<|c_n|\\le k+1\\bigr\\},\\qquad N_k:=|S_k|.\n\\]\n\nPart (a) – convergence for every exponent $\\,4+\\varepsilon$  \n----------------------------------------------------------\n\nStep 1 – Disjoint discs with shell-dependent radius.  \nFix $k\\ge 0$ and define  \n\\[\nr_k:=\\frac12\\bigl(1+(k+1)^2\\bigr)^{-1/2}.\n\\tag{1}\n\\]\nIf $r,s\\in S_k$ with $r\\neq s$ then $|c_r|,|c_s|\\le k+1$, and \\eqref{gap} gives  \n\\[\n|c_r-c_s|>\\bigl(1+|c_r|\\;|c_s|\\bigr)^{-1/2}\\ge\\bigl(1+(k+1)^2\\bigr)^{-1/2}=2r_k .\n\\]\nHence, for every $n\\in S_k$, the closed discs  \n\\[\n\\Delta_n:=\\bigl\\{z\\in\\Bbb C:\\,|z-c_n|\\le r_k\\bigr\\}\n\\tag{2}\n\\]\nare pair-wise disjoint.\n\nStep 2 – Locating the discs (for $k\\ge 1$).  \nBecause $|c_n|\\le k+1$ and $r_k\\le\\dfrac{1}{2(k+1)}<1$ when $k\\ge 1$, each $z\\in\\Delta_n$ satisfies  \n\\[\nk-r_k<|z|<k+1+r_k .\n\\]\nConsequently all discs with indices in $S_k$ lie in the annulus  \n\\[\n\\mathcal A_k:=\\bigl\\{z\\in\\Bbb C:\\,k-r_k<|z|<k+1+r_k\\bigr\\}.\n\\]\n\nStep 3 – A packing estimate for $N_k$ ($k\\ge 1$).  \nEvery disc $\\Delta_n$ has area $\\pi r_k^{2}$, whereas  \n\\[\n\\operatorname{area}(\\mathcal A_k)=\\pi\\bigl[(k+1+r_k)^2-(k-r_k)^2\\bigr]=(2k+1)\\pi(1+2r_k).\n\\]\nThus  \n\\[\nN_k\\,\\pi r_k^{2}\\le(2k+1)\\pi(1+2r_k)\n\\Longrightarrow\nN_k\\le\\frac{(2k+1)(1+2r_k)}{r_k^{2}} .\n\\tag{3}\n\\]\n\nFor every $k\\ge 1$\n\\[\n\\frac1{r_k^{2}}\n      =4\\bigl(1+(k+1)^2\\bigr)\n      \\le 8(k+1)^2,\n\\qquad\n2k+1\\le2(k+1),\n\\qquad\n1+2r_k\\le1+\\frac{1}{k+1}\\le\\frac32 .\n\\]\nInserting these bounds into \\eqref{3} yields  \n\\[\nN_k\\le 24\\,(k+1)^3\\qquad(k\\ge 1).\n\\tag{4}\n\\]\n\nStep 4 – The inner shell $k=0$.  \nFor $k=0$ the radius from \\eqref{1} is\n\\[\nr_0=\\frac{1}{2\\sqrt2}.\n\\]\nBecause \\eqref{gap} implies $|c_r-c_s|>1/\\sqrt2$ for $r\\neq s$, the discs  \n\\[\n\\bigl\\{z\\in\\Bbb C:\\,|z-c_n|\\le r_0\\bigr\\},\\qquad n\\in S_0,\n\\]\nare pair-wise disjoint.  They all lie in the larger disc $|z|\\le 1+r_0$.  Therefore  \n\\[\nN_0\\,\\pi r_0^{2}\\le\\pi(1+r_0)^{2}\n\\quad\\Longrightarrow\\quad\nN_0\\le\\frac{(1+r_0)^{2}}{r_0^{2}}<\\infty .\n\\]\n\nStep 5 – Estimating the tail of the series.  \nLet $\\varepsilon>0$.  For $k\\ge 1$ we obtain from \\eqref{4}  \n\\[\n\\sum_{n\\in S_k}\\frac1{|c_n|^{\\,4+\\varepsilon}}\n   \\le\\frac{N_k}{k^{\\,4+\\varepsilon}}\n   \\le\\frac{24(k+1)^3}{k^{\\,4+\\varepsilon}}\n   \\le 192\\,k^{-(1+\\varepsilon)} .\n\\]\nHence  \n\\[\n\\sum_{n=1}^{\\infty}\\frac1{|c_n|^{\\,4+\\varepsilon}}\n   =\\sum_{k=0}^{\\infty}\\sum_{n\\in S_k}\\frac1{|c_n|^{\\,4+\\varepsilon}}\n   \\le\\sum_{n\\in S_0}\\frac1{|c_n|^{\\,4+\\varepsilon}}\n        +192\\sum_{k=1}^{\\infty}k^{-(1+\\varepsilon)}<\\infty ,\n\\]\nbecause the outer series is a $p$-series with exponent $1+\\varepsilon>1$.  \nThus part (a) is proved.\n\n\n\nPart (b) – optimality of the exponent $\\,4$  \n-------------------------------------------\n\nConstruction of a critical sequence.  \nFor every integer $k\\ge 2$ put  \n\\[\nm_k:=k^{2},\\qquad\nr_{k,s}:=k+\\frac{s}{k},\\qquad s=0,1,\\dots ,k-1 .\n\\]\nDefine the lattice-type family  \n\\[\nd_{k,s,j}:=r_{k,s}\\,e^{2\\pi i j/m_k},\n\\qquad\nj=0,1,\\dots ,m_k-1 .\n\\]\nEnumerate the points $\\{d_{k,s,j}\\}_{k\\ge 2,\\;0\\le s\\le k-1,\\;0\\le j<m_k}$ arbitrarily and finally set $d_1:=1$.  \nThe resulting sequence $(d_n)_{n\\ge 1}$ is pair-wise distinct.\n\n1.  Verification of the separation condition \\eqref{gap}.  \n\nFix $k\\ge 2$.\n\n(a)  Points on the same circle $r_{k,s}$:  \n\\[\n|d_{k,s,j}-d_{k,s,\\ell}|\n   =2r_{k,s}\\sin\\!\\Bigl(\\frac{\\pi|j-\\ell|}{m_k}\\Bigr)\n   \\ge 2r_{k,s}\\sin\\!\\Bigl(\\frac{\\pi}{m_k}\\Bigr)\n   \\ge 2k\\cdot\\frac{2}{\\pi}\\cdot\\frac{\\pi}{m_k}\n   =\\frac{4}{k}\n   \\ge(1+k^{2})^{-1/2}.\n\\]\n\n(b)  Points on different circles within the same shell $k$:  \nIf $s\\neq t$ then $|r_{k,s}-r_{k,t}|=\\dfrac{|s-t|}{k}\\ge\\dfrac1{k}$, so  \n\\[\n|d_{k,s,j}-d_{k,t,\\ell}|\\ge\\frac1{k}\\ge(1+k^{2})^{-1/2}.\n\\]\n\n(c)  Points belonging to distinct shells $k\\neq k'$.  \nAssume $k'<k$.  The smallest possible distance of moduli occurs for $k'=k-1$ when the outermost circle of shell $k-1$ and the innermost circle of shell $k$ are chosen.  \nThe outermost admissible circle in shell $k-1$ is $r_{k-1,k-2}=k-\\dfrac1{k-1}$, whence  \n\\[\n\\bigl|\\,|d_{k,0,j}|-|d_{k-1,k-2,\\ell}|\\,\\bigr|\n   =r_{k,0}-r_{k-1,k-2}\n   =\\frac{1}{k-1}\\ge\\frac1{k}\\ge(1+k^{2})^{-1/2}.\n\\]\nIf $k'=1$ the lower bound is even larger.  \n\n(d)  The special point $d_{1}=1$ satisfies $|d_{1}-d_{k,s,j}|\\ge r_{k,s}-1\\ge k-1\\ge (1+k^{2})^{-1/2}$ for every $k\\ge 2$.  \n\nConsequently the full sequence $(d_n)$ satisfies \\eqref{gap}.\n\n2.  Divergence of the series with exponent $4$.  \nEvery $d_{k,s,j}$ has modulus $r_{k,s}\\in(k,k+1]$.  Thus  \n\\[\n\\sum_{n=1}^{\\infty}\\frac1{|d_n|^{4}}\n  \\ge\\sum_{k=2}^{\\infty}\\;\\sum_{s=0}^{k-1}\\;\\sum_{j=0}^{m_k-1}\n       \\frac1{(k+1)^{4}}\n  =\\sum_{k=2}^{\\infty}\\frac{k\\cdot m_k}{(k+1)^{4}}\n  =\\sum_{k=2}^{\\infty}\\frac{k\\cdot k^{2}}{(k+1)^{4}}\n  \\asymp\\sum_{k=2}^{\\infty}\\frac1{k}\n  =\\infty .\n\\]\nTherefore the exponent $4$ is critical: part (a) shows that any exponent $4+\\varepsilon$ ($\\varepsilon>0$) enforces convergence for all sequences satisfying \\eqref{gap}, whereas the explicit sequence $(d_n)$ makes the series with exponent $4$ diverge.  Part (b) is complete.",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.424645",
        "was_fixed": false,
        "difficulty_analysis": "1. Variable separation –  Unlike the original “constant–gap’’ condition \\(|a_r-a_s|>1\\), the inequality (\\*) allows the permissible distance between points to shrink like \\(O((|c_r|\\;|c_s|)^{-1/2})\\).  Handling discs whose radii now depend on the size of the centres requires a non-uniform geometric estimate and a careful comparison between the area of an annulus and the varying areas of the corresponding discs.\n\n2. Optimal exponent & two-sided result –  The problem no longer asks merely for a single convergence statement.  It demands:\n   • convergence for an entire one–parameter family of exponents \\(3+\\varepsilon\\), and  \n   • a constructive demonstration that the threshold \\(3\\) itself cannot be improved.  \n   The second requirement forces the solver to devise an explicit high-density configuration that still respects the shrinking–gap condition.\n\n3. Tighter counting bounds –  Because the disc radii vary with \\(|c_n|\\), bounding \\(|S_k|\\) needs a lower estimate for \\(\\rho_n\\) and therefore leads to a cubic growth bound \\(\\lvert S_k\\rvert\\ll k^{3}\\), rather than the linear bound in the original problem.\n\n4. Additional techniques –  \n   • Variable-radius packing arguments in the plane;  \n   • Careful asymptotics to verify (\\*) for the constructed divergent example;  \n   • Series tests that hinge on uniform geometric estimates rather than constant ones.\n\nOverall, the enhanced variant intertwines sophisticated geometric counting with an optimality argument, demanding deeper insight and several more steps than the original constant–gap convergence problem."
      }
    },
    "original_kernel_variant": {
      "question": "Let $\\,(c_n)_{n\\ge 1}$ be a sequence of pair-wise distinct, non–zero complex numbers that satisfies the variable–gap condition  \n\\[\n\\tag{\\ast}\\label{gap}\n|c_r-c_s|>\\bigl(1+|c_r|\\;|c_s|\\bigr)^{-1/2}\\qquad(r\\neq s).\n\\]\n\n(a) Prove that for every real number $\\varepsilon>0$ the series  \n\\[\n\\sum_{n=1}^{\\infty}\\frac{1}{|c_n|^{\\,4+\\varepsilon}}\n\\]\nconverges absolutely.\n\n(b) Show that the exponent $4$ is optimal by constructing an explicit sequence $\\,(d_n)_{n\\ge 1}$ that still satisfies \\eqref{gap} but for which  \n\\[\n\\sum_{n=1}^{\\infty}\\frac{1}{|d_n|^{\\,4}}\n\\]\ndiverges.",
      "solution": "Write, for every integer $k\\ge 0$,  \n\\[\nS_k:=\\bigl\\{n:\\,k<|c_n|\\le k+1\\bigr\\},\\qquad N_k:=|S_k|.\n\\]\n\nPart (a) – convergence for every exponent $\\,4+\\varepsilon$  \n----------------------------------------------------------\n\nStep 1 – Disjoint discs with shell-dependent radius.  \nFix $k\\ge 0$ and define  \n\\[\nr_k:=\\frac12\\bigl(1+(k+1)^2\\bigr)^{-1/2}.\n\\tag{1}\n\\]\nIf $r,s\\in S_k$ with $r\\neq s$ then $|c_r|,|c_s|\\le k+1$, and \\eqref{gap} gives  \n\\[\n|c_r-c_s|>\\bigl(1+|c_r|\\;|c_s|\\bigr)^{-1/2}\\ge\\bigl(1+(k+1)^2\\bigr)^{-1/2}=2r_k .\n\\]\nHence, for every $n\\in S_k$, the closed discs  \n\\[\n\\Delta_n:=\\bigl\\{z\\in\\Bbb C:\\,|z-c_n|\\le r_k\\bigr\\}\n\\tag{2}\n\\]\nare pair-wise disjoint.\n\nStep 2 – Locating the discs (for $k\\ge 1$).  \nBecause $|c_n|\\le k+1$ and $r_k\\le\\dfrac{1}{2(k+1)}<1$ when $k\\ge 1$, each $z\\in\\Delta_n$ satisfies  \n\\[\nk-r_k<|z|<k+1+r_k .\n\\]\nConsequently all discs with indices in $S_k$ lie in the annulus  \n\\[\n\\mathcal A_k:=\\bigl\\{z\\in\\Bbb C:\\,k-r_k<|z|<k+1+r_k\\bigr\\}.\n\\]\n\nStep 3 – A packing estimate for $N_k$ ($k\\ge 1$).  \nEvery disc $\\Delta_n$ has area $\\pi r_k^{2}$, whereas  \n\\[\n\\operatorname{area}(\\mathcal A_k)=\\pi\\bigl[(k+1+r_k)^2-(k-r_k)^2\\bigr]=(2k+1)\\pi(1+2r_k).\n\\]\nThus  \n\\[\nN_k\\,\\pi r_k^{2}\\le(2k+1)\\pi(1+2r_k)\n\\Longrightarrow\nN_k\\le\\frac{(2k+1)(1+2r_k)}{r_k^{2}} .\n\\tag{3}\n\\]\n\nFor every $k\\ge 1$\n\\[\n\\frac1{r_k^{2}}\n      =4\\bigl(1+(k+1)^2\\bigr)\n      \\le 8(k+1)^2,\n\\qquad\n2k+1\\le2(k+1),\n\\qquad\n1+2r_k\\le1+\\frac{1}{k+1}\\le\\frac32 .\n\\]\nInserting these bounds into \\eqref{3} yields  \n\\[\nN_k\\le 24\\,(k+1)^3\\qquad(k\\ge 1).\n\\tag{4}\n\\]\n\nStep 4 – The inner shell $k=0$.  \nFor $k=0$ the radius from \\eqref{1} is\n\\[\nr_0=\\frac{1}{2\\sqrt2}.\n\\]\nBecause \\eqref{gap} implies $|c_r-c_s|>1/\\sqrt2$ for $r\\neq s$, the discs  \n\\[\n\\bigl\\{z\\in\\Bbb C:\\,|z-c_n|\\le r_0\\bigr\\},\\qquad n\\in S_0,\n\\]\nare pair-wise disjoint.  They all lie in the larger disc $|z|\\le 1+r_0$.  Therefore  \n\\[\nN_0\\,\\pi r_0^{2}\\le\\pi(1+r_0)^{2}\n\\quad\\Longrightarrow\\quad\nN_0\\le\\frac{(1+r_0)^{2}}{r_0^{2}}<\\infty .\n\\]\n\nStep 5 – Estimating the tail of the series.  \nLet $\\varepsilon>0$.  For $k\\ge 1$ we obtain from \\eqref{4}  \n\\[\n\\sum_{n\\in S_k}\\frac1{|c_n|^{\\,4+\\varepsilon}}\n   \\le\\frac{N_k}{k^{\\,4+\\varepsilon}}\n   \\le\\frac{24(k+1)^3}{k^{\\,4+\\varepsilon}}\n   \\le 192\\,k^{-(1+\\varepsilon)} .\n\\]\nHence  \n\\[\n\\sum_{n=1}^{\\infty}\\frac1{|c_n|^{\\,4+\\varepsilon}}\n   =\\sum_{k=0}^{\\infty}\\sum_{n\\in S_k}\\frac1{|c_n|^{\\,4+\\varepsilon}}\n   \\le\\sum_{n\\in S_0}\\frac1{|c_n|^{\\,4+\\varepsilon}}\n        +192\\sum_{k=1}^{\\infty}k^{-(1+\\varepsilon)}<\\infty ,\n\\]\nbecause the outer series is a $p$-series with exponent $1+\\varepsilon>1$.  \nThus part (a) is proved.\n\n\n\nPart (b) – optimality of the exponent $\\,4$  \n-------------------------------------------\n\nConstruction of a critical sequence.  \nFor every integer $k\\ge 2$ put  \n\\[\nm_k:=k^{2},\\qquad\nr_{k,s}:=k+\\frac{s}{k},\\qquad s=0,1,\\dots ,k-1 .\n\\]\nDefine the lattice-type family  \n\\[\nd_{k,s,j}:=r_{k,s}\\,e^{2\\pi i j/m_k},\n\\qquad\nj=0,1,\\dots ,m_k-1 .\n\\]\nEnumerate the points $\\{d_{k,s,j}\\}_{k\\ge 2,\\;0\\le s\\le k-1,\\;0\\le j<m_k}$ arbitrarily and finally set $d_1:=1$.  \nThe resulting sequence $(d_n)_{n\\ge 1}$ is pair-wise distinct.\n\n1.  Verification of the separation condition \\eqref{gap}.  \n\nFix $k\\ge 2$.\n\n(a)  Points on the same circle $r_{k,s}$:  \n\\[\n|d_{k,s,j}-d_{k,s,\\ell}|\n   =2r_{k,s}\\sin\\!\\Bigl(\\frac{\\pi|j-\\ell|}{m_k}\\Bigr)\n   \\ge 2r_{k,s}\\sin\\!\\Bigl(\\frac{\\pi}{m_k}\\Bigr)\n   \\ge 2k\\cdot\\frac{2}{\\pi}\\cdot\\frac{\\pi}{m_k}\n   =\\frac{4}{k}\n   \\ge(1+k^{2})^{-1/2}.\n\\]\n\n(b)  Points on different circles within the same shell $k$:  \nIf $s\\neq t$ then $|r_{k,s}-r_{k,t}|=\\dfrac{|s-t|}{k}\\ge\\dfrac1{k}$, so  \n\\[\n|d_{k,s,j}-d_{k,t,\\ell}|\\ge\\frac1{k}\\ge(1+k^{2})^{-1/2}.\n\\]\n\n(c)  Points belonging to distinct shells $k\\neq k'$.  \nAssume $k'<k$.  The smallest possible distance of moduli occurs for $k'=k-1$ when the outermost circle of shell $k-1$ and the innermost circle of shell $k$ are chosen.  \nThe outermost admissible circle in shell $k-1$ is $r_{k-1,k-2}=k-\\dfrac1{k-1}$, whence  \n\\[\n\\bigl|\\,|d_{k,0,j}|-|d_{k-1,k-2,\\ell}|\\,\\bigr|\n   =r_{k,0}-r_{k-1,k-2}\n   =\\frac{1}{k-1}\\ge\\frac1{k}\\ge(1+k^{2})^{-1/2}.\n\\]\nIf $k'=1$ the lower bound is even larger.  \n\n(d)  The special point $d_{1}=1$ satisfies $|d_{1}-d_{k,s,j}|\\ge r_{k,s}-1\\ge k-1\\ge (1+k^{2})^{-1/2}$ for every $k\\ge 2$.  \n\nConsequently the full sequence $(d_n)$ satisfies \\eqref{gap}.\n\n2.  Divergence of the series with exponent $4$.  \nEvery $d_{k,s,j}$ has modulus $r_{k,s}\\in(k,k+1]$.  Thus  \n\\[\n\\sum_{n=1}^{\\infty}\\frac1{|d_n|^{4}}\n  \\ge\\sum_{k=2}^{\\infty}\\;\\sum_{s=0}^{k-1}\\;\\sum_{j=0}^{m_k-1}\n       \\frac1{(k+1)^{4}}\n  =\\sum_{k=2}^{\\infty}\\frac{k\\cdot m_k}{(k+1)^{4}}\n  =\\sum_{k=2}^{\\infty}\\frac{k\\cdot k^{2}}{(k+1)^{4}}\n  \\asymp\\sum_{k=2}^{\\infty}\\frac1{k}\n  =\\infty .\n\\]\nTherefore the exponent $4$ is critical: part (a) shows that any exponent $4+\\varepsilon$ ($\\varepsilon>0$) enforces convergence for all sequences satisfying \\eqref{gap}, whereas the explicit sequence $(d_n)$ makes the series with exponent $4$ diverge.  Part (b) is complete.",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.368878",
        "was_fixed": false,
        "difficulty_analysis": "1. Variable separation –  Unlike the original “constant–gap’’ condition \\(|a_r-a_s|>1\\), the inequality (\\*) allows the permissible distance between points to shrink like \\(O((|c_r|\\;|c_s|)^{-1/2})\\).  Handling discs whose radii now depend on the size of the centres requires a non-uniform geometric estimate and a careful comparison between the area of an annulus and the varying areas of the corresponding discs.\n\n2. Optimal exponent & two-sided result –  The problem no longer asks merely for a single convergence statement.  It demands:\n   • convergence for an entire one–parameter family of exponents \\(3+\\varepsilon\\), and  \n   • a constructive demonstration that the threshold \\(3\\) itself cannot be improved.  \n   The second requirement forces the solver to devise an explicit high-density configuration that still respects the shrinking–gap condition.\n\n3. Tighter counting bounds –  Because the disc radii vary with \\(|c_n|\\), bounding \\(|S_k|\\) needs a lower estimate for \\(\\rho_n\\) and therefore leads to a cubic growth bound \\(\\lvert S_k\\rvert\\ll k^{3}\\), rather than the linear bound in the original problem.\n\n4. Additional techniques –  \n   • Variable-radius packing arguments in the plane;  \n   • Careful asymptotics to verify (\\*) for the constructed divergent example;  \n   • Series tests that hinge on uniform geometric estimates rather than constant ones.\n\nOverall, the enhanced variant intertwines sophisticated geometric counting with an optimality argument, demanding deeper insight and several more steps than the original constant–gap convergence problem."
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}