{
  "index": "1973-A-2",
  "type": "ANA",
  "tag": [
    "ANA",
    "ALG"
  ],
  "difficulty": "",
  "question": "A-2. Consider an infinite series whose \\( n \\)th term is \\( \\pm(1 / n) \\), the \\( \\pm \\) signs being determined according to a pattern that repeats periodically in blocks of eight. [There are \\( 2^{8} \\) possible patterns of which two examples are:\n\\[\n\\begin{array}{l}\n++----++ \\\\\n+---+---\n\\end{array}\n\\]\n\nThe first example would generate the series\n\\[\n\\begin{array}{l}\n1+(1 / 2)-(1 / 3)-(1 / 4)-(1 / 5)-(1 / 6)+(1 / 7)+(1 / 8) \\\\\n+(1 / 9)+(1 / 10)-(1 / 11)-(1 / 12)-\\cdots .]\n\\end{array}\n\\]\n(a) Show that a sufficient condition for the series to be conditionally convergent is that there be four \"+\" signs and four \"-\" signs in the block of eight.\n(b) Is this sufficient condition also necessary?\n[Here \"convergent\" means \"convergent to a finite limit.\"]",
  "solution": "A-2. The ideas in both parts are similar and the answer in (b) is \"Yes.\" Let \\( u_{n} \\) be the \\( n \\)th term \\( \\pm 1 / n \\) and \\( S_{n}=u_{1}+\\cdots+u_{n} \\). Since \\( u_{n} \\rightarrow 0 \\) as \\( n \\rightarrow \\infty,\\left\\{S_{n}\\right\\} \\) will converge if and only if \\( \\left\\{S_{8 m}\\right\\} \\) does. Using the facts that\n\\[\n\\frac{1}{n}-\\frac{1}{n+k}=\\frac{k}{n(n+k)}\n\\]\nthat \\( \\Sigma\\left(1 / n^{2}\\right) \\) converges, and that \\( \\Sigma(1 / n) \\) diverges, one shows that with four \" + \" signs and four \" - \" signs in each block, \\( \\left\\{S_{8 m}\\right\\} \\) converges as the term-by-term sum of four convergent sequences while an imbalance of signs makes \\( \\left\\{S_{8 m}\\right\\} \\) divergent as the sum of a convergent and a divergent sequence.",
  "vars": [
    "n",
    "u_n",
    "S_n",
    "k",
    "m",
    "S_8m"
  ],
  "params": [],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "n": "indexvar",
        "u_n": "termvalue",
        "S_n": "partialsum",
        "k": "diffindex",
        "m": "blockindex",
        "S_8m": "partialsumblock"
      },
      "question": "A-2. Consider an infinite series whose \\( indexvar \\)th term is \\( \\pm(1 / indexvar) \\), the \\( \\pm \\) signs being determined according to a pattern that repeats periodically in blocks of eight. [There are \\( 2^{8} \\) possible patterns of which two examples are:\n\\[\n\\begin{array}{l}\n++----++ \\\\\n+---+---\n\\end{array}\n\\]\n\nThe first example would generate the series\n\\[\n\\begin{array}{l}\n1+(1 / 2)-(1 / 3)-(1 / 4)-(1 / 5)-(1 / 6)+(1 / 7)+(1 / 8) \\\\\n+(1 / 9)+(1 / 10)-(1 / 11)-(1 / 12)-\\cdots .\n\\end{array}\n\\]\n(a) Show that a sufficient condition for the series to be conditionally convergent is that there be four \"+\" signs and four \"-\" signs in the block of eight.\n(b) Is this sufficient condition also necessary?\n[Here \"convergent\" means \"convergent to a finite limit.\"]",
      "solution": "A-2. The ideas in both parts are similar and the answer in (b) is \"Yes.\" Let \\( termvalue \\) be the \\( indexvar \\)th term \\( \\pm 1 / indexvar \\) and \\( partialsum = u_{1}+\\cdots+u_{indexvar} \\). Since \\( termvalue \\rightarrow 0 \\) as \\( indexvar \\rightarrow \\infty,\\left\\{partialsum\\right\\} \\) will converge if and only if \\( \\left\\{partialsumblock\\right\\} \\) does. Using the facts that\n\\[\n\\frac{1}{indexvar}-\\frac{1}{indexvar+diffindex}=\\frac{diffindex}{indexvar(indexvar+diffindex)}\n\\]\nthat \\( \\Sigma\\left(1 / indexvar^{2}\\right) \\) converges, and that \\( \\Sigma(1 / indexvar) \\) diverges, one shows that with four \"+\" signs and four \"-\" signs in each block, \\( \\left\\{partialsumblock\\right\\} \\) converges as the term-by-term sum of four convergent sequences while an imbalance of signs makes \\( \\left\\{partialsumblock\\right\\} \\) divergent as the sum of a convergent and a divergent sequence."
    },
    "descriptive_long_confusing": {
      "map": {
        "n": "pinecone",
        "u_n": "sandalwood",
        "S_n": "lighthouse",
        "k": "butterfly",
        "m": "pineapple",
        "S_8m": "cinnamonroll"
      },
      "question": "A-2. Consider an infinite series whose \\( pinecone \\)th term is \\( \\pm(1 / pinecone) \\), the \\( \\pm \\) signs being determined according to a pattern that repeats periodically in blocks of eight. [There are \\( 2^{8} \\) possible patterns of which two examples are:\n\\[\n\\begin{array}{l}\n++----++ \\\\\n+---+---\n\\end{array}\n\\]\n\nThe first example would generate the series\n\\[\n\\begin{array}{l}\n1+(1 / 2)-(1 / 3)-(1 / 4)-(1 / 5)-(1 / 6)+(1 / 7)+(1 / 8) \\\\\n+(1 / 9)+(1 / 10)-(1 / 11)-(1 / 12)-\\cdots .]\n\\end{array}\n\\]\n(a) Show that a sufficient condition for the series to be conditionally convergent is that there be four \"+\" signs and four \"-\" signs in the block of eight.\n(b) Is this sufficient condition also necessary?\n[Here \"convergent\" means \"convergent to a finite limit.\"]",
      "solution": "A-2. The ideas in both parts are similar and the answer in (b) is \"Yes.\" Let \\( sandalwood \\) be the \\( pinecone \\)th term \\( \\pm 1 / pinecone \\) and \\( lighthouse =u_{1}+\\cdots+sandalwood \\). Since \\( sandalwood \\rightarrow 0 \\) as \\( pinecone \\rightarrow \\infty,\\left\\{lighthouse \\right\\} \\) will converge if and only if \\( \\left\\{cinnamonroll \\right\\} \\) does. Using the facts that\n\\[\n\\frac{1}{pinecone}-\\frac{1}{pinecone+butterfly}=\\frac{butterfly}{pinecone(pinecone+butterfly)}\n\\]\nthat \\( \\Sigma\\left(1 / pinecone^{2}\\right) \\) converges, and that \\( \\Sigma(1 / pinecone) \\) diverges, one shows that with four \" + \" signs and four \" - \" signs in each block, \\( \\left\\{cinnamonroll \\right\\} \\) converges as the term-by-term sum of four convergent sequences while an imbalance of signs makes \\( \\left\\{cinnamonroll \\right\\} \\) divergent as the sum of a convergent and a divergent sequence."
    },
    "descriptive_long_misleading": {
      "map": {
        "n": "continuous",
        "u_n": "stableterm",
        "S_n": "stagnanttotal",
        "k": "fixedamount",
        "m": "massivevalue",
        "S_8m": "eightfoldsummary"
      },
      "question": "A-2. Consider an infinite series whose \\( continuous \\)th term is \\( \\pm(1 / continuous) \\), the \\( \\pm \\) signs being determined according to a pattern that repeats periodically in blocks of eight. [There are \\( 2^{8} \\) possible patterns of which two examples are:\n\\[\n\\begin{array}{l}\n++----++ \\\\\n+---+---\n\\end{array}\n\\]\n\nThe first example would generate the series\n\\[\n\\begin{array}{l}\n1+(1 / 2)-(1 / 3)-(1 / 4)-(1 / 5)-(1 / 6)+(1 / 7)+(1 / 8) \\\\\n+(1 / 9)+(1 / 10)-(1 / 11)-(1 / 12)-\\cdots .]\n\\end{array}\n\\]\n(a) Show that a sufficient condition for the series to be conditionally convergent is that there be four \"+\" signs and four \"-\" signs in the block of eight.\n(b) Is this sufficient condition also necessary?\n[Here \"convergent\" means \"convergent to a finite limit.\"]",
      "solution": "A-2. The ideas in both parts are similar and the answer in (b) is \"Yes.\" Let \\( stableterm_{continuous} \\) be the \\( continuous \\)th term \\( \\pm 1 / continuous \\) and \\( stagnanttotal_{continuous}=stableterm_{1}+\\cdots+stableterm_{continuous} \\). Since \\( stableterm_{continuous} \\rightarrow 0 \\) as \\( continuous \\rightarrow \\infty,\\left\\{stagnanttotal_{continuous}\\right\\} \\) will converge if and only if \\( \\left\\{eightfoldsummary\\right\\} \\) does. Using the facts that\n\\[\n\\frac{1}{continuous}-\\frac{1}{continuous+fixedamount}=\\frac{fixedamount}{continuous(continuous+fixedamount)}\n\\]\nthat \\( \\Sigma\\left(1 / continuous^{2}\\right) \\) converges, and that \\( \\Sigma(1 / continuous) \\) diverges, one shows that with four \" + \" signs and four \" - \" signs in each block, \\( \\left\\{eightfoldsummary\\right\\} \\) converges as the term-by-term sum of four convergent sequences while an imbalance of signs makes \\( \\left\\{eightfoldsummary\\right\\} \\) divergent as the sum of a convergent and a divergent sequence."
    },
    "garbled_string": {
      "map": {
        "n": "qzxwvtnp",
        "u_n": "hjgrksla",
        "S_n": "bdcmrtyu",
        "k": "vploqjse",
        "m": "zkgfynrt",
        "S_8m": "lswxphqr"
      },
      "question": "A-2. Consider an infinite series whose \\( qzxwvtnp \\)th term is \\( \\pm(1 / qzxwvtnp) \\), the \\( \\pm \\) signs being determined according to a pattern that repeats periodically in blocks of eight. [There are \\( 2^{8} \\) possible patterns of which two examples are:\n\\[\n\\begin{array}{l}\n++----++ \\\\\n+---+---\n\\end{array}\n\\]\n\nThe first example would generate the series\n\\[\n\\begin{array}{l}\n1+(1 / 2)-(1 / 3)-(1 / 4)-(1 / 5)-(1 / 6)+(1 / 7)+(1 / 8) \\\\\n+(1 / 9)+(1 / 10)-(1 / 11)-(1 / 12)-\\cdots .]\n\\end{array}\n\\]\n(a) Show that a sufficient condition for the series to be conditionally convergent is that there be four \"+\" signs and four \"-\" signs in the block of eight.\n(b) Is this sufficient condition also necessary?\n[Here \"convergent\" means \"convergent to a finite limit.\"]",
      "solution": "A-2. The ideas in both parts are similar and the answer in (b) is \"Yes.\" Let \\( hjgrksla \\) be the \\( qzxwvtnp \\)th term \\( \\pm 1 / qzxwvtnp \\) and \\( bdcmrtyu=hjgrksla_{1}+\\cdots+hjgrksla \\). Since \\( hjgrksla \\rightarrow 0 \\) as \\( qzxwvtnp \\rightarrow \\infty,\\left\\{bdcmrtyu\\right\\} \\) will converge if and only if \\( \\left\\{lswxphqr\\right\\} \\) does. Using the facts that\n\\[\n\\frac{1}{qzxwvtnp}-\\frac{1}{qzxwvtnp+vploqjse}=\\frac{vploqjse}{qzxwvtnp(qzxwvtnp+vploqjse)}\n\\]\nthat \\( \\Sigma\\left(1 / qzxwvtnp^{2}\\right) \\) converges, and that \\( \\Sigma(1 / qzxwvtnp) \\) diverges, one shows that with four \" + \" signs and four \" - \" signs in each block, \\( \\left\\{lswxphqr\\right\\} \\) converges as the term-by-term sum of four convergent sequences while an imbalance of signs makes \\( \\left\\{lswxphqr\\right\\} \\) divergent as the sum of a convergent and a divergent sequence."
    },
    "kernel_variant": {
      "question": "Fix an integer $L\\ge 2$ and let $\\bigl(\\varepsilon_{n}\\bigr)_{n\\ge 1}$ be an $L$–periodic sequence of complex numbers of modulus $1$,\n\\[\n\\varepsilon_{n+L}=\\varepsilon_{n},\\qquad |\\varepsilon_{n}|=1 .\n\\]\nPut  \n\\[\nP(z):=\\varepsilon_{1}+\\varepsilon_{2}z+\\dots +\\varepsilon_{L}z^{L-1},\n\\qquad \n\\zeta_{L}:=e^{2\\pi i/L},\n\\]\nand consider the (in general only conditionally convergent) harmonic Dirichlet series  \n\\[\nS:=\\sum_{n=1}^{\\infty}\\frac{\\varepsilon_{n}}{n}.\n\\]\n\n(a)  Show that $S$ converges (necessarily only conditionally) if and only if the zero Fourier coefficient of one period vanishes, i.e.\\  \n\\[\n\\sum_{j=1}^{L}\\varepsilon_{j}=0 .\n\\]\n\n(b)  Assume from now on that $\\sum_{j=1}^{L}\\varepsilon_{j}=0$.  \nProve that $S$ can be written in the closed form\n\\[\n\\boxed{\\;\n          \\displaystyle\n          S=\n          \\frac{\\log L}{L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n          +\\frac{\\pi}{2L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n                       \\cot\\!\\Bigl(\\dfrac{\\pi j}{L}\\Bigr)\n          -\\frac{1}{L}\\sum_{k=1}^{L-1}\n                    \\Bigl(\\,\\sum_{j=1}^{L-1}\\varepsilon_{j}\n                                    \\zeta_{L}^{-jk}\\Bigr)\n                    \\log\\!\\bigl(1-\\zeta_{L}^{\\,k}\\bigr)\n     \\;}\n\\tag{$\\ast\\ast$}\n\\]\nwhere $\\log$ denotes the principal branch ($-\\pi<\\arg<\\pi$).\n\n(c)  Specialise to $L=12$ with \\emph{real} signs $\\varepsilon_{j}\\in\\{+1,-1\\}$.\n\n(i)  How many among the $2^{12}$ possible sign patterns make $S$ converge?\n\n(ii)  For the concrete \\emph{fully alternating} pattern\n\\[\n(+,-,+,-,+,-,+,-,+,-,+,-)\n\\quad\\bigl(\\text{equivalently }\\varepsilon_{j}=(-1)^{\\,j-1}\\bigr)\n\\]\nevaluate $S$ exactly.\n\n(Throughout, “convergent’’ means convergence to a finite complex limit.  \nAll logarithms are principal.)\n\n%--------------------------------------------------------------------",
      "solution": "Throughout $L\\ge 2$ is fixed, $\\bigl(\\varepsilon_{n}\\bigr)$ is $L$–periodic and  \n\\[\nA:=\\sum_{j=1}^{L}\\varepsilon_{j}\\qquad(\\text{the $0$–Fourier coefficient}).\n\\]\n\n------------------------------------------------------------------\n(a)  Convergence criterion.  \n\nGroup the terms of $S$ into successive complete blocks of length $L$:\n\\[\nS=\\sum_{m=0}^{\\infty}\\sum_{j=1}^{L}\\frac{\\varepsilon_{j}}{mL+j}.\n\\tag{1}\n\\]\nFor $m\\gg 1$\n\\[\n\\frac{1}{mL+j}= \\frac{1}{mL}-\\frac{j}{(mL)^{2}}\n               +\\mathcal{O}\\!\\bigl(m^{-3}\\bigr),\n\\tag{2}\n\\]\nhence\n\\[\nS=\\sum_{m=0}^{\\infty}\\Bigl(\\frac{A}{mL}\n             -\\frac{1}{(mL)^{2}}\\sum_{j=1}^{L}j\\,\\varepsilon_{j}\n             +\\mathcal{O}\\bigl(m^{-3}\\bigr)\\Bigr).\n\\tag{3}\n\\]\nIf $A\\neq 0$ the leading term is a non–zero multiple of the harmonic series\n$\\sum_{m\\ge 0}1/m$, so $S$ diverges.  \nIf $A=0$ the $1/m$–term disappears and the series is dominated by\n$\\sum m^{-2}$, hence convergent.  \nIn either case convergence is only \\emph{conditional} because\n$\\sum_{n\\ge1}|\\varepsilon_{n}|/n=\\sum_{n\\ge1}1/n$ diverges.\n\nTherefore  \n\\[\n\\boxed{\\;\n   S\\text{ converges } \\Longleftrightarrow \n   \\sum_{j=1}^{L}\\varepsilon_{j}=0.\n\\;}\n\\]\n\n------------------------------------------------------------------\n(b)  Evaluation of $S$ under the hypothesis $A=0$.  \n\nStep 1.  A convergent auxiliary series.  \n\nSeparate the $j=L$ column in (1) and insert  \n\\[\n\\frac{1}{mL+j}=\n       \\Bigl[\\frac{1}{mL+j}-\\frac{1}{mL+L}\\Bigr]+\\frac{1}{mL+L}.\n\\]\nBecause $\\sum_{j=1}^{L-1}\\varepsilon_{j}=-\\varepsilon_{L}$ (use $A=0$),\nthe potentially divergent $\\sum_{m\\ge0}1/(mL+L)$ terms cancel, giving\n\\[\nS=\\sum_{j=1}^{L-1}\\varepsilon_{j}\\sum_{m=0}^{\\infty}\n      \\Bigl(\\frac{1}{mL+j}-\\frac{1}{mL+L}\\Bigr).\n\\tag{4}\n\\]\n\nStep 2.  Digamma reduction.  \n\nWith the classical expansion of the digamma function\n\\[\n\\psi(z)=-\\gamma+\\sum_{n=0}^{\\infty}\n                 \\Bigl(\\frac{1}{n+1}-\\frac{1}{n+z}\\Bigr),\n\\]\nwe get for $1\\le j\\le L-1$\n\\[\n\\sum_{m=0}^{\\infty}\\Bigl(\\frac{1}{mL+j}-\\frac{1}{mL+L}\\Bigr)\n       =\\frac{1}{L}\\bigl[\\psi(1)-\\psi(j/L)\\bigr].\n\\tag{5}\n\\]\nHence\n\\[\nS=\\frac{1}{L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n          \\bigl[\\psi(1)-\\psi(j/L)\\bigr].\n\\tag{6}\n\\]\n\nStep 3.  Gauss’ multiplication (digamma) formula.  \n\nFor $1\\le r\\le L-1$\n\\[\n\\boxed{\\;\n\\psi\\!\\Bigl(\\frac{r}{L}\\Bigr)\n  =-\\gamma-\\log L\n   -\\frac{\\pi}{2}\\cot\\!\\Bigl(\\frac{\\pi r}{L}\\Bigr)\n   +\\sum_{k=1}^{L-1}\\zeta_{L}^{-rk}\n        \\log\\!\\bigl(1-\\zeta_{L}^{\\,k}\\bigr)\\; }.\n\\tag{7}\n\\]\n\nUsing $\\psi(1)=-\\gamma$ in (6) and substituting (7) gives\n\\[\n\\begin{aligned}\nS&=\\frac{1}{L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n     \\Bigl[\n        \\log L+\\frac{\\pi}{2}\\cot\\!\\Bigl(\\frac{\\pi j}{L}\\Bigr)\n        -\\sum_{k=1}^{L-1}\\zeta_{L}^{-jk}\n              \\log\\!\\bigl(1-\\zeta_{L}^{\\,k}\\bigr)\n     \\Bigr]                                                     \\\\\n &=\\frac{\\log L}{L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n   +\\frac{\\pi}{2L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n                  \\cot\\!\\Bigl(\\frac{\\pi j}{L}\\Bigr)              \\\\\n &\\quad-\\frac{1}{L}\\sum_{k=1}^{L-1}\n        \\Bigl(\\sum_{j=1}^{L-1}\\varepsilon_{j}\\zeta_{L}^{-jk}\\Bigr)\n        \\log\\!\\bigl(1-\\zeta_{L}^{\\,k}\\bigr),\n\\end{aligned}\\tag{8}\n\\]\nwhich is exactly the announced formula $(\\ast\\ast)$.\n\n------------------------------------------------------------------\n(c)  The real $L=12$ case.  \n\n----------------------------------------------------------------\n(i)  Counting convergent sign patterns.  \n\nWith $\\varepsilon_{j}\\in\\{+1,-1\\}$ the condition\n$\\sum_{j=1}^{12}\\varepsilon_{j}=0$ demands six $+1$’s and six $-1$’s\nin each period.  Hence\n\\[\n\\boxed{\\;924=\\binom{12}{6}\\;}\n\\]\npatterns give a convergent series.\n\n----------------------------------------------------------------\n(ii)  The fully alternating pattern $\\varepsilon_{j}=(-1)^{\\,j-1}$.  \n\nBecause the sign pattern has period $2$ rather than $12$, the series is\nprecisely the classical Dirichlet eta series:\n\\[\nS=\\sum_{n=1}^{\\infty}\\frac{(-1)^{\\,n-1}}{n}.\n\\]\n(No rearrangement is involved; the terms already occur in their natural\norder.)  Euler’s evaluation\n\\[\n\\sum_{n=1}^{\\infty}\\frac{(-1)^{\\,n-1}}{n}=\\log 2\n\\]\ngives\n\\[\n\\boxed{\\;S=\\log 2\\; }.\n\\]\n\nConsistency check with $(\\ast\\ast)$.  \nFor $\\varepsilon_{j}=(-1)^{\\,j-1}$ one has\n\\[\n\\sum_{j=1}^{11}\\varepsilon_{j}\\zeta_{12}^{-jk}\n      =\\begin{cases}\n         1, & k\\not\\equiv 6\\pmod{12},\\\\[4pt]\n        -11,& k\\equiv 6\\pmod{12},\n       \\end{cases}\n\\]\nwhereas the \\emph{full} Fourier coefficient\n$\\widehat{\\varepsilon}(k)=\\sum_{j=1}^{12}\\varepsilon_{j}\\zeta_{12}^{-jk}$\nequals $0$ for $k\\not\\equiv 6$ and $12$ for $k\\equiv 6$.  Inserting\nthose values into $(\\ast\\ast)$ shows that every $k\\not\\equiv 6$ term\ncancels between the cotangent and logarithmic parts, leaving  \n$S=\\log 2$ as required.\n\n\\hfill$\\square$\n\n%--------------------------------------------------------------------",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.607975",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher mathematical structures: the problem passes from real ±1 coefficients to general complex unit moduli and introduces the period polynomial, roots of unity, the digamma function, and Fourier techniques.  \n2. Additional constraints: convergence is tied to the vanishing of the zero Fourier mode, a subtler notion than merely “equal numbers of ±’s’’.  \n3. Deeper theory: evaluation of the sum requires special-function identities (Gauss–Digamma), complex logarithms, and discrete Fourier inversion, none of which appear in the original.  \n4. Technical length: proving sufficiency/necessity, deriving (∗), and executing a non-trivial explicit example demand several layers of argument (block grouping, asymptotic expansion, analytic continuation, Fourier analysis).  \n5. Quantitative output: the solver must not only decide convergence but also compute the exact finite value of the series in closed form.\n\nAll of these aspects render the enhanced variant substantially more difficult than both the original problem and the current kernel variant, meeting the stated guidelines."
      }
    },
    "original_kernel_variant": {
      "question": "Fix an integer $L\\ge 2$ and let $\\bigl(\\varepsilon_{n}\\bigr)_{n\\ge 1}$ be an $L$–periodic sequence of complex numbers of modulus $1$,\n\\[\n\\varepsilon_{n+L}=\\varepsilon_{n},\\qquad |\\varepsilon_{n}|=1 .\n\\]\nPut  \n\\[\nP(z):=\\varepsilon_{1}+\\varepsilon_{2}z+\\dots +\\varepsilon_{L}z^{L-1},\n\\qquad \n\\zeta_{L}:=e^{2\\pi i/L},\n\\]\nand consider the (in general only conditionally convergent) harmonic Dirichlet series  \n\\[\nS:=\\sum_{n=1}^{\\infty}\\frac{\\varepsilon_{n}}{n}.\n\\]\n\n(a)  Show that $S$ converges (necessarily only conditionally) if and only if the zero Fourier coefficient of one period vanishes, i.e.\\  \n\\[\n\\sum_{j=1}^{L}\\varepsilon_{j}=0 .\n\\]\n\n(b)  Assume from now on that $\\sum_{j=1}^{L}\\varepsilon_{j}=0$.  \nProve that $S$ can be written in the closed form\n\\[\n\\boxed{\\;\n          \\displaystyle\n          S=\n          \\frac{\\log L}{L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n          +\\frac{\\pi}{2L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n                       \\cot\\!\\Bigl(\\dfrac{\\pi j}{L}\\Bigr)\n          -\\frac{1}{L}\\sum_{k=1}^{L-1}\n                    \\Bigl(\\,\\sum_{j=1}^{L-1}\\varepsilon_{j}\n                                    \\zeta_{L}^{-jk}\\Bigr)\n                    \\log\\!\\bigl(1-\\zeta_{L}^{\\,k}\\bigr)\n     \\;}\n\\tag{$\\ast\\ast$}\n\\]\nwhere $\\log$ denotes the principal branch ($-\\pi<\\arg<\\pi$).\n\n(c)  Specialise to $L=12$ with \\emph{real} signs $\\varepsilon_{j}\\in\\{+1,-1\\}$.\n\n(i)  How many among the $2^{12}$ possible sign patterns make $S$ converge?\n\n(ii)  For the concrete \\emph{fully alternating} pattern\n\\[\n(+,-,+,-,+,-,+,-,+,-,+,-)\n\\quad\\bigl(\\text{equivalently }\\varepsilon_{j}=(-1)^{\\,j-1}\\bigr)\n\\]\nevaluate $S$ exactly.\n\n(Throughout, “convergent’’ means convergence to a finite complex limit.  \nAll logarithms are principal.)\n\n%--------------------------------------------------------------------",
      "solution": "Throughout $L\\ge 2$ is fixed, $\\bigl(\\varepsilon_{n}\\bigr)$ is $L$–periodic and  \n\\[\nA:=\\sum_{j=1}^{L}\\varepsilon_{j}\\qquad(\\text{the $0$–Fourier coefficient}).\n\\]\n\n------------------------------------------------------------------\n(a)  Convergence criterion.  \n\nGroup the terms of $S$ into successive complete blocks of length $L$:\n\\[\nS=\\sum_{m=0}^{\\infty}\\sum_{j=1}^{L}\\frac{\\varepsilon_{j}}{mL+j}.\n\\tag{1}\n\\]\nFor $m\\gg 1$\n\\[\n\\frac{1}{mL+j}= \\frac{1}{mL}-\\frac{j}{(mL)^{2}}\n               +\\mathcal{O}\\!\\bigl(m^{-3}\\bigr),\n\\tag{2}\n\\]\nhence\n\\[\nS=\\sum_{m=0}^{\\infty}\\Bigl(\\frac{A}{mL}\n             -\\frac{1}{(mL)^{2}}\\sum_{j=1}^{L}j\\,\\varepsilon_{j}\n             +\\mathcal{O}\\bigl(m^{-3}\\bigr)\\Bigr).\n\\tag{3}\n\\]\nIf $A\\neq 0$ the leading term is a non–zero multiple of the harmonic series\n$\\sum_{m\\ge 0}1/m$, so $S$ diverges.  \nIf $A=0$ the $1/m$–term disappears and the series is dominated by\n$\\sum m^{-2}$, hence convergent.  \nIn either case convergence is only \\emph{conditional} because\n$\\sum_{n\\ge1}|\\varepsilon_{n}|/n=\\sum_{n\\ge1}1/n$ diverges.\n\nTherefore  \n\\[\n\\boxed{\\;\n   S\\text{ converges } \\Longleftrightarrow \n   \\sum_{j=1}^{L}\\varepsilon_{j}=0.\n\\;}\n\\]\n\n------------------------------------------------------------------\n(b)  Evaluation of $S$ under the hypothesis $A=0$.  \n\nStep 1.  A convergent auxiliary series.  \n\nSeparate the $j=L$ column in (1) and insert  \n\\[\n\\frac{1}{mL+j}=\n       \\Bigl[\\frac{1}{mL+j}-\\frac{1}{mL+L}\\Bigr]+\\frac{1}{mL+L}.\n\\]\nBecause $\\sum_{j=1}^{L-1}\\varepsilon_{j}=-\\varepsilon_{L}$ (use $A=0$),\nthe potentially divergent $\\sum_{m\\ge0}1/(mL+L)$ terms cancel, giving\n\\[\nS=\\sum_{j=1}^{L-1}\\varepsilon_{j}\\sum_{m=0}^{\\infty}\n      \\Bigl(\\frac{1}{mL+j}-\\frac{1}{mL+L}\\Bigr).\n\\tag{4}\n\\]\n\nStep 2.  Digamma reduction.  \n\nWith the classical expansion of the digamma function\n\\[\n\\psi(z)=-\\gamma+\\sum_{n=0}^{\\infty}\n                 \\Bigl(\\frac{1}{n+1}-\\frac{1}{n+z}\\Bigr),\n\\]\nwe get for $1\\le j\\le L-1$\n\\[\n\\sum_{m=0}^{\\infty}\\Bigl(\\frac{1}{mL+j}-\\frac{1}{mL+L}\\Bigr)\n       =\\frac{1}{L}\\bigl[\\psi(1)-\\psi(j/L)\\bigr].\n\\tag{5}\n\\]\nHence\n\\[\nS=\\frac{1}{L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n          \\bigl[\\psi(1)-\\psi(j/L)\\bigr].\n\\tag{6}\n\\]\n\nStep 3.  Gauss’ multiplication (digamma) formula.  \n\nFor $1\\le r\\le L-1$\n\\[\n\\boxed{\\;\n\\psi\\!\\Bigl(\\frac{r}{L}\\Bigr)\n  =-\\gamma-\\log L\n   -\\frac{\\pi}{2}\\cot\\!\\Bigl(\\frac{\\pi r}{L}\\Bigr)\n   +\\sum_{k=1}^{L-1}\\zeta_{L}^{-rk}\n        \\log\\!\\bigl(1-\\zeta_{L}^{\\,k}\\bigr)\\; }.\n\\tag{7}\n\\]\n\nUsing $\\psi(1)=-\\gamma$ in (6) and substituting (7) gives\n\\[\n\\begin{aligned}\nS&=\\frac{1}{L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n     \\Bigl[\n        \\log L+\\frac{\\pi}{2}\\cot\\!\\Bigl(\\frac{\\pi j}{L}\\Bigr)\n        -\\sum_{k=1}^{L-1}\\zeta_{L}^{-jk}\n              \\log\\!\\bigl(1-\\zeta_{L}^{\\,k}\\bigr)\n     \\Bigr]                                                     \\\\\n &=\\frac{\\log L}{L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n   +\\frac{\\pi}{2L}\\sum_{j=1}^{L-1}\\varepsilon_{j}\n                  \\cot\\!\\Bigl(\\frac{\\pi j}{L}\\Bigr)              \\\\\n &\\quad-\\frac{1}{L}\\sum_{k=1}^{L-1}\n        \\Bigl(\\sum_{j=1}^{L-1}\\varepsilon_{j}\\zeta_{L}^{-jk}\\Bigr)\n        \\log\\!\\bigl(1-\\zeta_{L}^{\\,k}\\bigr),\n\\end{aligned}\\tag{8}\n\\]\nwhich is exactly the announced formula $(\\ast\\ast)$.\n\n------------------------------------------------------------------\n(c)  The real $L=12$ case.  \n\n----------------------------------------------------------------\n(i)  Counting convergent sign patterns.  \n\nWith $\\varepsilon_{j}\\in\\{+1,-1\\}$ the condition\n$\\sum_{j=1}^{12}\\varepsilon_{j}=0$ demands six $+1$’s and six $-1$’s\nin each period.  Hence\n\\[\n\\boxed{\\;924=\\binom{12}{6}\\;}\n\\]\npatterns give a convergent series.\n\n----------------------------------------------------------------\n(ii)  The fully alternating pattern $\\varepsilon_{j}=(-1)^{\\,j-1}$.  \n\nBecause the sign pattern has period $2$ rather than $12$, the series is\nprecisely the classical Dirichlet eta series:\n\\[\nS=\\sum_{n=1}^{\\infty}\\frac{(-1)^{\\,n-1}}{n}.\n\\]\n(No rearrangement is involved; the terms already occur in their natural\norder.)  Euler’s evaluation\n\\[\n\\sum_{n=1}^{\\infty}\\frac{(-1)^{\\,n-1}}{n}=\\log 2\n\\]\ngives\n\\[\n\\boxed{\\;S=\\log 2\\; }.\n\\]\n\nConsistency check with $(\\ast\\ast)$.  \nFor $\\varepsilon_{j}=(-1)^{\\,j-1}$ one has\n\\[\n\\sum_{j=1}^{11}\\varepsilon_{j}\\zeta_{12}^{-jk}\n      =\\begin{cases}\n         1, & k\\not\\equiv 6\\pmod{12},\\\\[4pt]\n        -11,& k\\equiv 6\\pmod{12},\n       \\end{cases}\n\\]\nwhereas the \\emph{full} Fourier coefficient\n$\\widehat{\\varepsilon}(k)=\\sum_{j=1}^{12}\\varepsilon_{j}\\zeta_{12}^{-jk}$\nequals $0$ for $k\\not\\equiv 6$ and $12$ for $k\\equiv 6$.  Inserting\nthose values into $(\\ast\\ast)$ shows that every $k\\not\\equiv 6$ term\ncancels between the cotangent and logarithmic parts, leaving  \n$S=\\log 2$ as required.\n\n\\hfill$\\square$\n\n%--------------------------------------------------------------------",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.486263",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher mathematical structures: the problem passes from real ±1 coefficients to general complex unit moduli and introduces the period polynomial, roots of unity, the digamma function, and Fourier techniques.  \n2. Additional constraints: convergence is tied to the vanishing of the zero Fourier mode, a subtler notion than merely “equal numbers of ±’s’’.  \n3. Deeper theory: evaluation of the sum requires special-function identities (Gauss–Digamma), complex logarithms, and discrete Fourier inversion, none of which appear in the original.  \n4. Technical length: proving sufficiency/necessity, deriving (∗), and executing a non-trivial explicit example demand several layers of argument (block grouping, asymptotic expansion, analytic continuation, Fourier analysis).  \n5. Quantitative output: the solver must not only decide convergence but also compute the exact finite value of the series in closed form.\n\nAll of these aspects render the enhanced variant substantially more difficult than both the original problem and the current kernel variant, meeting the stated guidelines."
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}