{
  "index": "1981-B-5",
  "type": "NT",
  "tag": [
    "NT",
    "ALG"
  ],
  "difficulty": "",
  "question": "Problem B-5\nLet \\( B(n) \\) be the number of ones in the base two expression for the positive integer \\( n \\). For example, \\( B(6)=B\\left(110_{2}\\right) \\) \\( =2 \\) and \\( B(15)=B\\left(1111_{2}\\right)=4 \\). Determine whether or not\n\\[\n\\exp \\left(\\sum_{n=1}^{\\infty} \\frac{B(n)}{n(n+1)}\\right)\n\\]\nis a rational number. Here \\( \\exp (x) \\) denotes \\( e^{x} \\).",
  "solution": "B-5.\nIf \\( n \\) has \\( d \\) digits in base \\( 2,2^{d-1} \\leq n \\) and so\n\\[\nB(n) \\leq d \\leq 1+\\log _{2} n .\n\\]\n\nThis readily implies that \\( \\sum_{n=1}^{\\infty}[B(n) / n(n+1)] \\) converges to a real number \\( S \\). Hence the manipulations below with convergent series are allowable in the two solutions which follow.\n\nEach \\( n \\) is uniquely expressible as \\( n_{0}+2 n_{1}+2^{2} n_{2}+\\cdots \\) with each \\( n_{1} \\) in \\( \\{0,1\\rangle \\) (and with \\( n_{1}=0 \\) for all but a finite set of \\( i \\) ). Since\n\\[\n1+2+2^{2}+\\cdots+2^{\\prime-1}=2^{\\prime}-1,\n\\]\none sees that \\( n_{1}=1 \\) if and only if \\( n \\) is of the form \\( k+2^{\\prime}+2^{\\prime+1} j \\) with \\( k \\) in \\( \\left\\{0,1, \\ldots, 2^{\\prime}-1\\right\\} \\) and \\( j \\) in \\( \\{0,1,2, \\ldots\\} \\). Thus\n\\[\n\\begin{aligned}\nS & =\\sum_{n=1}^{\\infty} \\frac{1}{n(n+1)} \\sum_{t=0}^{\\infty} n_{t} \\\\\n& =\\sum_{i=0}^{\\infty} \\sum_{j=0}^{\\infty} \\sum_{k=0}^{2^{\\prime}-1} \\frac{1}{\\left(k+2^{\\prime}+2^{i+1} j\\right)\\left(1+k+2^{\\prime}+2^{\\prime+1} j\\right)}\n\\end{aligned}\n\\]\n\nUsing \\( 1 / s(s+1)=1 / s-1 /(s+1) \\), the innermost sum telescopes and\n\\[\nS=\\sum_{i=0}^{\\infty} \\sum_{j=0}^{\\infty}\\left[\\frac{1}{2^{\\prime}(1+2 j)}-\\frac{1}{2^{\\prime}(2+2 j)}\\right]=\\sum_{i=0}^{\\infty} \\frac{1}{2^{\\prime}} \\sum_{j=0}^{\\infty}(-1)^{\\prime} \\frac{1}{j} .\n\\]\n\nSince it is well known that \\( 1-\\frac{1}{2}+\\frac{1}{3}-\\frac{1}{4}+\\cdots=\\ln 2 \\),\n\\[\nS=\\left(\\sum_{1=0}^{\\infty} 2^{-1}\\right) \\ln 2=2 \\ln 2=\\ln 4\n\\]\nand \\( e^{S} \\) is the rational number 4.\nAlternatively, we note that \\( B(2 m)=B(m), B(2 m+1)=1+B(2 m)=1+B(m) \\).\nThen\n\\[\n\\begin{aligned}\nS & =\\sum_{n=1}^{\\infty} \\frac{B(n)}{n(n+1)}=\\sum_{m=0}^{\\infty} \\frac{B(2 m+1)}{(2 m+1)(2 m+2)}+\\sum_{m=1}^{\\infty} \\frac{B(2 m)}{2 m(2 m+1)} \\\\\n& =\\sum_{m=0}^{\\infty} \\frac{1+B(m)}{(2 m+1)(2 m+2)}+\\sum_{m=1}^{\\infty} \\frac{B(m)}{2 m(2 m+1)} \\\\\n& =\\sum_{m=0}^{\\infty} \\frac{1}{(2 m+1)(2 m+2)}+\\sum_{m=1}^{\\infty} B(m)\\left[\\frac{1}{2 m(2 m+1)}+\\frac{1}{(2 m+1)(2 m+2)}\\right] \\\\\n& =\\ln 2+\\frac{1}{2} \\sum_{m=1}^{\\infty} \\frac{B(m)}{m(m+1)}=\\ln 2+\\frac{S}{2} .\n\\end{aligned}\n\\]\n\nHence \\( S / 2=\\ln 2, S=\\ln 4 \\), and \\( \\exp (S) \\) is the rational number 4 .",
  "vars": [
    "x",
    "n",
    "d",
    "S",
    "n_0",
    "n_1",
    "n_2",
    "n_t",
    "t",
    "i",
    "j",
    "k",
    "s",
    "m"
  ],
  "params": [
    "B"
  ],
  "sci_consts": [
    "e"
  ],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "genericvalue",
        "n": "positiveint",
        "d": "digitscount",
        "S": "seriesvalue",
        "n_0": "digitzerocoeff",
        "n_1": "digitonecoeff",
        "n_2": "digittwocoeff",
        "n_t": "digittcoeff",
        "t": "indextee",
        "i": "levelindex",
        "j": "indexjay",
        "k": "indexkay",
        "s": "tempvar",
        "m": "indexm",
        "B": "binaryones"
      },
      "question": "Problem B-5\nLet \\(binaryones(positiveint)\\) be the number of ones in the base two expression for the positive integer \\(positiveint\\). For example, \\(binaryones(6)=binaryones\\left(110_{2}\\right)=2\\) and \\(binaryones(15)=binaryones\\left(1111_{2}\\right)=4\\). Determine whether or not\n\\[\n\\exp \\left(\\sum_{positiveint=1}^{\\infty} \\frac{binaryones(positiveint)}{positiveint(positiveint+1)}\\right)\n\\]\nis a rational number. Here \\( \\exp (genericvalue) \\) denotes \\( e^{genericvalue} \\).",
      "solution": "B-5.\nIf \\( positiveint \\) has \\( digitscount \\) digits in base \\(2,2^{digitscount-1} \\leq positiveint\\) and so\n\\[\nbinaryones(positiveint) \\leq digitscount \\leq 1+\\log _{2} positiveint .\n\\]\n\nThis readily implies that \\( \\sum_{positiveint=1}^{\\infty}[binaryones(positiveint) / positiveint(positiveint+1)] \\) converges to a real number \\( seriesvalue \\). Hence the manipulations below with convergent series are allowable in the two solutions which follow.\n\nEach \\( positiveint \\) is uniquely expressible as \\( digitzerocoeff+2 digitonecoeff+2^{2} digittwocoeff+\\cdots \\) with each \\( digitonecoeff \\) in \\( \\{0,1\\rangle \\) (and with \\( digitonecoeff=0 \\) for all but a finite set of \\( levelindex \\) ). Since\n\\[\n1+2+2^{2}+\\cdots+2^{\\prime-1}=2^{\\prime}-1,\n\\]\none sees that \\( digitonecoeff=1 \\) if and only if \\( positiveint \\) is of the form \\( indexkay+2^{\\prime}+2^{\\prime+1} indexjay \\) with \\( indexkay \\) in \\( \\left\\{0,1, \\ldots, 2^{\\prime}-1\\right\\} \\) and \\( indexjay \\) in \\( \\{0,1,2, \\ldots\\} \\). Thus\n\\[\n\\begin{aligned}\nseriesvalue & =\\sum_{positiveint=1}^{\\infty} \\frac{1}{positiveint(positiveint+1)} \\sum_{indextee=0}^{\\infty} digittcoeff \\\\\n& =\\sum_{levelindex=0}^{\\infty} \\sum_{indexjay=0}^{\\infty} \\sum_{indexkay=0}^{2^{\\prime}-1} \\frac{1}{\\left(indexkay+2^{\\prime}+2^{levelindex+1} indexjay\\right)\\left(1+indexkay+2^{\\prime}+2^{\\prime+1} indexjay\\right)}\n\\end{aligned}\n\\]\n\nUsing \\( 1 / tempvar(tempvar+1)=1 / tempvar-1 /(tempvar+1) \\), the innermost sum telescopes and\n\\[\nseriesvalue=\\sum_{levelindex=0}^{\\infty} \\sum_{indexjay=0}^{\\infty}\\left[\\frac{1}{2^{\\prime}(1+2 indexjay)}-\\frac{1}{2^{\\prime}(2+2 indexjay)}\\right]=\\sum_{levelindex=0}^{\\infty} \\frac{1}{2^{\\prime}} \\sum_{indexjay=0}^{\\infty}(-1)^{\\prime} \\frac{1}{indexjay} .\n\\]\n\nSince it is well known that \\( 1-\\frac{1}{2}+\\frac{1}{3}-\\frac{1}{4}+\\cdots=\\ln 2 \\),\n\\[\nseriesvalue=\\left(\\sum_{1=0}^{\\infty} 2^{-1}\\right) \\ln 2=2 \\ln 2=\\ln 4\n\\]\nand \\( e^{seriesvalue} \\) is the rational number 4.\nAlternatively, we note that \\( binaryones(2 indexm)=binaryones(indexm),\\; binaryones(2 indexm+1)=1+binaryones(2 indexm)=1+binaryones(indexm) \\).\nThen\n\\[\n\\begin{aligned}\nseriesvalue & =\\sum_{positiveint=1}^{\\infty} \\frac{binaryones(positiveint)}{positiveint(positiveint+1)}=\\sum_{indexm=0}^{\\infty} \\frac{binaryones(2 indexm+1)}{(2 indexm+1)(2 indexm+2)}+\\sum_{indexm=1}^{\\infty} \\frac{binaryones(2 indexm)}{2 indexm(2 indexm+1)} \\\\\n& =\\sum_{indexm=0}^{\\infty} \\frac{1+binaryones(indexm)}{(2 indexm+1)(2 indexm+2)}+\\sum_{indexm=1}^{\\infty} \\frac{binaryones(indexm)}{2 indexm(2 indexm+1)} \\\\\n& =\\sum_{indexm=0}^{\\infty} \\frac{1}{(2 indexm+1)(2 indexm+2)}+\\sum_{indexm=1}^{\\infty} binaryones(indexm)\\left[\\frac{1}{2 indexm(2 indexm+1)}+\\frac{1}{(2 indexm+1)(2 indexm+2)}\\right] \\\\\n& =\\ln 2+\\frac{1}{2} \\sum_{indexm=1}^{\\infty} \\frac{binaryones(indexm)}{indexm(indexm+1)}=\\ln 2+\\frac{seriesvalue}{2} .\n\\end{aligned}\n\\]\n\nHence \\( seriesvalue / 2=\\ln 2,\\; seriesvalue=\\ln 4 \\), and \\( \\exp (seriesvalue) \\) is the rational number 4 ."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "bookshelf",
        "n": "lighthouse",
        "d": "pineapple",
        "S": "waterfall",
        "n_0": "butterfly",
        "n_1": "snowflake",
        "n_2": "tablespoon",
        "n_t": "candycane",
        "t": "skateboard",
        "i": "marshmallow",
        "j": "toothpick",
        "k": "raincloud",
        "s": "horseshoe",
        "m": "gingerbread",
        "B": "histogram"
      },
      "question": "Problem B-5\nLet \\( histogram(lighthouse) \\) be the number of ones in the base two expression for the positive integer \\( lighthouse \\). For example, \\( histogram(6)=histogram\\left(110_{2}\\right) =2 \\) and \\( histogram(15)=histogram\\left(1111_{2}\\right)=4 \\). Determine whether or not\n\\[\n\\exp \\left(\\sum_{lighthouse=1}^{\\infty} \\frac{histogram(lighthouse)}{lighthouse(lighthouse+1)}\\right)\n\\]\nis a rational number. Here \\( \\exp (bookshelf) \\) denotes \\( e^{bookshelf} \\).",
      "solution": "B-5.\nIf \\( lighthouse \\) has \\( pineapple \\) digits in base \\( 2,2^{pineapple-1} \\leq lighthouse \\) and so\n\\[\nhistogram(lighthouse) \\leq pineapple \\leq 1+\\log _{2} lighthouse .\n\\]\n\nThis readily implies that \\( \\sum_{lighthouse=1}^{\\infty}[histogram(lighthouse) / lighthouse(lighthouse+1)] \\) converges to a real number \\( waterfall \\). Hence the manipulations below with convergent series are allowable in the two solutions which follow.\n\nEach \\( lighthouse \\) is uniquely expressible as \\( butterfly+2 snowflake+2^{2} tablespoon+\\cdots \\) with each \\( snowflake \\) in \\( \\{0,1\\rangle \\) (and with \\( snowflake=0 \\) for all but a finite set of \\( marshmallow \\) ). Since\n\\[\n1+2+2^{2}+\\cdots+2^{marshmallow-1}=2^{marshmallow}-1,\n\\]\none sees that \\( snowflake=1 \\) if and only if \\( lighthouse \\) is of the form \\( raincloud+2^{marshmallow}+2^{marshmallow+1} toothpick \\) with \\( raincloud \\) in \\( \\left\\{0,1, \\ldots, 2^{marshmallow}-1\\right\\} \\) and \\( toothpick \\) in \\( \\{0,1,2, \\ldots\\} \\). Thus\n\\[\n\\begin{aligned}\nwaterfall & =\\sum_{lighthouse=1}^{\\infty} \\frac{1}{lighthouse(lighthouse+1)} \\sum_{skateboard=0}^{\\infty} candycane \\\\\n& =\\sum_{marshmallow=0}^{\\infty} \\sum_{toothpick=0}^{\\infty} \\sum_{raincloud=0}^{2^{marshmallow}-1} \\frac{1}{\\left(raincloud+2^{marshmallow}+2^{marshmallow+1} toothpick\\right)\\left(1+raincloud+2^{marshmallow}+2^{marshmallow+1} toothpick\\right)}\n\\end{aligned}\n\\]\n\nUsing \\( 1 / horseshoe(horseshoe+1)=1 / horseshoe-1 /(horseshoe+1) \\), the innermost sum telescopes and\n\\[\nwaterfall=\\sum_{marshmallow=0}^{\\infty} \\sum_{toothpick=0}^{\\infty}\\left[\\frac{1}{2^{marshmallow}(1+2 toothpick)}-\\frac{1}{2^{marshmallow}(2+2 toothpick)}\\right]=\\sum_{marshmallow=0}^{\\infty} \\frac{1}{2^{marshmallow}} \\sum_{toothpick=0}^{\\infty}(-1)^{toothpick} \\frac{1}{toothpick} .\n\\]\n\nSince it is well known that \\( 1-\\frac{1}{2}+\\frac{1}{3}-\\frac{1}{4}+\\cdots=\\ln 2 \\),\n\\[\nwaterfall=\\left(\\sum_{marshmallow=0}^{\\infty} 2^{-marshmallow}\\right) \\ln 2=2 \\ln 2=\\ln 4\n\\]\nand \\( e^{waterfall} \\) is the rational number 4.\nAlternatively, we note that \\( histogram(2 gingerbread)=histogram(gingerbread), histogram(2 gingerbread+1)=1+histogram(2 gingerbread)=1+histogram(gingerbread) \\).\nThen\n\\[\n\\begin{aligned}\nwaterfall & =\\sum_{lighthouse=1}^{\\infty} \\frac{histogram(lighthouse)}{lighthouse(lighthouse+1)}=\\sum_{gingerbread=0}^{\\infty} \\frac{histogram(2 gingerbread+1)}{(2 gingerbread+1)(2 gingerbread+2)}+\\sum_{gingerbread=1}^{\\infty} \\frac{histogram(2 gingerbread)}{2 gingerbread(2 gingerbread+1)} \\\\\n& =\\sum_{gingerbread=0}^{\\infty} \\frac{1+histogram(gingerbread)}{(2 gingerbread+1)(2 gingerbread+2)}+\\sum_{gingerbread=1}^{\\infty} \\frac{histogram(gingerbread)}{2 gingerbread(2 gingerbread+1)} \\\\\n& =\\sum_{gingerbread=0}^{\\infty} \\frac{1}{(2 gingerbread+1)(2 gingerbread+2)}+\\sum_{gingerbread=1}^{\\infty} histogram(gingerbread)\\left[\\frac{1}{2 gingerbread(2 gingerbread+1)}+\\frac{1}{(2 gingerbread+1)(2 gingerbread+2)}\\right] \\\\\n& =\\ln 2+\\frac{1}{2} \\sum_{gingerbread=1}^{\\infty} \\frac{histogram(gingerbread)}{gingerbread(gingerbread+1)}=\\ln 2+\\frac{waterfall}{2} .\n\\end{aligned}\n\\]\n\nHence \\( waterfall / 2=\\ln 2, waterfall=\\ln 4 \\), and \\( \\exp (waterfall) \\) is the rational number 4 ."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "blandconstant",
        "n": "continuousvalue",
        "d": "blankspace",
        "S": "boundless",
        "n_0": "fullcarry",
        "n_1": "voidcarry",
        "n_2": "overflow",
        "n_t": "stablebit",
        "t": "stationary",
        "i": "fraction",
        "j": "stillness",
        "k": "constantk",
        "s": "difference",
        "m": "staticvalue",
        "B": "zerocount"
      },
      "question": "Problem B-5\nLet \\( zerocount(continuousvalue) \\) be the number of ones in the base two expression for the positive integer \\( continuousvalue \\). For example, \\( zerocount(6)=zerocount\\left(110_{2}\\right) =2 \\) and \\( zerocount(15)=zerocount\\left(1111_{2}\\right)=4 \\). Determine whether or not\n\\[\n\\exp \\left(\\sum_{continuousvalue=1}^{\\infty} \\frac{zerocount(continuousvalue)}{continuousvalue(continuousvalue+1)}\\right)\n\\]\nis a rational number. Here \\( \\exp (\\blandconstant) \\) denotes \\( e^{\\blandconstant} \\).",
      "solution": "B-5.\nIf \\( continuousvalue \\) has \\( blankspace \\) digits in base \\( 2,2^{blankspace-1} \\leq continuousvalue \\) and so\n\\[\nzerocount(continuousvalue) \\leq blankspace \\leq 1+\\log _{2} continuousvalue .\n\\]\n\nThis readily implies that \\( \\sum_{continuousvalue=1}^{\\infty}[zerocount(continuousvalue) / continuousvalue(continuousvalue+1)] \\) converges to a real number \\( boundless \\). Hence the manipulations below with convergent series are allowable in the two solutions which follow.\n\nEach \\( continuousvalue \\) is uniquely expressible as \\( fullcarry+2 voidcarry+2^{2} overflow+\\cdots \\) with each \\( voidcarry \\) in \\( \\{0,1\\rangle \\) (and with \\( voidcarry=0 \\) for all but a finite set of \\( fraction \\) ). Since\n\\[\n1+2+2^{2}+\\cdots+2^{\\prime-1}=2^{\\prime}-1,\n\\]\none sees that \\( voidcarry=1 \\) if and only if \\( continuousvalue \\) is of the form \\( constantk+2^{\\prime}+2^{\\prime+1} stillness \\) with \\( constantk \\) in \\( \\left\\{0,1, \\ldots, 2^{\\prime}-1\\right\\} \\) and \\( stillness \\) in \\( \\{0,1,2, \\ldots\\} \\). Thus\n\\[\n\\begin{aligned}\nboundless & =\\sum_{continuousvalue=1}^{\\infty} \\frac{1}{continuousvalue(continuousvalue+1)} \\sum_{stationary=0}^{\\infty} stablebit \\\\\n& =\\sum_{fraction=0}^{\\infty} \\sum_{stillness=0}^{\\infty} \\sum_{constantk=0}^{2^{\\prime}-1} \\frac{1}{\\left(constantk+2^{\\prime}+2^{fraction+1} stillness\\right)\\left(1+constantk+2^{\\prime}+2^{\\prime+1} stillness\\right)}\n\\end{aligned}\n\\]\n\nUsing \\( 1 / difference(difference+1)=1 / difference-1 /(difference+1) \\), the innermost sum telescopes and\n\\[\nboundless=\\sum_{fraction=0}^{\\infty} \\sum_{stillness=0}^{\\infty}\\left[\\frac{1}{2^{\\prime}(1+2 stillness)}-\\frac{1}{2^{\\prime}(2+2 stillness)}\\right]=\\sum_{fraction=0}^{\\infty} \\frac{1}{2^{\\prime}} \\sum_{stillness=0}^{\\infty}(-1)^{\\prime} \\frac{1}{stillness} .\n\\]\n\nSince it is well known that \\( 1-\\frac{1}{2}+\\frac{1}{3}-\\frac{1}{4}+\\cdots=\\ln 2 \\),\n\\[\nboundless=\\left(\\sum_{fraction=0}^{\\infty} 2^{-1}\\right) \\ln 2=2 \\ln 2=\\ln 4\n\\]\nand \\( e^{boundless} \\) is the rational number 4.\n\nAlternatively, we note that \\( zerocount(2 staticvalue)=zerocount(staticvalue), zerocount(2 staticvalue+1)=1+zerocount(2 staticvalue)=1+zerocount(staticvalue) \\).\nThen\n\\[\n\\begin{aligned}\nboundless & =\\sum_{continuousvalue=1}^{\\infty} \\frac{zerocount(continuousvalue)}{continuousvalue(continuousvalue+1)}=\\sum_{staticvalue=0}^{\\infty} \\frac{zerocount(2 staticvalue+1)}{(2 staticvalue+1)(2 staticvalue+2)}+\\sum_{staticvalue=1}^{\\infty} \\frac{zerocount(2 staticvalue)}{2 staticvalue(2 staticvalue+1)} \\\\\n& =\\sum_{staticvalue=0}^{\\infty} \\frac{1+zerocount(staticvalue)}{(2 staticvalue+1)(2 staticvalue+2)}+\\sum_{staticvalue=1}^{\\infty} \\frac{zerocount(staticvalue)}{2 staticvalue(2 staticvalue+1)} \\\\\n& =\\sum_{staticvalue=0}^{\\infty} \\frac{1}{(2 staticvalue+1)(2 staticvalue+2)}+\\sum_{staticvalue=1}^{\\infty} zerocount(staticvalue)\\left[\\frac{1}{2 staticvalue(2 staticvalue+1)}+\\frac{1}{(2 staticvalue+1)(2 staticvalue+2)}\\right] \\\\\n& =\\ln 2+\\frac{1}{2} \\sum_{staticvalue=1}^{\\infty} \\frac{zerocount(staticvalue)}{staticvalue(staticvalue+1)}=\\ln 2+\\frac{boundless}{2} .\n\\end{aligned}\n\\]\n\nHence \\( boundless / 2=\\ln 2, boundless=\\ln 4 \\), and \\( \\exp (boundless) \\) is the rational number 4 ."
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "n": "hjgrksla",
        "d": "vctblfqe",
        "S": "mgzfrdoy",
        "n_0": "nyzxqvab",
        "n_1": "rlqmoptw",
        "n_2": "sbgjkvhe",
        "n_t": "fwdpclra",
        "t": "lszwernu",
        "i": "kbqxjvmo",
        "j": "dchpsnry",
        "k": "uvmaezti",
        "s": "oyhvcgbr",
        "m": "wzkrtbsa",
        "B": "uaxplneo"
      },
      "question": "Problem B-5\nLet \\( uaxplneo(hjgrksla) \\) be the number of ones in the base two expression for the positive integer \\( hjgrksla \\). For example, \\( uaxplneo(6)=uaxplneo\\left(110_{2}\\right)=2 \\) and \\( uaxplneo(15)=uaxplneo\\left(1111_{2}\\right)=4 \\). Determine whether or not\n\\[\n\\exp \\left(\\sum_{hjgrksla=1}^{\\infty} \\frac{uaxplneo(hjgrksla)}{hjgrksla(hjgrksla+1)}\\right)\n\\]\nis a rational number. Here \\( \\exp (qzxwvtnp) \\) denotes \\( e^{qzxwvtnp} \\).",
      "solution": "B-5.\nIf \\( hjgrksla \\) has \\( vctblfqe \\) digits in base \\( 2,2^{vctblfqe-1} \\leq hjgrksla \\) and so\n\\[\nuaxplneo(hjgrksla) \\leq vctblfqe \\leq 1+\\log _{2} hjgrksla .\n\\]\n\nThis readily implies that \\( \\sum_{hjgrksla=1}^{\\infty}[uaxplneo(hjgrksla) / hjgrksla(hjgrksla+1)] \\) converges to a real number \\( mgzfrdoy \\). Hence the manipulations below with convergent series are allowable in the two solutions which follow.\n\nEach \\( hjgrksla \\) is uniquely expressible as \\( nyzxqvab+2 rlqmoptw+2^{2} sbgjkvhe+\\cdots \\) with each \\( rlqmoptw \\) in \\( \\{0,1\\rangle \\) (and with \\( rlqmoptw=0 \\) for all but a finite set of \\( kbqxjvmo \\) ). Since\n\\[\n1+2+2^{2}+\\cdots+2^{\\prime-1}=2^{\\prime}-1,\n\\]\none sees that \\( rlqmoptw=1 \\) if and only if \\( hjgrksla \\) is of the form \\( uvmaezti+2^{\\prime}+2^{\\prime+1} dchpsnry \\) with \\( uvmaezti \\) in \\( \\left\\{0,1, \\ldots, 2^{\\prime}-1\\right\\} \\) and \\( dchpsnry \\) in \\( \\{0,1,2, \\ldots\\} \\). Thus\n\\[\n\\begin{aligned}\nmgzfrdoy & =\\sum_{hjgrksla=1}^{\\infty} \\frac{1}{hjgrksla(hjgrksla+1)} \\sum_{lszwernu=0}^{\\infty} fwdpclra \\\\\n& =\\sum_{kbqxjvmo=0}^{\\infty} \\sum_{dchpsnry=0}^{\\infty} \\sum_{uvmaezti=0}^{2^{\\prime}-1} \\frac{1}{\\left(uvmaezti+2^{\\prime}+2^{kbqxjvmo+1} dchpsnry\\right)\\left(1+uvmaezti+2^{\\prime}+2^{\\prime+1} dchpsnry\\right)}\n\\end{aligned}\n\\]\n\nUsing \\( 1 / oyhvcgbr(oyhvcgbr+1)=1 / oyhvcgbr-1 /(oyhvcgbr+1) \\), the innermost sum telescopes and\n\\[\nmgzfrdoy=\\sum_{kbqxjvmo=0}^{\\infty} \\sum_{dchpsnry=0}^{\\infty}\\left[\\frac{1}{2^{\\prime}(1+2 dchpsnry)}-\\frac{1}{2^{\\prime}(2+2 dchpsnry)}\\right]=\\sum_{kbqxjvmo=0}^{\\infty} \\frac{1}{2^{\\prime}} \\sum_{dchpsnry=0}^{\\infty}(-1)^{\\prime} \\frac{1}{dchpsnry} .\n\\]\n\nSince it is well known that \\( 1-\\frac{1}{2}+\\frac{1}{3}-\\frac{1}{4}+\\cdots=\\ln 2 \\),\n\\[\nmgzfrdoy=\\left(\\sum_{1=0}^{\\infty} 2^{-1}\\right) \\ln 2=2 \\ln 2=\\ln 4\n\\]\nand \\( e^{mgzfrdoy} \\) is the rational number 4.\nAlternatively, we note that \\( uaxplneo(2 wzkrtbsa)=uaxplneo(wzkrtbsa), uaxplneo(2 wzkrtbsa+1)=1+uaxplneo(2 wzkrtbsa)=1+uaxplneo(wzkrtbsa) \\).\nThen\n\\[\n\\begin{aligned}\nmgzfrdoy & =\\sum_{hjgrksla=1}^{\\infty} \\frac{uaxplneo(hjgrksla)}{hjgrksla(hjgrksla+1)}=\\sum_{wzkrtbsa=0}^{\\infty} \\frac{uaxplneo(2 wzkrtbsa+1)}{(2 wzkrtbsa+1)(2 wzkrtbsa+2)}+\\sum_{wzkrtbsa=1}^{\\infty} \\frac{uaxplneo(2 wzkrtbsa)}{2 wzkrtbsa(2 wzkrtbsa+1)} \\\\\n& =\\sum_{wzkrtbsa=0}^{\\infty} \\frac{1+uaxplneo(wzkrtbsa)}{(2 wzkrtbsa+1)(2 wzkrtbsa+2)}+\\sum_{wzkrtbsa=1}^{\\infty} \\frac{uaxplneo(wzkrtbsa)}{2 wzkrtbsa(2 wzkrtbsa+1)} \\\\\n& =\\sum_{wzkrtbsa=0}^{\\infty} \\frac{1}{(2 wzkrtbsa+1)(2 wzkrtbsa+2)}+\\sum_{wzkrtbsa=1}^{\\infty} uaxplneo(wzkrtbsa)\\left[\\frac{1}{2 wzkrtbsa(2 wzkrtbsa+1)}+\\frac{1}{(2 wzkrtbsa+1)(2 wzkrtbsa+2)}\\right] \\\\\n& =\\ln 2+\\frac{1}{2} \\sum_{wzkrtbsa=1}^{\\infty} \\frac{uaxplneo(wzkrtbsa)}{wzkrtbsa(wzkrtbsa+1)}=\\ln 2+\\frac{mgzfrdoy}{2} .\n\\end{aligned}\n\\]\n\nHence \\( mgzfrdoy / 2=\\ln 2, mgzfrdoy=\\ln 4 \\), and \\( \\exp (mgzfrdoy) \\) is the rational number 4 ."
    },
    "kernel_variant": {
      "question": "Fix a complex parameter $\\tau$ with $|\\tau|<2$.  \nFor every positive integer $n$ write its binary expansion\n\\[\nn=\\varepsilon_{0}+ \\varepsilon_{1}\\,2+\\varepsilon_{2}\\,2^{2}+\\dots ,\n\\qquad \\varepsilon_{i}\\in\\{0,1\\}\\text{ and only finitely many }\\varepsilon_{i}=1 .\n\\]\n\nDefine the $\\tau$–weighted binary digit–sum\n\\[\nB_{\\tau}(n)\\;:=\\;\\sum_{i\\ge 0}\\varepsilon_{i}\\,\\tau^{\\,i},\n\\]\nand set\n\\[\nS(\\tau):=\\sum_{n=1}^{\\infty}\\frac{B_{\\tau}(n)}{n(n+1)},\\qquad \nE(\\tau):=\\exp\\bigl(S(\\tau)\\bigr).\n\\]\n\nA.  Show that the series $S(\\tau)$ converges absolutely for every $\\tau$ with $|\\tau|<2$.\n\nB.  Prove the closed formulas\n\\[\nS(\\tau)=\\frac{\\ln 2}{1-\\tau/2},\n\\qquad\nE(\\tau)=2^{\\,1/(1-\\tau/2)}.\n\\]\n\nC.  Study the arithmetic nature of $E(\\tau)$.\n\n   (i)  Determine precisely those rational parameters $\\tau$ ($|\\tau|<2$) for which $E(\\tau)$ is rational, and give the corresponding values $E(\\tau)$.\n\n   (ii)  Let $\\tau$ be a rational number not covered by (i).\n         Prove that $E(\\tau)$ is algebraic but irrational and determine its minimal polynomial over $\\mathbb{Q}$.\n\n   (iii)  Show that if $\\tau$ is algebraic and irrational ($|\\tau|<2$) then $E(\\tau)$ is transcendental.\n\n   (The last assertion is an application of the Gelfond–Schneider theorem; no claim is made about transcendental $\\tau$.)",
      "solution": "Throughout write $\\alpha:=|\\tau|<2$.\n\n--------------------------------------------------------------------\nA.  Absolute convergence of $S(\\tau)$\n--------------------------------------------------------------------\nLet $m:=\\lfloor\\log_{2}n\\rfloor+1$ be the length of the binary\nexpansion of $n$, so that $2^{\\,m-1}\\le n<2^{\\,m}$.\n\n1.  A uniform bound for $B_{\\tau}(n)$.\n\nSince $|\\varepsilon_{i}|=1$ when it occurs,\n\\[\n|B_{\\tau}(n)|\n\\;=\\;\\bigl|\\sum_{i=0}^{m-1}\\varepsilon_{i}\\tau^{\\,i}\\bigr|\n\\;\\le\\;\\sum_{i=0}^{m-1}\\alpha^{\\,i}.\n\\]\nThus\n\\[\n|B_{\\tau}(n)|\\le\n\\begin{cases}\n\\dfrac{1-\\alpha^{\\,m}}{1-\\alpha}\\;<\\;\\dfrac{1}{1-\\alpha}, & 0<\\alpha<1,\\\\[6pt]\nm\\;=\\;\\log_{2}n+1, & \\alpha=1,\\\\[6pt]\n\\dfrac{\\alpha^{\\,m}-1}{\\alpha-1}\\;<\\;C_{0}\\,\\alpha^{\\,m}, & 1<\\alpha<2,\n\\end{cases}\n\\]\nwhere $C_{0}:=\\dfrac{1}{\\alpha-1}$ is a fixed constant depending only on $\\alpha$ in the last case.\n\n2.  Express $\\alpha^{\\,m}$ as a power of $n$ (case $1<\\alpha<2$).\n\nFor $1<\\alpha<2$ we have\n\\[\n\\alpha^{\\,m}\\le\n\\alpha\\,(2^{\\,m-1})^{\\log_{2}\\alpha}\n=\\alpha\\,n^{\\beta},\\qquad\n\\beta:=\\log_{2}\\alpha\\in(0,1).\n\\]\nCollecting the three ranges of $\\alpha$ we therefore have\n\\[\n|B_{\\tau}(n)|\n\\;\\le\\;\n\\begin{cases}\n\\dfrac{1}{1-\\alpha}, & 0<\\alpha<1,\\\\[6pt]\n\\log_{2}n+1, & \\alpha=1,\\\\[6pt]\nC\\,n^{\\beta}, & 1<\\alpha<2,\n\\end{cases}\n\\tag{1}\n\\]\nfor suitable absolute constants $C$.\n\n3.  Comparison test for $S(\\tau)$.\n\nUsing (1) we obtain\n\\[\n\\frac{|B_{\\tau}(n)|}{n(n+1)}\n\\;\\le\\;\n\\begin{cases}\n\\dfrac{C_{1}}{n^{2}}, & 0<\\alpha<1,\\\\[8pt]\n\\dfrac{C_{2}\\,\\log n}{n^{2}}, & \\alpha=1,\\\\[8pt]\n\\dfrac{C_{3}}{n^{2-\\beta}}, & 1<\\alpha<2.\n\\end{cases}\n\\]\nSince $2-\\beta>1$, each comparison series\n$\\sum_{n\\ge 1}n^{-2}$, $\\sum_{n\\ge 1}n^{-2}\\log n$ and\n$\\sum_{n\\ge 1}n^{-(2-\\beta)}$\nconverges.  Hence\n$\\sum_{n\\ge 1}|B_{\\tau}(n)|/[n(n+1)]$ converges absolutely and so does\n$S(\\tau)$. $\\square$\n\n--------------------------------------------------------------------\nB.  Closed form for $S(\\tau)$\n--------------------------------------------------------------------\nBecause $S(\\tau)$ is absolutely convergent we may rearrange\n\\[\nS(\\tau)\n=\\sum_{i\\ge 0}\\tau^{\\,i}\n\\underbrace{\\sum_{n\\ge 1}\\frac{\\varepsilon_{i}(n)}{n(n+1)}}_{=:T_{i}},\n\\tag{2}\n\\]\nwhere $\\varepsilon_{i}(n)$ is the $i$-th binary digit of $n$.\nFix $i\\ge 0$ and put $d:=2^{\\,i}$.\nInside each block\n\\[\n\\{2d\\,m,\\,2d\\,m+1,\\,\\dots,\\,2d\\,m+2d-1\\},\\qquad m\\ge 0,\n\\]\nexactly the last $d$ integers have $\\varepsilon_{i}(n)=1$.\nWriting $n=2d\\,m+d+k$ with $0\\le k<d$ gives\n\\[\nT_{i}\n=\\sum_{m\\ge 0}\\sum_{k=0}^{d-1}\n\\frac{1}{(2d\\,m+d+k)(2d\\,m+d+k+1)}.\n\\]\nSince $\\dfrac{1}{s(s+1)}=\\dfrac{1}{s}-\\dfrac{1}{s+1}$, the inner sum\ntelescopes:\n\\[\n\\sum_{k=0}^{d-1}\\frac{1}{(A+k)(A+k+1)}\n=\\frac{1}{A}-\\frac{1}{A+d},\n\\qquad A:=2d\\,m+d.\n\\]\nHence\n\\[\nT_{i}\n=\\sum_{m\\ge 0}\\Bigl(\\frac{1}{2d\\,m+d}-\\frac{1}{2d\\,m+2d}\\Bigr)\n=\\frac{1}{d}\\sum_{m\\ge 0}\\Bigl(\\frac{1}{2m+1}-\\frac{1}{2m+2}\\Bigr).\n\\]\nThe inner series is the alternating harmonic series\n$\\sum_{m\\ge 0}(-1)^{m}/(m+1)=\\ln 2$, therefore\n\\[\nT_{i}=\\frac{\\ln 2}{2^{\\,i}}.\n\\]\nInsert this into (2):\n\\[\nS(\\tau)\n=\\ln 2\\sum_{i\\ge 0}\\Bigl(\\frac{\\tau}{2}\\Bigr)^{i}\n=\\frac{\\ln 2}{1-\\tau/2},\n\\qquad |\\tau|<2.\n\\]\nExponentiation gives\n\\[\nE(\\tau)=2^{\\,1/(1-\\tau/2)}.\n\\quad\\square\n\\]\n\n--------------------------------------------------------------------\nC.  Arithmetic nature of $E(\\tau)$\n--------------------------------------------------------------------\nPut\n\\[\n\\rho(\\tau):=\\frac{1}{1-\\tau/2},\\qquad\\text{so }E(\\tau)=2^{\\rho(\\tau)}.\n\\]\n\n----------------------------------------------------------------\n(i)  Rational $\\tau$ with $E(\\tau)\\in\\mathbb{Q}$\n----------------------------------------------------------------\nWrite $\\tau=a/b$ in lowest terms with $b>0$ and $|a/b|<2$.\nThen $\\rho=2b/(2b-a)\\in\\mathbb{Q}$.\nThe equality $2^{\\rho}\\in\\mathbb{Q}$ holds exactly when $\\rho\\in\\mathbb{Z}$,\nbecause:\n\nLemma 1.  \n$2^{r}$ is irrational for every rational $r\\notin\\mathbb{Z}$.\n\nProof.  Write $r=p/q$ with $\\gcd(p,q)=1$, $q\\ge 2$.\nIf $2^{p/q}$ were rational, then $(2^{p/q})^{q}=2^{p}$ would be rational\nand hence an integer, forcing $2^{p/q}$ itself to be an integer.\nBut an integer perfect $q$-th power of $2$ is $2^{k}$ with $k\\in\\mathbb{Z}$,\ncontradicting $q\\ge 2$. $\\square$\n\nThus $\\rho=k\\in\\mathbb{Z}$ and\n$\\tau=2\\bigl(1-\\dfrac{1}{k}\\bigr)$.\nCondition $|\\tau|<2$ implies $k\\ge 1$.\nConsequently\n\\[\n\\tau=2-\\frac{2}{k},\\qquad k=1,2,3,\\dots,\\qquad\nE(\\tau)=2^{\\,k}.\n\\]\n\n----------------------------------------------------------------\n(ii)  Rational $\\tau$ not covered by (i)\n----------------------------------------------------------------\nNow $\\tau$ is rational with $|\\tau|<2$ and\n$\\rho=p/q\\in\\mathbb{Q}\\setminus\\mathbb{Z}$ in lowest terms ($q>1$).\nThen\n\\[\nE(\\tau)=2^{p/q}.\n\\]\nRaising both sides to the $q$-th power gives\n\\[\n\\bigl(E(\\tau)\\bigr)^{q}=2^{p},\n\\]\nso $E(\\tau)$ is a root of\n\\[\nf(x):=x^{q}-2^{p}\\in\\mathbb{Q}[x].\n\\]\n\nIrreducibility.  \nThe polynomial $x^{q}-2$ is Eisenstein at the prime $2$, hence irreducible.\nConsequently $2^{1/q}$ has degree $q$ over $\\mathbb{Q}$.\nBecause $\\gcd(p,q)=1$ there exist integers $u,v$ with $up+vq=1$,\nand therefore\n$2^{1/q}=(2^{p/q})^{u}\\,2^{v}\\in\\mathbb{Q}(2^{p/q})$.\nThus\n$\\mathbb{Q}(2^{p/q})=\\mathbb{Q}(2^{1/q})$ and\n$[\\mathbb{Q}(2^{p/q}):\\mathbb{Q}]=q$.\nHence $f(x)$, which has degree $q$ and vanishes at $E(\\tau)$, must be the\nminimal polynomial of $E(\\tau)$ over $\\mathbb{Q}$.\nSince $q\\ge 2$, $E(\\tau)$ is algebraic but irrational. $\\square$\n\n----------------------------------------------------------------\n(iii)  Algebraic, irrational $\\tau$\n----------------------------------------------------------------\nHere $\\tau$ is algebraic but not rational, so\n$\\rho(\\tau)$ is algebraic and irrational.\nBecause the base $2$ is algebraic and $2\\ne 0,1$, the Gelfond–Schneider\ntheorem implies that the number\n\\[\nE(\\tau)=2^{\\rho(\\tau)}\n\\]\nis transcendental. $\\square$",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.660074",
        "was_fixed": false,
        "difficulty_analysis": "• Multiple Parameters:  The problem now contains a complex parameter τ whose\narithmetic nature (rational / irrational algebraic / transcendental) must be\ntracked throughout.\n\n• General Classification:  Instead of a single yes/no rationality test, the\nsolver must give a full classification of E(τ)—rational, irrational algebraic,\nor transcendental—depending on τ.\n\n• Advanced Tools:  Beyond elementary telescoping, the solution invokes\nproperties of harmonic numbers, convergence in the complex plane, the structure\nof algebraic powers 2^{α}, and the Lindemann–Weierstrass theorem.\n\n• Higher Conceptual Load:  The argument must disentangle absolute convergence,\ndouble–series rearrangements, binary–digit combinatorics, geometric-series sums\nin complex variables, and algebraic-number theory in one coherent chain.\n\n• Broader Insight Required:  The original problem asks only for “rational or\nirrational.”  The enhanced variant demands an explicit closed form and a\ndetailed arithmetic classification, substantially increasing both technical\ncomplexity and conceptual depth."
      }
    },
    "original_kernel_variant": {
      "question": "Fix a complex parameter $\\tau$ with $|\\tau|<2$.  \nFor every positive integer $n$ write its binary expansion\n\\[\nn=\\varepsilon_{0}+ \\varepsilon_{1}\\,2+\\varepsilon_{2}\\,2^{2}+\\dots ,\n\\qquad \\varepsilon_{i}\\in\\{0,1\\}\\text{ and only finitely many }\\varepsilon_{i}=1 .\n\\]\n\nDefine the $\\tau$–weighted binary digit–sum\n\\[\nB_{\\tau}(n)\\;:=\\;\\sum_{i\\ge 0}\\varepsilon_{i}\\,\\tau^{\\,i},\n\\]\nand set\n\\[\nS(\\tau):=\\sum_{n=1}^{\\infty}\\frac{B_{\\tau}(n)}{n(n+1)},\\qquad \nE(\\tau):=\\exp\\bigl(S(\\tau)\\bigr).\n\\]\n\nA.  Show that the series $S(\\tau)$ converges absolutely for every $\\tau$ with $|\\tau|<2$.\n\nB.  Prove the closed formulas\n\\[\nS(\\tau)=\\frac{\\ln 2}{1-\\tau/2},\n\\qquad\nE(\\tau)=2^{\\,1/(1-\\tau/2)}.\n\\]\n\nC.  Study the arithmetic nature of $E(\\tau)$.\n\n   (i)  Determine precisely those rational parameters $\\tau$ ($|\\tau|<2$) for which $E(\\tau)$ is rational, and give the corresponding values $E(\\tau)$.\n\n   (ii)  Let $\\tau$ be a rational number not covered by (i).\n         Prove that $E(\\tau)$ is algebraic but irrational and determine its minimal polynomial over $\\mathbb{Q}$.\n\n   (iii)  Show that if $\\tau$ is algebraic and irrational ($|\\tau|<2$) then $E(\\tau)$ is transcendental.\n\n   (The last assertion is an application of the Gelfond–Schneider theorem; no claim is made about transcendental $\\tau$.)",
      "solution": "Throughout write $\\alpha:=|\\tau|<2$.\n\n--------------------------------------------------------------------\nA.  Absolute convergence of $S(\\tau)$\n--------------------------------------------------------------------\nLet $m:=\\lfloor\\log_{2}n\\rfloor+1$ be the length of the binary\nexpansion of $n$, so that $2^{\\,m-1}\\le n<2^{\\,m}$.\n\n1.  A uniform bound for $B_{\\tau}(n)$.\n\nSince $|\\varepsilon_{i}|=1$ when it occurs,\n\\[\n|B_{\\tau}(n)|\n\\;=\\;\\bigl|\\sum_{i=0}^{m-1}\\varepsilon_{i}\\tau^{\\,i}\\bigr|\n\\;\\le\\;\\sum_{i=0}^{m-1}\\alpha^{\\,i}.\n\\]\nThus\n\\[\n|B_{\\tau}(n)|\\le\n\\begin{cases}\n\\dfrac{1-\\alpha^{\\,m}}{1-\\alpha}\\;<\\;\\dfrac{1}{1-\\alpha}, & 0<\\alpha<1,\\\\[6pt]\nm\\;=\\;\\log_{2}n+1, & \\alpha=1,\\\\[6pt]\n\\dfrac{\\alpha^{\\,m}-1}{\\alpha-1}\\;<\\;C_{0}\\,\\alpha^{\\,m}, & 1<\\alpha<2,\n\\end{cases}\n\\]\nwhere $C_{0}:=\\dfrac{1}{\\alpha-1}$ is a fixed constant depending only on $\\alpha$ in the last case.\n\n2.  Express $\\alpha^{\\,m}$ as a power of $n$ (case $1<\\alpha<2$).\n\nFor $1<\\alpha<2$ we have\n\\[\n\\alpha^{\\,m}\\le\n\\alpha\\,(2^{\\,m-1})^{\\log_{2}\\alpha}\n=\\alpha\\,n^{\\beta},\\qquad\n\\beta:=\\log_{2}\\alpha\\in(0,1).\n\\]\nCollecting the three ranges of $\\alpha$ we therefore have\n\\[\n|B_{\\tau}(n)|\n\\;\\le\\;\n\\begin{cases}\n\\dfrac{1}{1-\\alpha}, & 0<\\alpha<1,\\\\[6pt]\n\\log_{2}n+1, & \\alpha=1,\\\\[6pt]\nC\\,n^{\\beta}, & 1<\\alpha<2,\n\\end{cases}\n\\tag{1}\n\\]\nfor suitable absolute constants $C$.\n\n3.  Comparison test for $S(\\tau)$.\n\nUsing (1) we obtain\n\\[\n\\frac{|B_{\\tau}(n)|}{n(n+1)}\n\\;\\le\\;\n\\begin{cases}\n\\dfrac{C_{1}}{n^{2}}, & 0<\\alpha<1,\\\\[8pt]\n\\dfrac{C_{2}\\,\\log n}{n^{2}}, & \\alpha=1,\\\\[8pt]\n\\dfrac{C_{3}}{n^{2-\\beta}}, & 1<\\alpha<2.\n\\end{cases}\n\\]\nSince $2-\\beta>1$, each comparison series\n$\\sum_{n\\ge 1}n^{-2}$, $\\sum_{n\\ge 1}n^{-2}\\log n$ and\n$\\sum_{n\\ge 1}n^{-(2-\\beta)}$\nconverges.  Hence\n$\\sum_{n\\ge 1}|B_{\\tau}(n)|/[n(n+1)]$ converges absolutely and so does\n$S(\\tau)$. $\\square$\n\n--------------------------------------------------------------------\nB.  Closed form for $S(\\tau)$\n--------------------------------------------------------------------\nBecause $S(\\tau)$ is absolutely convergent we may rearrange\n\\[\nS(\\tau)\n=\\sum_{i\\ge 0}\\tau^{\\,i}\n\\underbrace{\\sum_{n\\ge 1}\\frac{\\varepsilon_{i}(n)}{n(n+1)}}_{=:T_{i}},\n\\tag{2}\n\\]\nwhere $\\varepsilon_{i}(n)$ is the $i$-th binary digit of $n$.\nFix $i\\ge 0$ and put $d:=2^{\\,i}$.\nInside each block\n\\[\n\\{2d\\,m,\\,2d\\,m+1,\\,\\dots,\\,2d\\,m+2d-1\\},\\qquad m\\ge 0,\n\\]\nexactly the last $d$ integers have $\\varepsilon_{i}(n)=1$.\nWriting $n=2d\\,m+d+k$ with $0\\le k<d$ gives\n\\[\nT_{i}\n=\\sum_{m\\ge 0}\\sum_{k=0}^{d-1}\n\\frac{1}{(2d\\,m+d+k)(2d\\,m+d+k+1)}.\n\\]\nSince $\\dfrac{1}{s(s+1)}=\\dfrac{1}{s}-\\dfrac{1}{s+1}$, the inner sum\ntelescopes:\n\\[\n\\sum_{k=0}^{d-1}\\frac{1}{(A+k)(A+k+1)}\n=\\frac{1}{A}-\\frac{1}{A+d},\n\\qquad A:=2d\\,m+d.\n\\]\nHence\n\\[\nT_{i}\n=\\sum_{m\\ge 0}\\Bigl(\\frac{1}{2d\\,m+d}-\\frac{1}{2d\\,m+2d}\\Bigr)\n=\\frac{1}{d}\\sum_{m\\ge 0}\\Bigl(\\frac{1}{2m+1}-\\frac{1}{2m+2}\\Bigr).\n\\]\nThe inner series is the alternating harmonic series\n$\\sum_{m\\ge 0}(-1)^{m}/(m+1)=\\ln 2$, therefore\n\\[\nT_{i}=\\frac{\\ln 2}{2^{\\,i}}.\n\\]\nInsert this into (2):\n\\[\nS(\\tau)\n=\\ln 2\\sum_{i\\ge 0}\\Bigl(\\frac{\\tau}{2}\\Bigr)^{i}\n=\\frac{\\ln 2}{1-\\tau/2},\n\\qquad |\\tau|<2.\n\\]\nExponentiation gives\n\\[\nE(\\tau)=2^{\\,1/(1-\\tau/2)}.\n\\quad\\square\n\\]\n\n--------------------------------------------------------------------\nC.  Arithmetic nature of $E(\\tau)$\n--------------------------------------------------------------------\nPut\n\\[\n\\rho(\\tau):=\\frac{1}{1-\\tau/2},\\qquad\\text{so }E(\\tau)=2^{\\rho(\\tau)}.\n\\]\n\n----------------------------------------------------------------\n(i)  Rational $\\tau$ with $E(\\tau)\\in\\mathbb{Q}$\n----------------------------------------------------------------\nWrite $\\tau=a/b$ in lowest terms with $b>0$ and $|a/b|<2$.\nThen $\\rho=2b/(2b-a)\\in\\mathbb{Q}$.\nThe equality $2^{\\rho}\\in\\mathbb{Q}$ holds exactly when $\\rho\\in\\mathbb{Z}$,\nbecause:\n\nLemma 1.  \n$2^{r}$ is irrational for every rational $r\\notin\\mathbb{Z}$.\n\nProof.  Write $r=p/q$ with $\\gcd(p,q)=1$, $q\\ge 2$.\nIf $2^{p/q}$ were rational, then $(2^{p/q})^{q}=2^{p}$ would be rational\nand hence an integer, forcing $2^{p/q}$ itself to be an integer.\nBut an integer perfect $q$-th power of $2$ is $2^{k}$ with $k\\in\\mathbb{Z}$,\ncontradicting $q\\ge 2$. $\\square$\n\nThus $\\rho=k\\in\\mathbb{Z}$ and\n$\\tau=2\\bigl(1-\\dfrac{1}{k}\\bigr)$.\nCondition $|\\tau|<2$ implies $k\\ge 1$.\nConsequently\n\\[\n\\tau=2-\\frac{2}{k},\\qquad k=1,2,3,\\dots,\\qquad\nE(\\tau)=2^{\\,k}.\n\\]\n\n----------------------------------------------------------------\n(ii)  Rational $\\tau$ not covered by (i)\n----------------------------------------------------------------\nNow $\\tau$ is rational with $|\\tau|<2$ and\n$\\rho=p/q\\in\\mathbb{Q}\\setminus\\mathbb{Z}$ in lowest terms ($q>1$).\nThen\n\\[\nE(\\tau)=2^{p/q}.\n\\]\nRaising both sides to the $q$-th power gives\n\\[\n\\bigl(E(\\tau)\\bigr)^{q}=2^{p},\n\\]\nso $E(\\tau)$ is a root of\n\\[\nf(x):=x^{q}-2^{p}\\in\\mathbb{Q}[x].\n\\]\n\nIrreducibility.  \nThe polynomial $x^{q}-2$ is Eisenstein at the prime $2$, hence irreducible.\nConsequently $2^{1/q}$ has degree $q$ over $\\mathbb{Q}$.\nBecause $\\gcd(p,q)=1$ there exist integers $u,v$ with $up+vq=1$,\nand therefore\n$2^{1/q}=(2^{p/q})^{u}\\,2^{v}\\in\\mathbb{Q}(2^{p/q})$.\nThus\n$\\mathbb{Q}(2^{p/q})=\\mathbb{Q}(2^{1/q})$ and\n$[\\mathbb{Q}(2^{p/q}):\\mathbb{Q}]=q$.\nHence $f(x)$, which has degree $q$ and vanishes at $E(\\tau)$, must be the\nminimal polynomial of $E(\\tau)$ over $\\mathbb{Q}$.\nSince $q\\ge 2$, $E(\\tau)$ is algebraic but irrational. $\\square$\n\n----------------------------------------------------------------\n(iii)  Algebraic, irrational $\\tau$\n----------------------------------------------------------------\nHere $\\tau$ is algebraic but not rational, so\n$\\rho(\\tau)$ is algebraic and irrational.\nBecause the base $2$ is algebraic and $2\\ne 0,1$, the Gelfond–Schneider\ntheorem implies that the number\n\\[\nE(\\tau)=2^{\\rho(\\tau)}\n\\]\nis transcendental. $\\square$",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.519854",
        "was_fixed": false,
        "difficulty_analysis": "• Multiple Parameters:  The problem now contains a complex parameter τ whose\narithmetic nature (rational / irrational algebraic / transcendental) must be\ntracked throughout.\n\n• General Classification:  Instead of a single yes/no rationality test, the\nsolver must give a full classification of E(τ)—rational, irrational algebraic,\nor transcendental—depending on τ.\n\n• Advanced Tools:  Beyond elementary telescoping, the solution invokes\nproperties of harmonic numbers, convergence in the complex plane, the structure\nof algebraic powers 2^{α}, and the Lindemann–Weierstrass theorem.\n\n• Higher Conceptual Load:  The argument must disentangle absolute convergence,\ndouble–series rearrangements, binary–digit combinatorics, geometric-series sums\nin complex variables, and algebraic-number theory in one coherent chain.\n\n• Broader Insight Required:  The original problem asks only for “rational or\nirrational.”  The enhanced variant demands an explicit closed form and a\ndetailed arithmetic classification, substantially increasing both technical\ncomplexity and conceptual depth."
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}