{
  "index": "2014-A-5",
  "type": "NT",
  "tag": [
    "NT",
    "ALG",
    "ANA"
  ],
  "difficulty": "",
  "question": "Let\n\\[\nP_n(x) = 1 + 2 x  + 3 x^2 + \\cdots + n x^{n-1}.\n\\]\nProve that the polynomials $P_j(x)$\nand $P_k(x)$ are relatively prime\nfor all positive integers $j$ and $k$ with $j \\neq k$.",
  "solution": "\\textbf{First solution:}\nSuppose to the contrary that there exist positive integers $i \\neq j$ and a complex number $z$ such that $P_i(z) = P_j(z) = 0$. Note that $z$ cannot be a nonnegative real number or else $P_i(z), P_j(z) > 0$; we may put $w = z^{-1} \\neq 0,1$. For $n \\in \\{i+1,j+1\\}$ we compute that\n\\[\nw^n = n w - n + 1,\n\\qquad \\overline{w}^n =  n \\overline{w} - n + 1;\n\\]\nnote crucially that these equations also hold for $n \\in \\{0,1\\}$.\nTherefore, the function $f: [0, +\\infty) \\to \\RR$ given by\n\\[\nf(t) = \\left| w \\right|^{2t} - t^2 \\left| w \\right|^2 + 2t(t-1)\\mathrm{Re}(w) - (t-1)^2\n\\]\nsatisfies $f(t) = 0$ for $t \\in \\{0,1,i+1,j+1\\}$. On the other hand, for all $t \\geq 0$ we have\n\\[\nf'''(t) = (2 \\log \\left| w \\right|)^3 \\left| w \\right|^{2t} > 0,\n\\]\nso by Rolle's theorem, the equation $f^{(3-k)}(t) = 0$ has at most $k$ distinct solutions for $k=0,1,2,3$. This yields the desired contradiction.\n\n\\noindent\n\\textbf{Remark:}\nBy similar reasoning, an equation of the form $e^{x} = P(x)$ in which $P$ is a real polynomial of degree $d$ has at most $d+1$ real solutions. This turns out to be closely related to a concept in mathematical logic known as \\emph{o-minimality}, which in turn has deep consequences for the solution of Diophantine equations.\n\n\\noindent\n\\textbf{Second solution:}\n\\setcounter{lemma}{0}\n(by Noam Elkies)\nWe recall a result commonly known as the \\emph{Enestr\\\"om-Kakeya theorem}.\n\\begin{lemma}\nLet \n\\[\nf(x) = a_0 + a_1 x + \\cdots + a_n x^n\n\\]\nbe a polynomial with real coefficients such that $0 < a_0 \\leq a_1 \\leq \\cdots \\leq a_n$.\nThen every root $z \\in \\CC$ of $f$ satisfies $|z| \\leq 1$.\n\\end{lemma}\n\\begin{proof}\nIf $f(z) = 0$, then we may rearrange the equality $0 = f(z)(z-1)$ to obtain\n\\[\na_n z^{n+1} = (a_n - a_{n-1}) z^n + \\cdots + (a_1 - a_0)z + a_0.\n\\]\nBut if $|z| > 1$, then \n\\[\n|a_n z^{n+1}| \\leq (|a_n - a_{n-1}| + \\cdots + |a_1 - a_0|) |z|^{n}\n\\leq |a_n z^{n}|,\n\\]\ncontradiction.\n\\end{proof}\n\\begin{cor}\nLet \n\\[\nf(x) = a_0 + a_1 x + \\cdots + a_n x^n\n\\]\nbe a polynomial with positive real coefficients. Then every root $z \\in \\CC$ of $f$ satisfies $r \\leq |z| \\leq R$ for\n\\begin{align*}\nr &= \\min\\{a_0/a_1, \\dots, a_{n-1}/a_n\\} \\\\\nR &= \\max\\{a_0/a_1, \\dots, a_{n-1}/a_n\\}.\n\\end{align*}\n\\end{cor}\n\\begin{proof}\nThe bound $|z| \\leq R$ follows by applying the lemma to the polynomial $f(x/R)$.\nThe bound $|z| \\geq r$ follows by applying the lemma to the reverse of the polynomial $f(x/r)$.\n\\end{proof}\nSuppose now that $P_i(z) = P_j(z) = 0$ for some $z \\in \\CC$ and some integers $i < j$. \nWe clearly cannot have $j = i+1$, as then $P_i(0) \\neq 0$ and so $P_j(z) - P_i(z) = (i+1) z^i \\neq 0$; we thus have $j-i \\geq 2$.\nBy applying Corollary~2 to $P_i(x)$, we see that $|z| \\leq 1 - \\frac{1}{i}$. On the other hand, by applying Corollary~2 to $(P_j(x) - P_i(x))/x^{i-1}$, we see that $|z| \\geq 1 - \\frac{1}{i+2}$, contradiction.\n\n\\noindent\n\\textbf{Remark:} Elkies also reports that this problem is his submission, dating back to 2005 and arising from work of Joe Harris.\nIt dates back further to Example 3.7 in: Hajime Kaji,\nOn the tangentially degenerate curves,\n\\textit{J. London Math. Soc. (2)} \\textbf{33} (1986), 430--440, in which the second solution is given.\n\n\\noindent\n\\textbf{Remark:}\nElkies points out a mild generalization which may be treated using the first solution but not the second: for integers $a<b<c<d$ and $z \\in \\CC$\nwhich is neither zero nor a root of unity, the matrix\n\\[\n\\begin{pmatrix}\n1 & 1 & 1 & 1 \\\\\na & b & c & d \\\\\nz^a & z^b & z^c & z^d\n\\end{pmatrix}\n\\]\nhas rank 3 (the problem at hand being the case $a=0, b=1, c=i+1, d=j+1$).\n\n\\noindent\n\\textbf{Remark:}\nIt seems likely that the individual polynomials $P_k(x)$ are all irreducible, but this appears difficult to prove.\n\n\\noindent\n\\textbf{Third solution:}\n(by David Feldman)\nNote that\n\\[\nP_n(x)(1-x) = 1 + x + \\cdots + x^{n-1} - nx^n.\n\\]\nIf $|z| \\geq 1$, then\n\\[\nn|z|^n \\geq |z|^{n-1} + \\cdots + 1 \\geq |z^{n-1} + \\cdots + 1|,\n\\]\nwith the first equality occurring only if $|z| = 1$ and the second equality occurring only if $z$ is a positive real number. Hence the equation $P_n(z)(1-z) = 0$ has no solutions  with $|z| \\geq 1$ other than the trivial solution $z=1$. Since\n\\[\nP_n(x)(1-x)^2 = 1 - (n+1) x^n + nx^{n+1},\n\\]\nit now suffices to check that the curves\n\\[\nC_n = \\{ z \\in \\CC: 0 < |z| < 1, \\left| z \\right|^n \\left| n+1 - zn \\right| = 1 \\}\n\\]\nare pairwise disjoint as $n$ varies over positive integers.\n\nWrite $z = u+iv$; we may assume without loss of generality that $v \\geq 0$.\nDefine the function\n\\[\nE_z(n) = n \\log |z| + \\log |n+1-zn|.\n\\]\nOne computes that for $n \\in \\RR$, $E_z''(n) < 0$ if and only if\n\\[\n\\frac{u-v-1}{(1-u)^2 + v^2} < n <\n\\frac{u+v-1}{(1-u)^2 + v^2}.\n\\]\nIn addition, $E_z(0) = 0$ and \n\\[\nE_z'(0) = \\frac{1}{2} \\log (u^2+v^2) + (1-u) \\geq \\log (u) + 1 - u \\geq 0\n\\]\nsince $\\log(u)$ is concave. From this, it follows that the equation $E_z(n) = 0$\ncan have at most one solution with $n>0$.\n\n\\noindent\n\\textbf{Remark:}\nThe reader may notice a strong similarity between this solution and the first solution. The primary difference is we compute that $E'_z(0) \\geq 0$ instead of discovering that $E_z(-1) = 0$.\n\n\\noindent\n\\textbf{Remark:}\nIt is also possible to solve this problem using a $p$-adic valuation on the field of algebraic numbers in place of the complex absolute value; however, this leads to a substantially more complicated solution. In lieu of including such a solution here,\nwe refer to the approach described by Victor Wang \nhere: \\url{http://www.artofproblemsolving.com/Forum/viewtopic.php?f=80&t=616731}.",
  "vars": [
    "C_n",
    "E_z",
    "R",
    "f",
    "r",
    "t",
    "u",
    "v",
    "w",
    "x",
    "z"
  ],
  "params": [
    "P_i",
    "P_j",
    "P_k",
    "P_n",
    "a",
    "a_0",
    "a_1",
    "a_n-1",
    "a_n",
    "b",
    "c",
    "d",
    "i",
    "j",
    "k",
    "n"
  ],
  "sci_consts": [
    "e"
  ],
  "variants": {
    "descriptive_long": {
      "map": {
        "C_n": "curvefamily",
        "E_z": "energyfunc",
        "R": "radiusrr",
        "f": "funcmain",
        "r": "radiuslo",
        "t": "variablet",
        "u": "realpart",
        "v": "imagpart",
        "w": "inversez",
        "x": "variablex",
        "z": "complexz",
        "P_i": "polyindexi",
        "P_j": "polyindexj",
        "P_k": "polyindexk",
        "P_n": "polyindexn",
        "a": "indexa",
        "a_0": "coeffazero",
        "a_1": "coeffaone",
        "a_n-1": "coeffaminus",
        "a_n": "coeffan",
        "b": "indexb",
        "c": "indexc",
        "d": "indexd",
        "i": "indexi",
        "j": "indexj",
        "k": "indexk",
        "n": "indexn"
      },
      "question": "Let\n\\[\npolyindexn(variablex) = 1 + 2 \\, variablex  + 3 \\, variablex^2 + \\cdots + indexn \\, variablex^{indexn-1}.\n\\]\nProve that the polynomials $polyindexj(variablex)$\nand $polyindexk(variablex)$ are relatively prime\nfor all positive integers $indexj$ and $indexk$ with $indexj \\neq indexk$.",
      "solution": "\\textbf{First solution:}\nSuppose to the contrary that there exist positive integers $indexi \\neq indexj$ and a complex number $complexz$ such that $polyindexi(complexz) = polyindexj(complexz) = 0$. Note that $complexz$ cannot be a nonnegative real number or else $polyindexi(complexz), polyindexj(complexz) > 0$; we may put $inversez = complexz^{-1} \\neq 0,1$. For $indexn \\in \\{indexi+1,indexj+1\\}$ we compute that\n\\[\ninversez^{indexn} = indexn \\, inversez - indexn + 1,\n\\qquad \\overline{inversez}^{indexn} =  indexn \\, \\overline{inversez} - indexn + 1;\n\\]\nnote crucially that these equations also hold for $indexn \\in \\{0,1\\}$.  Therefore, the function $funcmain: [0, +\\infty) \\to \\RR$ given by\n\\[\nfuncmain(variablet) = \\left| inversez \\right|^{2variablet} - variablet^2 \\left| inversez \\right|^2 + 2variablet(variablet-1)\\mathrm{Re}(inversez) - (variablet-1)^2\n\\]\nsatisfies $funcmain(variablet) = 0$ for $variablet \\in \\{0,1,indexi+1,indexj+1\\}$. On the other hand, for all $variablet \\geq 0$ we have\n\\[\nfuncmain'''(variablet) = (2 \\log \\left| inversez \\right|)^3 \\left| inversez \\right|^{2variablet} > 0,\n\\]\nso by Rolle's theorem, the equation $funcmain^{(3-indexk)}(variablet) = 0$ has at most $indexk$ distinct solutions for $indexk=0,1,2,3$. This yields the desired contradiction.\n\n\\noindent\n\\textbf{Remark:}\nBy similar reasoning, an equation of the form $e^{variablex} = P(variablex)$ in which $P$ is a real polynomial of degree $d$ has at most $d+1$ real solutions. This turns out to be closely related to a concept in mathematical logic known as \\emph{o-minimality}, which in turn has deep consequences for the solution of Diophantine equations.\n\n\\noindent\n\\textbf{Second solution:}\n(by Noam Elkies)\nWe recall a result commonly known as the \\emph{Enestr\\\"om-Kakeya theorem}.\n\\begin{lemma}\nLet \n\\[\nfuncmain(variablex) = coeffazero + coeffaone \\, variablex + \\cdots + coeffan \\, variablex^{indexn}\n\\]\nbe a polynomial with real coefficients such that $0 < coeffazero \\leq coeffaone \\leq \\cdots \\leq coeffan$.\nThen every root $complexz \\in \\CC$ of $funcmain$ satisfies $|complexz| \\leq 1$.\n\\end{lemma}\n\\begin{proof}\nIf $funcmain(complexz) = 0$, then we may rearrange the equality $0 = funcmain(complexz)(complexz-1)$ to obtain\n\\[\ncoeffan \\, complexz^{indexn+1} = (coeffan - coeffaminus) \\, complexz^{indexn} + \\cdots + (coeffaone - coeffazero)complexz + coeffazero.\n\\]\nBut if $|complexz| > 1$, then \n\\[\n|coeffan \\, complexz^{indexn+1}| \\leq (|coeffan - coeffaminus| + \\cdots + |coeffaone - coeffazero|) |complexz|^{indexn}\n\\leq |coeffan \\, complexz^{indexn}|,\n\\]\ncontradiction.\n\\end{proof}\n\\begin{cor}\nLet \n\\[\nfuncmain(variablex) = coeffazero + coeffaone \\, variablex + \\cdots + coeffan \\, variablex^{indexn}\n\\]\nbe a polynomial with positive real coefficients. Then every root $complexz \\in \\CC$ of $funcmain$ satisfies $radiuslo \\leq |complexz| \\leq radiusrr$ for\n\\[\nradiuslo = \\min\\{coeffazero/coeffaone, \\dots, coeffaminus/coeffan\\}, \\qquad\nradiusrr = \\max\\{coeffazero/coeffaone, \\dots, coeffaminus/coeffan\\}.\n\\]\n\\end{cor}\n\\begin{proof}\nThe bound $|complexz| \\leq radiusrr$ follows by applying the lemma to the polynomial $funcmain(variablex/radiusrr)$.\nThe bound $|complexz| \\geq radiuslo$ follows by applying the lemma to the reverse of the polynomial $funcmain(variablex/radiuslo)$.\n\\end{proof}\nSuppose now that $polyindexi(complexz) = polyindexj(complexz) = 0$ for some $complexz \\in \\CC$ and some integers $indexi < indexj$. \nWe clearly cannot have $indexj = indexi+1$, as then $polyindexi(0) \\neq 0$ and so $polyindexj(complexz) - polyindexi(complexz) = (indexi+1) \\, complexz^{indexi} \\neq 0$; we thus have $indexj-indexi \\geq 2$.\nBy applying Corollary~2 to $polyindexi(variablex)$, we see that $|complexz| \\leq 1 - \\frac{1}{indexi}$. On the other hand, by applying Corollary~2 to $(polyindexj(variablex) - polyindexi(variablex))/variablex^{indexi-1}$, we see that $|complexz| \\geq 1 - \\frac{1}{indexi+2}$, contradiction.\n\n\\noindent\n\\textbf{Remark:} Elkies also reports that this problem is his submission, dating back to 2005 and arising from work of Joe Harris.\nIt dates back further to Example 3.7 in: Hajime Kaji,\n\\emph{On the tangentially degenerate curves},\n\\textit{J. London Math. Soc. (2)} \\textbf{33} (1986), 430--440, in which the second solution is given.\n\n\\noindent\n\\textbf{Remark:}\nElkies points out a mild generalization which may be treated using the first solution but not the second: for integers indexa<indexb<indexc<indexd and $complexz \\in \\CC$\nwhich is neither zero nor a root of unity, the matrix\n\\[\n\\begin{pmatrix}\n1 & 1 & 1 & 1 \\\\\nindexa & indexb & indexc & indexd \\\\\ncomplexz^{indexa} & complexz^{indexb} & complexz^{indexc} & complexz^{indexd}\n\\end{pmatrix}\n\\]\nhas rank 3 (the problem at hand being the case indexa=0, indexb=1, indexc=indexi+1, indexd=indexj+1).\n\n\\noindent\n\\textbf{Remark:}\nIt seems likely that the individual polynomials $polyindexk(variablex)$ are all irreducible, but this appears difficult to prove.\n\n\\noindent\n\\textbf{Third solution:}\n(by David Feldman)\nNote that\n\\[\npolyindexn(variablex)(1-variablex) = 1 + variablex + \\cdots + variablex^{indexn-1} - indexn\\,variablex^{indexn}.\n\\]\nIf $|complexz| \\geq 1$, then\n\\[\nindexn|complexz|^{indexn} \\geq |complexz|^{indexn-1} + \\cdots + 1 \\geq |complexz^{indexn-1} + \\cdots + 1|,\n\\]\nwith the first equality occurring only if $|complexz| = 1$ and the second equality occurring only if $complexz$ is a positive real number. Hence the equation $polyindexn(complexz)(1-complexz) = 0$ has no solutions  with $|complexz| \\geq 1$ other than the trivial solution $complexz=1$. Since\n\\[\npolyindexn(variablex)(1-variablex)^2 = 1 - (indexn+1) \\, variablex^{indexn} + indexn\\,variablex^{indexn+1},\n\\]\nit now suffices to check that the curves\n\\[\ncurvefamily = \\{ complexz \\in \\CC: 0 < |complexz| < 1, | complexz |^{indexn} \\, | indexn+1 - complexz\\,indexn | = 1 \\}\n\\]\nare pairwise disjoint as $indexn$ varies over positive integers.\n\nWrite $complexz = realpart+ i\\,imagpart$; we may assume without loss of generality that $imagpart \\geq 0$.\nDefine the function\n\\[\nenergyfunc_{complexz}(indexn) = indexn \\log |complexz| + \\log |indexn+1-complexz\\,indexn|.\n\\]\nOne computes that for $indexn \\in \\RR$, $energyfunc_{complexz}''(indexn) < 0$ if and only if\n\\[\n\\frac{realpart-imagpart-1}{(1-realpart)^2 + imagpart^2} < indexn <\n\\frac{realpart+imagpart-1}{(1-realpart)^2 + imagpart^2}.\n\\]\nIn addition, $energyfunc_{complexz}(0) = 0$ and \n\\[\nenergyfunc_{complexz}'(0) = \\frac{1}{2} \\log (realpart^2+imagpart^2) + (1-realpart) \\geq \\log (realpart) + 1 - realpart \\geq 0\n\\]\nsince $\\log(realpart)$ is concave. From this, it follows that the equation $energyfunc_{complexz}(indexn) = 0$\ncan have at most one solution with $indexn>0$.\n\n\\noindent\n\\textbf{Remark:}\nThe reader may notice a strong similarity between this solution and the first solution. The primary difference is we compute that $energyfunc'_{complexz}(0) \\geq 0$ instead of discovering that $energyfunc_{complexz}(-1) = 0$.\n\n\\noindent\n\\textbf{Remark:}\nIt is also possible to solve this problem using a $p$-adic valuation on the field of algebraic numbers in place of the complex absolute value; however, this leads to a substantially more complicated solution. In lieu of including such a solution here,\nwe refer to the approach described by Victor Wang \nhere: http://www.artofproblemsolving.com/Forum/viewtopic.php?f=80&t=616731."
    },
    "descriptive_long_confusing": {
      "map": {
        "C_n": "orchardleaf",
        "E_z": "daisyfield",
        "R": "pineconess",
        "f": "kaleidoscope",
        "r": "marigoldbloom",
        "t": "lanternshade",
        "u": "silverspoon",
        "v": "thistlebrush",
        "w": "parchmentroll",
        "x": "ambergrisresin",
        "z": "willowbranch",
        "P_i": "sandcastles",
        "P_j": "driftwoodlog",
        "P_k": "honeyedmead",
        "P_n": "moonlitshore",
        "a": "crystalflame",
        "a_0": "hazelgrouse",
        "a_1": "emberfirefly",
        "a_n-1": "sparrowfeather",
        "a_n": "cedarwoodmist",
        "b": "riveroctopus",
        "c": "cloudberryjam",
        "d": "stardustveil",
        "i": "i",
        "j": "parchmentquill",
        "k": "foxglovelure",
        "n": "thunderstrike"
      },
      "question": "Let\n\\[\nmoonlitshore(ambergrisresin) = 1 + 2 ambergrisresin  + 3 ambergrisresin^2 + \\cdots + thunderstrike ambergrisresin^{thunderstrike-1}.\n\\]\nProve that the polynomials $driftwoodlog(ambergrisresin)$\nand $honeyedmead(ambergrisresin)$ are relatively prime\nfor all positive integers parchmentquill and foxglovelure with parchmentquill \\neq foxglovelure.",
      "solution": "\\textbf{First solution:}\nSuppose to the contrary that there exist positive integers $i \\neq parchmentquill$ and a complex number $willowbranch$ such that $sandcastles(willowbranch) = driftwoodlog(willowbranch) = 0$. Note that $willowbranch$ cannot be a nonnegative real number or else $sandcastles(willowbranch),\\ driftwoodlog(willowbranch) > 0$; we may put $parchmentroll = willowbranch^{-1} \\neq 0,1$. For $thunderstrike \\in \\{i+1,parchmentquill+1\\}$ we compute that\n\\[\nparchmentroll^{thunderstrike} = thunderstrike \\, parchmentroll - thunderstrike + 1,\n\\qquad \\overline{parchmentroll}^{\\,\\thunderstrike} =  thunderstrike \\, \\overline{parchmentroll} - thunderstrike + 1;\n\\]\nnote crucially that these equations also hold for $thunderstrike \\in \\{0,1\\}$. Therefore, the function $kaleidoscope: [0, +\\infty) \\to \\RR$ given by\n\\[\nkaleidoscope(lanternshade) = \\left| parchmentroll \\right|^{2\\,lanternshade} - lanternshade^2 \\left| parchmentroll \\right|^2 + 2\\,lanternshade(lanternshade-1)\\mathrm{Re}(parchmentroll) - (lanternshade-1)^2\n\\]\nsatisfies $kaleidoscope(lanternshade) = 0$ for $lanternshade \\in \\{0,1,i+1,parchmentquill+1\\}$. On the other hand, for all $lanternshade \\geq 0$ we have\n\\[\nkaleidoscope'''(lanternshade) = (2 \\log \\left| parchmentroll \\right|)^3 \\left| parchmentroll \\right|^{2\\,lanternshade} > 0,\n\\]\nso by Rolle's theorem, the equation $kaleidoscope^{(3-foxglovelure)}(lanternshade) = 0$ has at most $foxglovelure$ distinct solutions for $foxglovelure=0,1,2,3$. This yields the desired contradiction.\n\n\\noindent\n\\textbf{Remark:}\nBy similar reasoning, an equation of the form $e^{x} = P(x)$ in which $P$ is a real polynomial of degree $d$ has at most $d+1$ real solutions. This turns out to be closely related to a concept in mathematical logic known as \\emph{o-minimality}, which in turn has deep consequences for the solution of Diophantine equations.\n\n\\noindent\n\\textbf{Second solution:}\n\\setcounter{lemma}{0}\n(by Noam Elkies)\nWe recall a result commonly known as the \\emph{Enestr\"om-Kakeya theorem}.\n\\begin{lemma}\nLet \n\\[\nkaleidoscope(ambergrisresin) = hazelgrouse + emberfirefly \\, ambergrisresin + \\cdots + cedarwoodmist \\, ambergrisresin^{thunderstrike}\n\\]\nbe a polynomial with real coefficients such that $0 < hazelgrouse \\leq emberfirefly \\leq \\cdots \\leq cedarwoodmist$.\nThen every root $willowbranch \\in \\CC$ of $kaleidoscope$ satisfies $|willowbranch| \\leq 1$.\n\\end{lemma}\n\\begin{proof}\nIf $kaleidoscope(willowbranch) = 0$, then we may rearrange the equality $0 = kaleidoscope(willowbranch)(willowbranch-1)$ to obtain\n\\[\ncedarwoodmist \\, willowbranch^{thunderstrike+1} = (cedarwoodmist - sparrowfeather) \\, willowbranch^{\\thunderstrike} + \\cdots + (emberfirefly - hazelgrouse) \\, willowbranch + hazelgrouse.\n\\]\nBut if $|willowbranch| > 1$, then \n\\[\n|cedarwoodmist \\, willowbranch^{\\thunderstrike+1}| \\leq (|cedarwoodmist - sparrowfeather| + \\cdots + |emberfirefly - hazelgrouse|) |willowbranch|^{\\thunderstrike}\n\\leq |cedarwoodmist \\, willowbranch^{\\thunderstrike}|,\n\\]\ncontradiction.\n\\end{proof}\n\\begin{cor}\nLet \n\\[\nkaleidoscope(ambergrisresin) = hazelgrouse + emberfirefly \\, ambergrisresin + \\cdots + cedarwoodmist \\, ambergrisresin^{\\thunderstrike}\n\\]\nbe a polynomial with positive real coefficients. Then every root $willowbranch \\in \\CC$ of $kaleidoscope$ satisfies $marigoldbloom \\leq |willowbranch| \\leq pineconess$ for\n\\begin{align*}\nmarigoldbloom &= \\min\\{hazelgrouse/emberfirefly, \\dots, sparrowfeather/cedarwoodmist\\} \\\\\npineconess &= \\max\\{hazelgrouse/emberfirefly, \\dots, sparrowfeather/cedarwoodmist\\}.\n\\end{align*}\n\\end{cor}\n\\begin{proof}\nThe bound $|willowbranch| \\leq pineconess$ follows by applying the lemma to the polynomial $kaleidoscope(ambergrisresin/pineconess)$.\nThe bound $|willowbranch| \\geq marigoldbloom$ follows by applying the lemma to the reverse of the polynomial $kaleidoscope(ambergrisresin/marigoldbloom)$.\n\\end{proof}\nSuppose now that $sandcastles(willowbranch) = driftwoodlog(willowbranch) = 0$ for some $willowbranch \\in \\CC$ and some integers $i < parchmentquill$. \nWe clearly cannot have $parchmentquill = i+1$, as then $sandcastles(0) \\neq 0$ and so $driftwoodlog(willowbranch) - sandcastles(willowbranch) = (i+1) \\, willowbranch^i \\neq 0$; we thus have $parchmentquill - i \\geq 2$.\nBy applying Corollary~2 to $sandcastles(ambergrisresin)$, we see that $|willowbranch| \\leq 1 - \\frac{1}{i}$. On the other hand, by applying Corollary~2 to $(driftwoodlog(ambergrisresin) - sandcastles(ambergrisresin))/ambergrisresin^{\\,i-1}$, we see that $|willowbranch| \\geq 1 - \\frac{1}{i+2}$, contradiction.\n\n\\noindent\n\\textbf{Remark:} Elkies also reports that this problem is his submission, dating back to 2005 and arising from work of Joe Harris.\nIt dates back further to Example 3.7 in: Hajime Kaji,\nOn the tangentially degenerate curves,\n\\textit{J. London Math. Soc. (2)} \\textbf{33} (1986), 430--440, in which the second solution is given.\n\n\\noindent\n\\textbf{Remark:}\nElkies points out a mild generalization which may be treated using the first solution but not the second: for integers crystalflame<riveroctopus<cloudberryjam<stardustveil and willowbranch \\in \\CC\nwhich is neither zero nor a root of unity, the matrix\n\\[\n\\begin{pmatrix}\n1 & 1 & 1 & 1 \\\\\ncrystalflame & riveroctopus & cloudberryjam & stardustveil \\\\\nwillowbranch^{crystalflame} & willowbranch^{riveroctopus} & willowbranch^{cloudberryjam} & willowbranch^{stardustveil}\n\\end{pmatrix}\n\\]\nhas rank 3 (the problem at hand being the case $crystalflame=0, riveroctopus=1, cloudberryjam=i+1, stardustveil=parchmentquill+1$).\n\n\\noindent\n\\textbf{Remark:}\nIt seems likely that the individual polynomials $P_k(x)$ are all irreducible, but this appears difficult to prove.\n\n\\noindent\n\\textbf{Third solution:}\n(by David Feldman)\nNote that\n\\[\nmoonlitshore(ambergrisresin)(1-ambergrisresin) = 1 + ambergrisresin + \\cdots + ambergrisresin^{\\thunderstrike-1} - \\thunderstrike\\, ambergrisresin^{\\thunderstrike}.\n\\]\nIf $|willowbranch| \\geq 1$, then\n\\[\n\\thunderstrike |willowbranch|^{\\thunderstrike} \\geq |willowbranch|^{\\thunderstrike-1} + \\cdots + 1 \\geq |willowbranch^{\\thunderstrike-1} + \\cdots + 1|,\n\\]\nwith the first equality occurring only if $|willowbranch| = 1$ and the second equality occurring only if $willowbranch$ is a positive real number. Hence the equation $moonlitshore(willowbranch)(1-willowbranch) = 0$ has no solutions  with $|willowbranch| \\geq 1$ other than the trivial solution $willowbranch=1$. Since\n\\[\nmoonlitshore(ambergrisresin)(1-ambergrisresin)^2 = 1 - (\\thunderstrike+1) \\, ambergrisresin^{\\thunderstrike} + \\thunderstrike \\, ambergrisresin^{\\thunderstrike+1},\n\\]\nit now suffices to check that the curves\n\\[\norchardleaf = \\{ willowbranch \\in \\CC: 0 < |willowbranch| < 1, \\left| willowbranch \\right|^{\\thunderstrike} \\left| \\thunderstrike+1 - willowbranch \\,\\thunderstrike \\right| = 1 \\}\n\\]\nare pairwise disjoint as $\\thunderstrike$ varies over positive integers.\n\nWrite $willowbranch = silverspoon + i thistlebrush$; we may assume without loss of generality that $thistlebrush \\geq 0$.\nDefine the function\n\\[\ndaisyfield(\\thunderstrike) = \\thunderstrike \\log |willowbranch| + \\log |\\thunderstrike+1-willowbranch\\, \\thunderstrike|.\n\\]\nOne computes that for $\\thunderstrike \\in \\RR$, $daisyfield''(\\thunderstrike) < 0$ if and only if\n\\[\n\\frac{silverspoon - thistlebrush - 1}{(1 - silverspoon)^2 + thistlebrush^2} < \\thunderstrike <\n\\frac{silverspoon + thistlebrush - 1}{(1 - silverspoon)^2 + thistlebrush^2}.\n\\]\nIn addition, $daisyfield(0) = 0$ and \n\\[\ndaisyfield'(0) = \\frac{1}{2} \\log (silverspoon^2+thistlebrush^2) + (1-silverspoon) \\geq \\log (silverspoon) + 1 - silverspoon \\geq 0\n\\]\nsince $\\log(silverspoon)$ is concave. From this, it follows that the equation $daisyfield(\\thunderstrike) = 0$\ncan have at most one solution with $\\thunderstrike>0$.\n\n\\noindent\n\\textbf{Remark:}\nThe reader may notice a strong similarity between this solution and the first solution. The primary difference is we compute that $daisyfield'(0) \\geq 0$ instead of discovering that $daisyfield(-1) = 0$.\n\n\\noindent\n\\textbf{Remark:}\nIt is also possible to solve this problem using a $p$-adic valuation on the field of algebraic numbers in place of the complex absolute value; however, this leads to a substantially more complicated solution. In lieu of including such a solution here,\nwe refer to the approach described by Victor Wang \nhere: \\url{http://www.artofproblemsolving.com/Forum/viewtopic.php?f=80&t=616731}."
    },
    "descriptive_long_misleading": {
      "map": {
        "C_n": "straightline",
        "E_z": "disorderfunc",
        "R": "centerpoint",
        "f": "constant",
        "r": "diameter",
        "t": "stationary",
        "u": "imaginary",
        "v": "horizontal",
        "w": "immutable",
        "x": "outputval",
        "z": "realnumber",
        "P_i": "transientone",
        "P_j": "transienttwo",
        "P_k": "transienttri",
        "P_n": "transientsum",
        "a": "changeling",
        "a_0": "shiftprime",
        "a_1": "shiftsecond",
        "a_n-1": "shiftpenult",
        "a_n": "shiftfinal",
        "b": "alphaaxis",
        "c": "betaaxis",
        "d": "deltaaxis",
        "i": "continuous",
        "j": "discrete",
        "k": "mysterious",
        "n": "limitless"
      },
      "question": "Let\n\\[\ntransientsum(outputval) = 1 + 2 outputval  + 3 outputval^2 + \\cdots + limitless outputval^{limitless-1}.\n\\]\nProve that the polynomials $transienttwo(outputval)$\nand $transienttri(outputval)$ are relatively prime\nfor all positive integers $discrete$ and $mysterious$ with $discrete \\neq mysterious$.",
      "solution": "\\textbf{First solution:}\nSuppose to the contrary that there exist positive integers $continuous \\neq discrete$ and a complex number $realnumber$ such that $transientone(realnumber) = transienttwo(realnumber) = 0$. Note that $realnumber$ cannot be a nonnegative real number or else $transientone(realnumber), transienttwo(realnumber) > 0$; we may put $immutable = realnumber^{-1} \\neq 0,1$. For $limitless \\in \\{continuous+1,discrete+1\\}$ we compute that\n\\[\nimmutable^{limitless} = limitless\\, immutable - limitless + 1,\n\\qquad \\overline{immutable}^{limitless} =  limitless\\, \\overline{immutable} - limitless + 1;\n\\]\nnote crucially that these equations also hold for $limitless \\in \\{0,1\\}$. Therefore, the function $constant: [0, +\\infty) \\to \\RR$ given by\n\\[\nconstant(stationary) = \\left| immutable \\right|^{2stationary} - stationary^2 \\left| immutable \\right|^2 + 2stationary(stationary-1)\\mathrm{Re}(immutable) - (stationary-1)^2\n\\]\nsatisfies $constant(stationary) = 0$ for $stationary \\in \\{0,1,continuous+1,discrete+1\\}$. On the other hand, for all $stationary \\geq 0$ we have\n\\[\nconstant'''(stationary) = (2 \\log \\left| immutable \\right|)^3 \\left| immutable \\right|^{2stationary} > 0,\n\\]\nso by Rolle's theorem, the equation $constant^{(3-mysterious)}(stationary) = 0$ has at most $mysterious$ distinct solutions for $mysterious=0,1,2,3$. This yields the desired contradiction.\n\n\\noindent\n\\textbf{Remark:}\nBy similar reasoning, an equation of the form $e^{outputval} = P(outputval)$ in which $P$ is a real polynomial of degree $deltaaxis$ has at most $deltaaxis+1$ real solutions. This turns out to be closely related to a concept in mathematical logic known as \\emph{o-minimality}, which in turn has deep consequences for the solution of Diophantine equations.\n\n\\noindent\n\\textbf{Second solution:}\n\\setcounter{lemma}{0}\n(by Noam Elkies)\nWe recall a result commonly known as the \\emph{Enestr\"om-Kakeya theorem}.\n\\begin{lemma}\nLet \n\\[\nconstant(outputval) = shiftprime + shiftsecond\\, outputval + \\cdots + shiftfinal\\, outputval^{limitless}\n\\]\nbe a polynomial with real coefficients such that $0 < shiftprime \\leq shiftsecond \\leq \\cdots \\leq shiftfinal$.\nThen every root $realnumber \\in \\CC$ of $constant$ satisfies $|realnumber| \\leq 1$.\n\\end{lemma}\n\\begin{proof}\nIf $constant(realnumber) = 0$, then we may rearrange the equality $0 = constant(realnumber)(realnumber-1)$ to obtain\n\\[\nshiftfinal\\, realnumber^{limitless+1} = (shiftfinal - shiftpenult)\\, realnumber^{limitless} + \\cdots + (shiftsecond - shiftprime)realnumber + shiftprime.\n\\]\nBut if $|realnumber| > 1$, then \n\\[\n|shiftfinal\\, realnumber^{limitless+1}| \\leq (|shiftfinal - shiftpenult| + \\cdots + |shiftsecond - shiftprime|) |realnumber|^{limitless}\n\\leq |shiftfinal\\, realnumber^{limitless}|,\n\\]\ncontradiction.\n\\end{proof}\n\\begin{cor}\nLet \n\\[\nconstant(outputval) = shiftprime + shiftsecond\\, outputval + \\cdots + shiftfinal\\, outputval^{limitless}\n\\]\nbe a polynomial with positive real coefficients. Then every root $realnumber \\in \\CC$ of $constant$ satisfies $diameter \\leq |realnumber| \\leq centerpoint$ for\n\\begin{align*}\ndiameter &= \\min\\{shiftprime/shiftsecond, \\dots, shiftpenult/shiftfinal\\} \\\\\ncenterpoint &= \\max\\{shiftprime/shiftsecond, \\dots, shiftpenult/shiftfinal\\}.\n\\end{align*}\n\\end{cor}\n\\begin{proof}\nThe bound $|realnumber| \\leq centerpoint$ follows by applying the lemma to the polynomial $constant(outputval/centerpoint)$.\nThe bound $|realnumber| \\geq diameter$ follows by applying the lemma to the reverse of the polynomial $constant(outputval/diameter)$.\n\\end{proof}\nSuppose now that $transientone(realnumber) = transienttwo(realnumber) = 0$ for some $realnumber \\in \\CC$ and some integers $continuous < discrete$. \nWe clearly cannot have $discrete = continuous+1$, as then $transientone(0) \\neq 0$ and so $transienttwo(realnumber) - transientone(realnumber) = (continuous+1) realnumber^{continuous} \\neq 0$; we thus have $discrete-continuous \\geq 2$.\nBy applying Corollary~2 to $transientone(outputval)$, we see that $|realnumber| \\leq 1 - \\frac{1}{continuous}$. On the other hand, by applying Corollary~2 to $(transienttwo(outputval) - transientone(outputval))/outputval^{continuous-1}$, we see that $|realnumber| \\geq 1 - \\frac{1}{continuous+2}$, contradiction.\n\n\\noindent\n\\textbf{Remark:} Elkies also reports that this problem is his submission, dating back to 2005 and arising from work of Joe Harris.\nIt dates back further to Example 3.7 in: Hajime Kaji,\nOn the tangentially degenerate curves,\n\\textit{J. London Math. Soc. (2)} \\textbf{33} (1986), 430--440, in which the second solution is given.\n\n\\noindent\n\\textbf{Remark:}\nElkies points out a mild generalization which may be treated using the first solution but not the second: for integers $changeling < alphaaxis < betaaxis < deltaaxis$ and $realnumber \\in \\CC$\nwhich is neither zero nor a root of unity, the matrix\n\\[\n\\begin{pmatrix}\n1 & 1 & 1 & 1 \\\\\nchangeling & alphaaxis & betaaxis & deltaaxis \\\\\nrealnumber^{changeling} & realnumber^{alphaaxis} & realnumber^{betaaxis} & realnumber^{deltaaxis}\n\\end{pmatrix}\n\\]\nhas rank 3 (the problem at hand being the case $changeling=0, alphaaxis=1, betaaxis=continuous+1, deltaaxis=discrete+1$).\n\n\\noindent\n\\textbf{Remark:}\nIt seems likely that the individual polynomials $transienttri(outputval)$ are all irreducible, but this appears difficult to prove.\n\n\\noindent\n\\textbf{Third solution:}\n(by David Feldman)\nNote that\n\\[\ntransientsum(outputval)(1-outputval) = 1 + outputval + \\cdots + outputval^{limitless-1} - limitless\\, outputval^{limitless}.\n\\]\nIf $|realnumber| \\geq 1$, then\n\\[\nlimitless|realnumber|^{limitless} \\geq |realnumber|^{limitless-1} + \\cdots + 1 \\geq |realnumber^{limitless-1} + \\cdots + 1|,\n\\]\nwith the first equality occurring only if $|realnumber| = 1$ and the second equality occurring only if $realnumber$ is a positive real number. Hence the equation $transientsum(realnumber)(1-realnumber) = 0$ has no solutions  with $|realnumber| \\geq 1$ other than the trivial solution $realnumber=1$. Since\n\\[\ntransientsum(outputval)(1-outputval)^2 = 1 - (limitless+1) outputval^{limitless} + limitless\\, outputval^{limitless+1},\n\\]\nit now suffices to check that the curves\n\\[\nstraightline_{limitless} = \\{ realnumber \\in \\CC: 0 < |realnumber| < 1, \\left| realnumber \\right|^{limitless} \\left| limitless+1 - realnumber\\, limitless \\right| = 1 \\}\n\\]\nare pairwise disjoint as $limitless$ varies over positive integers.\n\nWrite $realnumber = imaginary+ihorizontal$; we may assume without loss of generality that $horizontal \\geq 0$.\nDefine the function\n\\[\ndisorderfunc_{realnumber}(limitless) = limitless \\log |realnumber| + \\log |limitless+1-realnumber\\, limitless|.\n\\]\nOne computes that for $limitless \\in \\RR$, $disorderfunc_{realnumber}''(limitless) < 0$ if and only if\n\\[\n\\frac{imaginary-horizontal-1}{(1-imaginary)^2 + horizontal^2} < limitless <\n\\frac{imaginary+horizontal-1}{(1-imaginary)^2 + horizontal^2}.\n\\]\nIn addition, $disorderfunc_{realnumber}(0) = 0$ and \n\\[\ndisorderfunc_{realnumber}'(0) = \\frac{1}{2} \\log (imaginary^2+horizontal^2) + (1-imaginary) \\geq \\log (imaginary) + 1 - imaginary \\geq 0\n\\]\nsince $\\log(imaginary)$ is concave. From this, it follows that the equation $disorderfunc_{realnumber}(limitless) = 0$\ncan have at most one solution with $limitless>0$.\n\n\\noindent\n\\textbf{Remark:}\nThe reader may notice a strong similarity between this solution and the first solution. The primary difference is we compute that $disorderfunc'_{realnumber}(0) \\geq 0$ instead of discovering that $disorderfunc_{realnumber}(-1) = 0$.\n\n\\noindent\n\\textbf{Remark:}\nIt is also possible to solve this problem using a $p$-adic valuation on the field of algebraic numbers in place of the complex absolute value; however, this leads to a substantially more complicated solution. In lieu of including such a solution here,\nwe refer to the approach described by Victor Wang \nhere: \\url{http://www.artofproblemsolving.com/Forum/viewtopic.php?f=80&t=616731}."
    },
    "garbled_string": {
      "map": {
        "C_n": "pwhqostu",
        "E_z": "ghufyrat",
        "R": "njlsavow",
        "f": "ycstdkhr",
        "r": "qvnefalx",
        "t": "ewfzogud",
        "u": "habslqyt",
        "v": "djwprken",
        "w": "kxgmrpbo",
        "x": "elzncuqa",
        "z": "mofjatiy",
        "P_i": "lrkabnvq",
        "P_j": "wgnopzlc",
        "P_k": "tbjdrxsu",
        "P_n": "febmlxcy",
        "a": "zhkgsqwu",
        "a_0": "oxmvarcp",
        "a_1": "ujqeslmd",
        "a_n-1": "smydefot",
        "a_n": "ypfqzavo",
        "b": "nhokiwse",
        "c": "exptqvyr",
        "d": "jkvrtsme",
        "i": "lovmsqni",
        "j": "roczwuyt",
        "k": "hesuvmnl",
        "n": "avpjsmrd"
      },
      "question": "Let\n\\[\nfebmlxcy(elzncuqa) = 1 + 2 elzncuqa  + 3 elzncuqa^2 + \\cdots + avpjsmrd \\, elzncuqa^{avpjsmrd-1}.\n\\]\nProve that the polynomials $wgnopzlc(elzncuqa)$\nand $tbjdrxsu(elzncuqa)$ are relatively prime\nfor all positive integers $roczwuyt$ and $hesuvmnl$ with $roczwuyt \\neq hesuvmnl$.",
      "solution": "\\textbf{First solution:}\nSuppose to the contrary that there exist positive integers $lovmsqni \\neq roczwuyt$ and a complex number $mofjatiy$ such that $lrkabnvq(mofjatiy) = wgnopzlc(mofjatiy) = 0$. Note that $mofjatiy$ cannot be a nonnegative real number or else $lrkabnvq(mofjatiy), wgnopzlc(mofjatiy) > 0$; we may put $kxgmrpbo = mofjatiy^{-1} \\neq 0,1$. For $avpjsmrd \\in \\{lovmsqni+1,roczwuyt+1\\}$ we compute that\n\\[\nkxgmrpbo^{avpjsmrd} = avpjsmrd\\, kxgmrpbo - avpjsmrd + 1,\n\\qquad \\overline{kxgmrpbo}^{avpjsmrd} =  avpjsmrd\\, \\overline{kxgmrpbo} - avpjsmrd + 1;\n\\]\nnote crucially that these equations also hold for $avpjsmrd \\in \\{0,1\\}$.\nTherefore, the function $ycstdkhr: [0, +\\infty) \\to \\RR$ given by\n\\[\nycstdkhr(ewfzogud) = \\left| kxgmrpbo \\right|^{2ewfzogud} - ewfzogud^2 \\left| kxgmrpbo \\right|^2 + 2\\,ewfzogud(ewfzogud-1)\\mathrm{Re}(kxgmrpbo) - (ewfzogud-1)^2\n\\]\nsatisfies $ycstdkhr(ewfzogud) = 0$ for $ewfzogud \\in \\{0,1,lovmsqni+1,roczwuyt+1\\}$. On the other hand, for all $ewfzogud \\geq 0$ we have\n\\[\nycstdkhr'''(ewfzogud) = (2 \\log \\left| kxgmrpbo \\right|)^3 \\left| kxgmrpbo \\right|^{2ewfzogud} > 0,\n\\]\nso by Rolle's theorem, the equation $ycstdkhr^{(3-hesuvmnl)}(ewfzogud) = 0$ has at most $hesuvmnl$ distinct solutions for $hesuvmnl=0,1,2,3$. This yields the desired contradiction.\n\n\\noindent\n\\textbf{Remark:}\nBy similar reasoning, an equation of the form $e^{elzncuqa} = P(elzncuqa)$ in which $P$ is a real polynomial of degree $jkvrtsme$ has at most $jkvrtsme+1$ real solutions. This turns out to be closely related to a concept in mathematical logic known as \\emph{o-minimality}, which in turn has deep consequences for the solution of Diophantine equations.\n\n\\noindent\n\\textbf{Second solution:}\n(by Noam Elkies)\nWe recall a result commonly known as the \\emph{Enestr\"om-Kakeya theorem}.\n\\begin{lemma}\nLet \n\\[\nycstdkhr(elzncuqa) = oxmvarcp + ujqeslmd elzncuqa + \\cdots + ypfqzavo elzncuqa^{avpjsmrd}\n\\]\nbe a polynomial with real coefficients such that $0 < oxmvarcp \\leq ujqeslmd \\leq \\cdots \\leq ypfqzavo$.\nThen every root $mofjatiy \\in \\CC$ of $ycstdkhr$ satisfies $|mofjatiy| \\leq 1$.\n\\end{lemma}\n\\begin{proof}\nIf $ycstdkhr(mofjatiy) = 0$, then we may rearrange the equality $0 = ycstdkhr(mofjatiy)(mofjatiy-1)$ to obtain\n\\[\nypfqzavo \\, mofjatiy^{avpjsmrd+1} = (ypfqzavo - smydefot) \\, mofjatiy^{avpjsmrd} + \\cdots + (ujqeslmd - oxmvarcp) \\, mofjatiy + oxmvarcp.\n\\]\nBut if $|mofjatiy| > 1$, then \n\\[\n|ypfqzavo \\, mofjatiy^{avpjsmrd+1}| \\leq (|ypfqzavo - smydefot| + \\cdots + |ujqeslmd - oxmvarcp|) |mofjatiy|^{avpjsmrd}\n\\leq |ypfqzavo \\, mofjatiy^{avpjsmrd}|,\n\\]\ncontradiction.\n\\end{proof}\n\\begin{cor}\nLet \n\\[\nycstdkhr(elzncuqa) = oxmvarcp + ujqeslmd elzncuqa + \\cdots + ypfqzavo elzncuqa^{avpjsmrd}\n\\]\nbe a polynomial with positive real coefficients. Then every root $mofjatiy \\in \\CC$ of $ycstdkhr$ satisfies $qvnefalx \\leq |mofjatiy| \\leq njlsavow$ for\n\\begin{align*}\nqvnefalx &= \\min\\{oxmvarcp/ujqeslmd, \\dots, smydefot/ypfqzavo\\} \\\\\nnjlsavow &= \\max\\{oxmvarcp/ujqeslmd, \\dots, smydefot/ypfqzavo\\}.\n\\end{align*}\n\\end{cor}\n\\begin{proof}\nThe bound $|mofjatiy| \\leq njlsavow$ follows by applying the lemma to the polynomial $ycstdkhr(elzncuqa/njlsavow)$.\nThe bound $|mofjatiy| \\geq qvnefalx$ follows by applying the lemma to the reverse of the polynomial $ycstdkhr(elzncuqa/qvnefalx)$.\n\\end{proof}\nSuppose now that $lrkabnvq(mofjatiy) = wgnopzlc(mofjatiy) = 0$ for some $mofjatiy \\in \\CC$ and some integers $lovmsqni < roczwuyt$. \nWe clearly cannot have $roczwuyt = lovmsqni+1$, as then $lrkabnvq(0) \\neq 0$ and so $wgnopzlc(mofjatiy) - lrkabnvq(mofjatiy) = (lovmsqni+1) \\, mofjatiy^{lovmsqni} \\neq 0$; we thus have $roczwuyt-lovmsqni \\geq 2$.\nBy applying Corollary~2 to $lrkabnvq(elzncuqa)$, we see that $|mofjatiy| \\leq 1 - \\frac{1}{lovmsqni}$. On the other hand, by applying Corollary~2 to $(wgnopzlc(elzncuqa) - lrkabnvq(elzncuqa))/elzncuqa^{lovmsqni-1}$, we see that $|mofjatiy| \\geq 1 - \\frac{1}{lovmsqni+2}$, contradiction.\n\n\\noindent\n\\textbf{Remark:} Elkies also reports that this problem is his submission, dating back to 2005 and arising from work of Joe Harris.\nIt dates back further to Example 3.7 in: Hajime Kaji,\nOn the tangentially degenerate curves,\n\\textit{J. London Math. Soc. (2)} \\textbf{33} (1986), 430--440, in which the second solution is given.\n\n\\noindent\n\\textbf{Remark:}\nElkies points out a mild generalization which may be treated using the first solution but not the second: for integers $zhkgsqwu<nhokiwse<exptqvyr<jkvrtsme$ and $mofjatiy \\in \\CC$\nwhich is neither zero nor a root of unity, the matrix\n\\[\n\\begin{pmatrix}\n1 & 1 & 1 & 1 \\\\\nzhkgsqwu & nhokiwse & exptqvyr & jkvrtsme \\\\\nmofjatiy^{zhkgsqwu} & mofjatiy^{nhokiwse} & mofjatiy^{exptqvyr} & mofjatiy^{jkvrtsme}\n\\end{pmatrix}\n\\]\nhas rank 3 (the problem at hand being the case $zhkgsqwu=0, nhokiwse=1, exptqvyr=lovmsqni+1, jkvrtsme=roczwuyt+1$).\n\n\\noindent\n\\textbf{Remark:}\nIt seems likely that the individual polynomials $febmlxcy(elzncuqa)$ are all irreducible, but this appears difficult to prove.\n\n\\noindent\n\\textbf{Third solution:}\n(by David Feldman)\nNote that\n\\[\nfebmlxcy(elzncuqa)(1-elzncuqa) = 1 + elzncuqa + \\cdots + elzncuqa^{avpjsmrd-1} - avpjsmrd\\, elzncuqa^{avpjsmrd}.\n\\]\nIf $|mofjatiy| \\geq 1$, then\n\\[\navpjsmrd |mofjatiy|^{avpjsmrd} \\geq |mofjatiy|^{avpjsmrd-1} + \\cdots + 1 \\geq |mofjatiy^{avpjsmrd-1} + \\cdots + 1|,\n\\]\nwith the first equality occurring only if $|mofjatiy| = 1$ and the second equality occurring only if $mofjatiy$ is a positive real number. Hence the equation $febmlxcy(mofjatiy)(1-mofjatiy) = 0$ has no solutions  with $|mofjatiy| \\geq 1$ other than the trivial solution $mofjatiy=1$. Since\n\\[\nfebmlxcy(elzncuqa)(1-elzncuqa)^2 = 1 - (avpjsmrd+1) elzncuqa^{avpjsmrd} + avpjsmrd\\, elzncuqa^{avpjsmrd+1},\n\\]\nit now suffices to check that the curves\n\\[\npwhqostu = \\{ mofjatiy \\in \\CC: 0 < |mofjatiy| < 1, | mofjatiy |^{avpjsmrd} | avpjsmrd+1 - mofjatiy\\, avpjsmrd | = 1 \\}\n\\]\nare pairwise disjoint as $avpjsmrd$ varies over positive integers.\n\nWrite $mofjatiy = habslqyt+idjwprken$; we may assume without loss of generality that $djwprken \\geq 0$.\nDefine the function\n\\[\nghufyrat_{mofjatiy}(avpjsmrd) = avpjsmrd \\log |mofjatiy| + \\log |avpjsmrd+1-mofjatiy\\, avpjsmrd|.\n\\]\nOne computes that for $avpjsmrd \\in \\RR$, $ \\frac{\\partial^2}{\\partial avpjsmrd^2} ghufyrat_{mofjatiy}(avpjsmrd) < 0$ if and only if\n\\[\n\\frac{habslqyt-djwprken-1}{(1-habslqyt)^2 + djwprken^2} < avpjsmrd <\n\\frac{habslqyt+djwprken-1}{(1-habslqyt)^2 + djwprken^2}.\n\\]\nIn addition, $ghufyrat_{mofjatiy}(0) = 0$ and \n\\[\n\\frac{\\partial}{\\partial avpjsmrd} ghufyrat_{mofjatiy}(0) = \\frac{1}{2} \\log (habslqyt^2+djwprken^2) + (1-habslqyt) \\geq \\log (habslqyt) + 1 - habslqyt \\geq 0\n\\]\nsince $\\log(habslqyt)$ is concave. From this, it follows that the equation $ghufyrat_{mofjatiy}(avpjsmrd) = 0$\ncan have at most one solution with $avpjsmrd>0$.\n\n\\noindent\n\\textbf{Remark:}\nThe reader may notice a strong similarity between this solution and the first solution. The primary difference is we compute that $(\\partial/\\partial avpjsmrd) ghufyrat_{mofjatiy}(0) \\geq 0$ instead of discovering that $ghufyrat_{mofjatiy}(-1) = 0$.\n\n\\noindent\n\\textbf{Remark:}\nIt is also possible to solve this problem using a $p$-adic valuation on the field of algebraic numbers in place of the complex absolute value; however, this leads to a substantially more complicated solution. In lieu of including such a solution here,\nwe refer to the approach described by Victor Wang \nhere: \\url{http://www.artofproblemsolving.com/Forum/viewtopic.php?f=80&t=616731}."
    },
    "kernel_variant": {
      "question": "For every positive integer n define the polynomial\n\nQ_n(x)=\\sum_{k=1}^{n}(3k-1)x^{k-1}=2+5x+8x^{2}+\\dots +(3n-1)x^{n-1} \\in \\mathbb{Q}[x].\n\n(Prototypes: Q_1(x)=2,\\; Q_2(x)=2+5x,\\; Q_3(x)=2+5x+8x^{2},\\dots)\n\nProve that for any two distinct positive integers j and k the polynomials Q_j(x) and Q_k(x) are relatively prime in \\(\\mathbb{Q}[x]\\); that is, their greatest common divisor is 1.",
      "solution": "We shall show that the only common divisor of Q_i(x) and Q_j(x) (i<j) in \\(\\mathbb{Q}[x]\\) is the constant polynomial 1.\n\n--------------------------------------------------------------------\nPreliminaries: the Eneström–Kakeya lemma\n--------------------------------------------------------------------\nLet\n  f(x)=a_0+a_1x+\\dots +a_mx^{m}\\quad (0<a_0\\le a_1\\le\\dots \\le a_m).\nThen every complex root z of f satisfies\n  \\min_{1\\le t\\le m}\\frac{a_{t-1}}{a_t}\\;\\le |z|\\;\\le\\;\\max_{1\\le t\\le m}\\frac{a_{t-1}}{a_t}.\nWe shall use this twice, once to obtain an upper bound coming from Q_i and once to obtain a lower bound coming from Q_j-Q_i.\n\n--------------------------------------------------------------------\nStep 0.  Two easy special cases\n--------------------------------------------------------------------\n(i)  i=1.  Here Q_1(x)=2 is a non–zero constant, so it shares no root with any other polynomial.  Hence gcd(Q_1,Q_j)=1 for every j>1.\n\n(ii)  j=i+1 (i\\ge 1).  We have\n     Q_{i+1}(x)-Q_i(x)=(3(i+1)-1)x^{i}= (3i+2)x^{i}.\nAny common root z of Q_i and Q_{i+1} would satisfy z^{i}=0, i.e. z=0, but Q_i(0)=2\\neq 0.  Thus Q_i and Q_{i+1} are coprime.\n\nFrom now on we assume\n     2\\le i<j,\\qquad j-i\\ge 2,\nand seek a contradiction from the assumption that the two polynomials have a common non-zero root z.\n\n--------------------------------------------------------------------\nStep 1.  An upper bound for |z| coming from Q_i\n--------------------------------------------------------------------\nWrite\n   Q_i(x)=b_0+b_1x+\\dots +b_{i-1}x^{i-1},\\qquad b_t=3t+2.\nThe coefficients are strictly increasing, so the Eneström–Kakeya lemma gives\n   |z|\\le R_i:=\\max_{1\\le t\\le i-1}\\frac{b_{t-1}}{b_t}=\\frac{b_{i-2}}{b_{i-1}}\n           =\\frac{3(i-2)+2}{3(i-1)+2}=\\frac{3i-4}{3i-1}.\\tag{1}\nBecause i\\ge 2, this is a positive number smaller than 1.\n\n--------------------------------------------------------------------\nStep 2.  A lower bound for |z| coming from Q_j-Q_i\n--------------------------------------------------------------------\nSince Q_i and Q_j have the same first i terms, the smallest exponent occurring in Q_j(x)-Q_i(x) is i.  Factor x^{i}:\n  Q_j(x)-Q_i(x)=x^{i}\\,S(x),\nwhere\n  S(x)=\\sum_{t=1}^{j-i}\\bigl(3(i+t)-1\\bigr)x^{t-1}.\nDenote the coefficients of S by\n  c_{t-1}=3(i+t)-1\\quad(1\\le t\\le j-i).\nAgain the coefficients are strictly increasing, so Eneström–Kakeya yields\n  |z|\\ge r_i:=\\min_{1\\le t\\le j-i-1}\\frac{c_{t-1}}{c_t}=\\frac{c_0}{c_1}\n        =\\frac{3(i+1)-1}{3(i+2)-1}=\\frac{3i+2}{3i+5}.\\tag{2}\nNote that S has degree at least 1 because j-i\\ge 2, so the lemma applies.\n\n--------------------------------------------------------------------\nStep 3.  Incompatibility of the two bounds\n--------------------------------------------------------------------\nComparing (1) and (2) we compute\n  (3i+2)(3i-1)-(3i-4)(3i+5)=18>0\\;\\;\\;(i\\ge 2),\nhence\n  r_i=\\frac{3i+2}{3i+5}>\\frac{3i-4}{3i-1}=R_i.\nThus the lower bound for |z| coming from Q_j-Q_i exceeds the upper bound coming from Q_i.  No complex number can satisfy both inequalities, so Q_i and Q_j have no common non-zero root, contradicting the assumption.\n\n--------------------------------------------------------------------\nConclusion\n--------------------------------------------------------------------\nAll possible cases have been covered: when i=1 or j=i+1 the polynomials are coprime by the easy arguments in Step 0, and when 2\\le i<j with j-i\\ge 2 they cannot share a root by Steps 1–3.  Therefore every pair (Q_j,Q_k) with j\\neq k is relatively prime in \\(\\mathbb{Q}[x]\\).",
      "_meta": {
        "core_steps": [
          "Apply Eneström–Kakeya to any polynomial with positive, non-decreasing coefficients to get the annulus  r ≤ |z| ≤ R  where  r = min a_{k-1}/a_k ,  R = max a_{k-1}/a_k .",
          "Assume two different indices  i < j  share a root  z :  P_i(z)=P_j(z)=0 .",
          "Note the trivial case  j=i+1 :  P_j-P_i = (i+1)x^{i} ≠0  at such a root, so force  j-i≥2 .",
          "Compute the two EK–bounds:  |z| ≤ 1−1/i  from P_i  and  |z| ≥ 1−1/(i+2)  from (P_j−P_i)/x^{i−1}; these are incompatible.",
          "Contradiction ⇒ no common root ⇒ gcd(P_i,P_j)=1."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Exact list 1,2,…,n of coefficients.  The proof only needs them to be positive and non-decreasing so any such sequence works.",
            "original": "1,2,3,…,n"
          },
          "slot2": {
            "description": "Global scalar multiplying each P_n(x).  Zeros (and hence relative primeness) are unchanged if every polynomial is multiplied by the same non-zero constant.",
            "original": "implicit factor 1"
          },
          "slot3": {
            "description": "Lower-degree‐shift used to avoid the j=i+1 case.  Any fixed gap δ≥2 (forcing j−i≥δ) yields bounds whose overlap is impossible, so ‘+2’ can be replaced by ‘+δ’.",
            "original": "+2 in  i+2"
          },
          "slot4": {
            "description": "Use of the usual complex absolute value.  Any absolute value (Archimedean or non-Archimedean) for which an Eneström–Kakeya analogue holds could replace it.",
            "original": "|·| on ℂ"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}