{
  "index": "1947-B-4",
  "type": "ANA",
  "tag": [
    "ANA",
    "ALG",
    "NT"
  ],
  "difficulty": "",
  "question": "10. Given \\( P(z)=z^{2}+a z+b \\), a quadratic polynomial of the complex variable \\( z \\) with complex coefficients \\( a, b \\). Suppose that \\( |P(z)|=1 \\) for every \\( z \\) such that \\( |z|=1 \\). Prove that \\( a=b=0 \\).",
  "solution": "First Solution. Let \\( P(z)=z^{2}+a z+b \\). Let \\( a=p+i q, b=r+i s \\), where \\( p, q, r, s \\) are real. We are given that \\( |P(1)|=|P(-1)|=|P(i)| \\) \\( =|P(-i)|=1 \\), and we find\n\\[\n\\begin{aligned}\n|P(1)|^{2} & =|1+p+i q+r+i s|^{2} \\\\\n& =1+p^{2}+q^{2}+r^{2}+s^{2}+2 p+2 r+2 p r+2 q s \\\\\n|P(-1)|^{2} & =|1-p-i q+r+i s|^{2} \\\\\n& =1+p^{2}+q^{2}+r^{2}+s^{2}-2 p+2 r-2 p r-2 q s \\\\\n|P(i)|^{2} & =|-1+i p-q+r+i s|^{2} \\\\\n& =1+p^{2}+q^{2}+r^{2}+s^{2}+2 q-2 r-2 q r+2 p s \\\\\n|P(-i)|^{2} & =|-1-i p+q+r+i s|^{2} \\\\\n& =1+p^{2}+q^{2}+r^{2}+s^{2}-2 q-2 r+2 q r-2 p s .\n\\end{aligned}\n\\]\n\nAdding these equations, we get\n\\[\n4=4+4\\left(p^{2}+q^{2}+r^{2}+s^{2}\\right)\n\\]\n\nIt follows that \\( p=q=r=s=0, \\quad \\) so \\( a=b=0 \\).\nWe can equally well use a different set of roots of one. If \\( \\boldsymbol{\\xi} \\) is a primitive \\( n \\)th root of one, \\( n>2 \\), then\n\\[\nn=\\sum_{k=1}^{n}\\left|P\\left(\\xi^{k}\\right)\\right|^{2}=n\\left(1+|a|^{2}+|b|^{2}\\right),\n\\]\nand \\( a=b=0 \\) follows. We can also replace the sum by an integral. For all real \\( \\theta \\), we have\n\\[\n\\begin{aligned}\n1=\\left|P\\left(e^{i \\theta}\\right)\\right|^{2}= & \\left(e^{2 i \\theta}+a e^{i \\theta}+b\\right)\\left(e^{-2 i \\theta}+\\bar{a} e^{-i \\theta}+\\bar{b}\\right) \\\\\n= & \\bar{b} e^{2 i \\theta}+(\\bar{a}+a \\bar{b}) e^{i \\theta}+1+|a|^{2}+|b|^{2} \\\\\n& +(a+\\bar{a} \\bar{b}) e^{-i \\theta}+b e^{-2 i \\theta} .\n\\end{aligned}\n\\]\n\nIf we integrate this over \\( [0,2 \\pi] \\), we get\n\\[\n2 \\pi=2 \\pi\\left(1+|a|^{2}+|b|^{2}\\right)\n\\]\nand \\( a=b=0 \\), as before.\nSecond Solution. If \\( \\alpha, \\beta, \\gamma \\) are complex numbers such that \\( |\\alpha|=|\\beta| \\) \\( =|\\gamma|=1 \\) and \\( \\alpha+\\beta+\\gamma=3 \\), then \\( \\alpha=\\beta=\\gamma=1 \\). Let \\( \\omega= \\) \\( (-1+i \\sqrt{3}) / 2 \\), a cube root of one. Then \\( |P(1)|=|\\omega P(\\omega)|=\\left|\\omega^{2} P\\left(\\omega^{2}\\right)\\right| \\) \\( =1 \\) and \\( P(1)+\\omega P(\\omega)+\\omega^{2} P\\left(\\omega^{2}\\right)=3 \\), so\n\\[\nP(1)=\\omega P(\\omega)=\\omega^{2} P\\left(\\omega^{2}\\right)=1\n\\]\nand \\( P(1)=1, P(\\omega)=\\omega^{2}, P\\left(\\omega^{2}\\right)=\\omega=\\omega^{4} \\). But there is only one polynomial of degree less than three that takes these values at these points, and it is \\( P(z)=z^{2} \\).\n\nThis argument also works using higher roots of one or by integration.\nThird Solution. Put \\( Q(z)=1+a z+b z^{2} \\). Then for \\( |z|=1 \\), we have\n\\[\n|Q(z)|=\\left|Q(z) z^{-2}\\right|=|P(z)|=1 .\n\\]\n\nSince \\( Q(0)=1 \\), we see that \\( Q \\) has the same absolute value at an interior point of the disc as its maximum absolute value on the boundary. By the maximum modulus principle, \\( Q \\) is constant; so \\( Q(z)=1 \\) and \\( a=b=0 \\).\n\nRemark. All three of these proofs can be adapted to prove a more general result, namely, if\n\\[\nP(z)=z^{n}+a_{1} z^{n-1}+a_{2} z^{n-2}+\\cdots+a_{n}\n\\]\nand \\( |P(z)|=1 \\) for all \\( z \\) such that \\( |z|=1 \\), then\n\\[\na_{1}=a_{2}=\\cdots=a_{n}=0\n\\]\n\nBlaschke studied functions \\( f \\) analytic on the open unit disc and continuous on the closed disc such that \\( |f(z)|=1 \\) when \\( |z|=1 \\). He showed that all such functions have the form\n\\[\nf(z)=\\sigma \\prod_{k=1}^{n} \\frac{z-b_{k}}{1-\\bar{b}_{k} z}\n\\]\nwhere \\( b_{1}, b_{2}, \\ldots, b_{n} \\) satisfy \\( \\left|b_{k}\\right|<1 \\) and \\( \\sigma \\) is a constant of absolute value 1. See, for example, J. L. Walsh, Interpolation and Approximation in the Complex Domain. American Mathematical Society, Providence, R.I., 1935, pp. 281 ff.",
  "vars": [
    "P",
    "z",
    "\\\\xi",
    "\\\\theta",
    "\\\\omega",
    "Q",
    "f",
    "k",
    "\\\\alpha",
    "\\\\beta"
  ],
  "params": [
    "a",
    "b",
    "p",
    "q",
    "r",
    "s",
    "n",
    "\\\\sigma",
    "a_1",
    "a_2",
    "a_n",
    "b_k"
  ],
  "sci_consts": [
    "i",
    "e"
  ],
  "variants": {
    "descriptive_long": {
      "map": {
        "P": "polyfun",
        "z": "compvar",
        "\\\\xi": "primroot",
        "\\\\theta": "anglevar",
        "\\\\omega": "cuberoot",
        "Q": "polyaux",
        "f": "analyticfun",
        "k": "counterk",
        "\\\\alpha": "alphavar",
        "\\\\beta": "betavar",
        "a": "coeffa",
        "b": "coeffb",
        "p": "realp",
        "q": "realq",
        "r": "realr",
        "s": "reals",
        "n": "posintn",
        "\\\\sigma": "unitconst",
        "a_1": "coeffaone",
        "a_2": "coeffatwo",
        "a_n": "coeffan",
        "b_k": "coeffbvar"
      },
      "question": "10. Given \\( polyfun(compvar)=compvar^{2}+coeffa\\,compvar+coeffb \\), a quadratic polynomial of the complex variable \\( compvar \\) with complex coefficients \\( coeffa, coeffb \\). Suppose that \\( |polyfun(compvar)|=1 \\) for every \\( compvar \\) such that \\( |compvar|=1 \\). Prove that \\( coeffa=coeffb=0 \\).",
      "solution": "First Solution. Let \\( polyfun(compvar)=compvar^{2}+coeffa\\,compvar+coeffb \\). Let \\( coeffa=realp+i\\,realq,\\;coeffb=realr+i\\,reals \\), where \\( realp,realq,realr,reals \\) are real. We are given that \\( |polyfun(1)|=|polyfun(-1)|=|polyfun(i)|=|polyfun(-i)|=1 \\), and we find\n\\[\n\\begin{aligned}\n|polyfun(1)|^{2}&=|1+realp+i\\,realq+realr+i\\,reals|^{2}\\\\\n&=1+realp^{2}+realq^{2}+realr^{2}+reals^{2}+2\\,realp+2\\,realr+2\\,realp\\,realr+2\\,realq\\,reals\\\\\n|polyfun(-1)|^{2}&=|1-realp-i\\,realq+realr+i\\,reals|^{2}\\\\\n&=1+realp^{2}+realq^{2}+realr^{2}+reals^{2}-2\\,realp+2\\,realr-2\\,realp\\,realr-2\\,realq\\,reals\\\\\n|polyfun(i)|^{2}&=|-1+i\\,realp-realq+realr+i\\,reals|^{2}\\\\\n&=1+realp^{2}+realq^{2}+realr^{2}+reals^{2}+2\\,realq-2\\,realr-2\\,realq\\,realr+2\\,realp\\,reals\\\\\n|polyfun(-i)|^{2}&=|-1-i\\,realp+realq+realr+i\\,reals|^{2}\\\\\n&=1+realp^{2}+realq^{2}+realr^{2}+reals^{2}-2\\,realq-2\\,realr+2\\,realq\\,realr-2\\,realp\\,reals.\n\\end{aligned}\n\\]\nAdding these equations, we get\n\\[4=4+4\\bigl(realp^{2}+realq^{2}+realr^{2}+reals^{2}\\bigr)\\]\nIt follows that \\( realp=realq=realr=reals=0 \\), so \\( coeffa=coeffb=0 \\).\n\nWe can equally well use a different set of roots of one. If \\( \\boldsymbol{primroot} \\) is a primitive \\( posintn \\)th root of one, \\( posintn>2 \\), then\n\\[posintn=\\sum_{counterk=1}^{posintn}\\left|polyfun\\bigl(primroot^{counterk}\\bigr)\\right|^{2}=posintn\\bigl(1+|coeffa|^{2}+|coeffb|^{2}\\bigr),\\]\nand \\( coeffa=coeffb=0 \\) follows. We can also replace the sum by an integral. For all real \\( anglevar \\), we have\n\\[\n\\begin{aligned}\n1=\\left|polyfun\\left(e^{i\\,anglevar}\\right)\\right|^{2}\n&=\\bigl(e^{2i\\,anglevar}+coeffa\\,e^{i\\,anglevar}+coeffb\\bigr)\\bigl(e^{-2i\\,anglevar}+\\overline{coeffa}\\,e^{-i\\,anglevar}+\\overline{coeffb}\\bigr)\\\\\n&=\\overline{coeffb}\\,e^{2i\\,anglevar}+\\bigl(\\overline{coeffa}+coeffa\\,\\overline{coeffb}\\bigr)e^{i\\,anglevar}+1+|coeffa|^{2}+|coeffb|^{2}\\\\\n&\\quad+\\bigl(coeffa+\\overline{coeffa}\\,\\overline{coeffb}\\bigr)e^{-i\\,anglevar}+coeffb\\,e^{-2i\\,anglevar}.\n\\end{aligned}\n\\]\nIf we integrate this over \\([0,2\\pi]\\), we get\n\\[2\\pi=2\\pi\\bigl(1+|coeffa|^{2}+|coeffb|^{2}\\bigr)\\]\nand \\( coeffa=coeffb=0 \\), as before.\n\nSecond Solution. If \\( alphavar,betavar,\\gamma \\) are complex numbers such that \\(|alphavar|=|betavar|=|\\gamma|=1\\) and \\( alphavar+betavar+\\gamma=3 \\), then \\( alphavar=betavar=\\gamma=1 \\). Let \\( cuberoot=(-1+i\\,\\sqrt{3})/2 \\), a cube root of one. Then \\( |polyfun(1)|=|cuberoot\\,polyfun(cuberoot)|=\\bigl|cuberoot^{2}\\,polyfun(cuberoot^{2})\\bigr|=1 \\) and \\( polyfun(1)+cuberoot\\,polyfun(cuberoot)+cuberoot^{2}\\,polyfun(cuberoot^{2})=3 \\), so\n\\[polyfun(1)=cuberoot\\,polyfun(cuberoot)=cuberoot^{2}\\,polyfun(cuberoot^{2})=1\\]\nand \\( polyfun(1)=1,\\;polyfun(cuberoot)=cuberoot^{2},\\;polyfun(cuberoot^{2})=cuberoot=cuberoot^{4} \\). But there is only one polynomial of degree less than three that takes these values at these points, and it is \\( polyfun(compvar)=compvar^{2} \\).\n\nThis argument also works using higher roots of one or by integration.\n\nThird Solution. Put \\( polyaux(compvar)=1+coeffa\\,compvar+coeffb\\,compvar^{2} \\). Then for \\( |compvar|=1 \\), we have\n\\[|polyaux(compvar)|=\\left|\\,polyaux(compvar)\\,compvar^{-2}\\right|=|polyfun(compvar)|=1.\\]\nSince \\( polyaux(0)=1 \\), we see that \\( polyaux \\) has the same absolute value at an interior point of the disc as its maximum absolute value on the boundary. By the maximum modulus principle, \\( polyaux \\) is constant; so \\( polyaux(compvar)=1 \\) and \\( coeffa=coeffb=0 \\).\n\nRemark. All three of these proofs can be adapted to prove a more general result, namely, if\n\\[polyfun(compvar)=compvar^{posintn}+coeffaone\\,compvar^{posintn-1}+coeffatwo\\,compvar^{posintn-2}+\\cdots+coeffan\\]\nand \\( |polyfun(compvar)|=1 \\) for all \\( compvar \\) such that \\( |compvar|=1 \\), then\n\\[coeffaone=coeffatwo=\\cdots=coeffan=0\\]\n\nBlaschke studied functions \\( analyticfun \\) analytic on the open unit disc and continuous on the closed disc such that \\( |analyticfun(compvar)|=1 \\) when \\( |compvar|=1 \\). He showed that all such functions have the form\n\\[analyticfun(compvar)=unitconst\\,\\prod_{counterk=1}^{posintn}\\frac{compvar-coeffbvar}{1-\\overline{coeffbvar}\\,compvar}\\]\nwhere \\( coeffbvar \\) satisfy \\(|coeffbvar|<1\\) and \\( unitconst \\) is a constant of absolute value 1. See, for example, J. L. Walsh, Interpolation and Approximation in the Complex Domain. American Mathematical Society, Providence, R.I., 1935, pp. 281 ff."
    },
    "descriptive_long_confusing": {
      "map": {
        "P": "lanternum",
        "z": "quartzite",
        "\\\\xi": "stargazer",
        "\\\\theta": "hinterlog",
        "\\\\omega": "cobblenet",
        "Q": "rosebloom",
        "f": "duskflower",
        "k": "larkspore",
        "\\\\alpha": "brickdust",
        "\\\\beta": "moonwhale",
        "a": "willowark",
        "b": "elmbranch",
        "p": "flintrock",
        "q": "sagebrush",
        "r": "thunderer",
        "s": "meadowlark",
        "n": "riverstone",
        "\\\\sigma": "gossamer",
        "a_1": "ambergris",
        "a_2": "briarwood",
        "a_n": "ironforge",
        "b_k": "maplekey"
      },
      "question": "10. Given \\( lanternum(quartzite)=quartzite^{2}+willowark\\, quartzite+elmbranch \\), a quadratic polynomial of the complex variable \\( quartzite \\) with complex coefficients \\( willowark, elmbranch \\). Suppose that \\( |lanternum(quartzite)|=1 \\) for every \\( quartzite \\) such that \\( |quartzite|=1 \\). Prove that \\( willowark=elmbranch=0 \\).",
      "solution": "First Solution. Let \\( lanternum(quartzite)=quartzite^{2}+willowark\\, quartzite+elmbranch \\). Let \\( willowark=flintrock+i sagebrush, elmbranch=thunderer+i meadowlark \\), where \\( flintrock, sagebrush, thunderer, meadowlark \\) are real. We are given that \\( |lanternum(1)|=|lanternum(-1)|=|lanternum(i)|=|lanternum(-i)|=1 \\), and we find\n\\[\n\\begin{aligned}\n|lanternum(1)|^{2} & =|1+flintrock+i sagebrush+thunderer+i meadowlark|^{2} \\\\\n& =1+flintrock^{2}+sagebrush^{2}+thunderer^{2}+meadowlark^{2}+2 flintrock+2 thunderer+2 flintrock thunderer+2 sagebrush meadowlark \\\\\n|lanternum(-1)|^{2} & =|1-flintrock-i sagebrush+thunderer+i meadowlark|^{2} \\\\\n& =1+flintrock^{2}+sagebrush^{2}+thunderer^{2}+meadowlark^{2}-2 flintrock+2 thunderer-2 flintrock thunderer-2 sagebrush meadowlark \\\\\n|lanternum(i)|^{2} & =|-1+i flintrock-sagebrush+thunderer+i meadowlark|^{2} \\\\\n& =1+flintrock^{2}+sagebrush^{2}+thunderer^{2}+meadowlark^{2}+2 sagebrush-2 thunderer-2 sagebrush thunderer+2 flintrock meadowlark \\\\\n|lanternum(-i)|^{2} & =|-1-i flintrock+sagebrush+thunderer+i meadowlark|^{2} \\\\\n& =1+flintrock^{2}+sagebrush^{2}+thunderer^{2}+meadowlark^{2}-2 sagebrush-2 thunderer+2 sagebrush thunderer-2 flintrock meadowlark .\n\\end{aligned}\n\\]\n\nAdding these equations, we get\n\\[\n4=4+4\\left(flintrock^{2}+sagebrush^{2}+thunderer^{2}+meadowlark^{2}\\right)\n\\]\n\nIt follows that \\( flintrock=sagebrush=thunderer=meadowlark=0, \\quad \\) so \\( willowark=elmbranch=0 \\).\n\nWe can equally well use a different set of roots of one. If \\( stargazer \\) is a primitive \\( riverstone \\)th root of one, \\( riverstone>2 \\), then\n\\[\nriverstone=\\sum_{larkspore=1}^{riverstone}\\left|lanternum\\left(stargazer^{larkspore}\\right)\\right|^{2}=riverstone\\left(1+|willowark|^{2}+|elmbranch|^{2}\\right),\n\\]\nand \\( willowark=elmbranch=0 \\) follows. We can also replace the sum by an integral. For all real \\( hinterlog \\), we have\n\\[\n\\begin{aligned}\n1=\\left|lanternum\\left(e^{i hinterlog}\\right)\\right|^{2}= & \\left(e^{2 i hinterlog}+willowark e^{i hinterlog}+elmbranch\\right)\\left(e^{-2 i hinterlog}+\\overline{willowark} e^{-i hinterlog}+\\overline{elmbranch}\\right) \\\\\n= & \\overline{elmbranch} e^{2 i hinterlog}+(\\overline{willowark}+willowark \\overline{elmbranch}) e^{i hinterlog}+1+|willowark|^{2}+|elmbranch|^{2} \\\\\n& +(willowark+\\overline{willowark} \\overline{elmbranch}) e^{-i hinterlog}+elmbranch e^{-2 i hinterlog} .\n\\end{aligned}\n\\]\n\nIf we integrate this over \\( [0,2 \\pi] \\), we get\n\\[\n2 \\pi=2 \\pi\\left(1+|willowark|^{2}+|elmbranch|^{2}\\right)\n\\]\nand \\( willowark=elmbranch=0 \\), as before.\n\nSecond Solution. If \\( brickdust, moonwhale, \\gamma \\) are complex numbers such that \\( |brickdust|=|moonwhale|=|\\gamma|=1 \\) and \\( brickdust+moonwhale+\\gamma=3 \\), then \\( brickdust=moonwhale=\\gamma=1 \\). Let \\( cobblenet= (-1+i \\sqrt{3}) / 2 \\), a cube root of one. Then \\( |lanternum(1)|=|cobblenet\\, lanternum(cobblenet)|=\\left|cobblenet^{2} lanternum\\left(cobblenet^{2}\\right)\\right| =1 \\) and \\( lanternum(1)+cobblenet\\, lanternum(cobblenet)+cobblenet^{2} lanternum\\left(cobblenet^{2}\\right)=3 \\), so\n\\[\nlanternum(1)=cobblenet\\, lanternum(cobblenet)=cobblenet^{2} lanternum\\left(cobblenet^{2}\\right)=1\n\\]\nand \\( lanternum(1)=1, lanternum(cobblenet)=cobblenet^{2}, lanternum\\left(cobblenet^{2}\\right)=cobblenet=cobblenet^{4} \\). But there is only one polynomial of degree less than three that takes these values at these points, and it is \\( lanternum(quartzite)=quartzite^{2} \\).\n\nThird Solution. Put \\( rosebloom(quartzite)=1+willowark\\, quartzite+elmbranch\\, quartzite^{2} \\). Then for \\( |quartzite|=1 \\), we have\n\\[\n|rosebloom(quartzite)|=\\left|rosebloom(quartzite)\\, quartzite^{-2}\\right|=|lanternum(quartzite)|=1 .\n\\]\n\nSince \\( rosebloom(0)=1 \\), we see that \\( rosebloom \\) has the same absolute value at an interior point of the disc as its maximum absolute value on the boundary. By the maximum modulus principle, \\( rosebloom \\) is constant; so \\( rosebloom(quartzite)=1 \\) and \\( willowark=elmbranch=0 \\).\n\nRemark. All three of these proofs can be adapted to prove a more general result, namely, if\n\\[\nlanternum(quartzite)=quartzite^{riverstone}+ambergris\\, quartzite^{riverstone-1}+briarwood\\, quartzite^{riverstone-2}+\\cdots+ironforge\n\\]\nand \\( |lanternum(quartzite)|=1 \\) for all \\( quartzite \\) such that \\( |quartzite|=1 \\), then\n\\[\nambergris=briarwood=\\cdots=ironforge=0\n\\]\n\nBlaschke studied functions \\( duskflower \\) analytic on the open unit disc and continuous on the closed disc such that \\( |duskflower(quartzite)|=1 \\) when \\( |quartzite|=1 \\). He showed that all such functions have the form\n\\[\nduskflower(quartzite)=gossamer \\prod_{larkspore=1}^{riverstone} \\frac{quartzite-maplekey}{1-\\overline{maplekey}\\, quartzite}\n\\]\nwhere \\( b_{1}, b_{2}, \\ldots, b_{riverstone} \\) satisfy \\( \\left|b_{k}\\right|<1 \\) and \\( gossamer \\) is a constant of absolute value 1. See, for example, J. L. Walsh, Interpolation and Approximation in the Complex Domain. American Mathematical Society, Providence, R.I., 1935, pp. 281 ff."
    },
    "descriptive_long_misleading": {
      "map": {
        "P": "noisefunc",
        "z": "realaxis",
        "\\xi": "stationary",
        "\\theta": "flatplane",
        "\\omega": "inertness",
        "Q": "constantval",
        "f": "misfunction",
        "k": "continuum",
        "\\alpha": "terminal",
        "\\beta": "finality",
        "a": "emptiness",
        "b": "zerohood",
        "p": "stillness",
        "q": "calmness",
        "r": "reststate",
        "s": "quietude",
        "n": "infinite",
        "\\sigma": "variability",
        "a_1": "fullnessone",
        "a_2": "fullnesstwo",
        "a_n": "fullnessmax",
        "b_k": "emptymark"
      },
      "question": "10. Given \\( noisefunc(realaxis)=realaxis^{2}+emptiness realaxis+zerohood \\), a quadratic polynomial of the complex variable \\( realaxis \\) with complex coefficients \\( emptiness, zerohood \\). Suppose that \\( |noisefunc(realaxis)|=1 \\) for every \\( realaxis \\) such that \\( |realaxis|=1 \\). Prove that \\( emptiness=zerohood=0 \\).",
      "solution": "First Solution. Let \\( noisefunc(realaxis)=realaxis^{2}+emptiness realaxis+zerohood \\). Let \\( emptiness=stillness+i calmness, zerohood=reststate+i quietude \\), where \\( stillness, calmness, reststate, quietude \\) are real. We are given that \\( |noisefunc(1)|=|noisefunc(-1)|=|noisefunc(i)|=|noisefunc(-i)|=1 \\), and we find\n\\[\n\\begin{aligned}\n|noisefunc(1)|^{2} & =|1+stillness+i calmness+reststate+i quietude|^{2} \\\\\n& =1+stillness^{2}+calmness^{2}+reststate^{2}+quietude^{2}+2 stillness+2 reststate+2 stillness reststate+2 calmness quietude \\\\\n|noisefunc(-1)|^{2} & =|1-stillness-i calmness+reststate+i quietude|^{2} \\\\\n& =1+stillness^{2}+calmness^{2}+reststate^{2}+quietude^{2}-2 stillness+2 reststate-2 stillness reststate-2 calmness quietude \\\\\n|noisefunc(i)|^{2} & =|-1+i stillness-calmness+reststate+i quietude|^{2} \\\\\n& =1+stillness^{2}+calmness^{2}+reststate^{2}+quietude^{2}+2 calmness-2 reststate-2 calmness reststate+2 stillness quietude \\\\\n|noisefunc(-i)|^{2} & =|-1-i stillness+calmness+reststate+i quietude|^{2} \\\\\n& =1+stillness^{2}+calmness^{2}+reststate^{2}+quietude^{2}-2 calmness-2 reststate+2 calmness reststate-2 stillness quietude .\n\\end{aligned}\n\\]\n\nAdding these equations, we get\n\\[\n4=4+4\\left(stillness^{2}+calmness^{2}+reststate^{2}+quietude^{2}\\right)\n\\]\n\nIt follows that \\( stillness=calmness=reststate=quietude=0, \\quad \\) so \\( emptiness=zerohood=0 \\).\n\nWe can equally well use a different set of roots of one. If \\( stationary \\) is a primitive \\( infinite \\)th root of one, \\( infinite>2 \\), then\n\\[\ninfinite=\\sum_{continuum=1}^{infinite}\\left|noisefunc\\left(stationary^{continuum}\\right)\\right|^{2}=infinite\\left(1+|emptiness|^{2}+|zerohood|^{2}\\right),\n\\]\nand \\( emptiness=zerohood=0 \\) follows. We can also replace the sum by an integral. For all real \\( flatplane \\), we have\n\\[\n\\begin{aligned}\n1=\\left|noisefunc\\left(e^{i flatplane}\\right)\\right|^{2}= & \\left(e^{2 i flatplane}+emptiness e^{i flatplane}+zerohood\\right)\\left(e^{-2 i flatplane}+\\overline{emptiness} e^{-i flatplane}+\\overline{zerohood}\\right) \\\\\n= & \\overline{zerohood} e^{2 i flatplane}+(\\overline{emptiness}+emptiness \\overline{zerohood}) e^{i flatplane}+1+|emptiness|^{2}+|zerohood|^{2} \\\\\n& +(emptiness+\\overline{emptiness} \\overline{zerohood}) e^{-i flatplane}+zerohood e^{-2 i flatplane} .\n\\end{aligned}\n\\]\n\nIf we integrate this over \\( [0,2 \\pi] \\), we get\n\\[\n2 \\pi=2 \\pi\\left(1+|emptiness|^{2}+|zerohood|^{2}\\right)\n\\]\nand \\( emptiness=zerohood=0 \\), as before.\n\nSecond Solution. If \\( terminal, finality, \\gamma \\) are complex numbers such that \\( |terminal|=|finality|=|\\gamma|=1 \\) and \\( terminal+finality+\\gamma=3 \\), then \\( terminal=finality=\\gamma=1 \\). Let \\( inertness= (-1+i \\sqrt{3}) / 2 \\), a cube root of one. Then \\( |noisefunc(1)|=|inertness noisefunc(inertness)|=\\left|inertness^{2} noisefunc\\left(inertness^{2}\\right)\\right|=1 \\) and \\( noisefunc(1)+inertness noisefunc(inertness)+inertness^{2} noisefunc\\left(inertness^{2}\\right)=3 \\), so\n\\[\nnoisefunc(1)=inertness noisefunc(inertness)=inertness^{2} noisefunc\\left(inertness^{2}\\right)=1\n\\]\n\nand \\( noisefunc(1)=1, noisefunc(inertness)=inertness^{2}, noisefunc\\left(inertness^{2}\\right)=inertness=inertness^{4} \\). But there is only one polynomial of degree less than three that takes these values at these points, and it is \\( noisefunc(realaxis)=realaxis^{2} \\).\n\nThis argument also works using higher roots of one or by integration.\n\nThird Solution. Put \\( constantval(realaxis)=1+emptiness realaxis+zerohood realaxis^{2} \\). Then for \\( |realaxis|=1 \\), we have\n\\[\n|constantval(realaxis)|=\\left|constantval(realaxis) realaxis^{-2}\\right|=|noisefunc(realaxis)|=1 .\n\\]\n\nSince \\( constantval(0)=1 \\), we see that \\( constantval \\) has the same absolute value at an interior point of the disc as its maximum absolute value on the boundary. By the maximum modulus principle, \\( constantval \\) is constant; so \\( constantval(realaxis)=1 \\) and \\( emptiness=zerohood=0 \\).\n\nRemark. All three of these proofs can be adapted to prove a more general result, namely, if\n\\[\nnoisefunc(realaxis)=realaxis^{infinite}+fullnessone realaxis^{infinite-1}+fullnesstwo realaxis^{infinite-2}+\\cdots+fullnessmax\n\\]\n\nand \\( |noisefunc(realaxis)|=1 \\) for all \\( realaxis \\) such that \\( |realaxis|=1 \\), then\n\\[\nfullnessone=fullnesstwo=\\cdots=fullnessmax=0\n\\]\n\nBlaschke studied functions \\( misfunction \\) analytic on the open unit disc and continuous on the closed disc such that \\( |misfunction(realaxis)|=1 \\) when \\( |realaxis|=1 \\). He showed that all such functions have the form\n\\[\nmisfunction(realaxis)=variability \\prod_{continuum=1}^{infinite} \\frac{realaxis-emptymark}{1-\\overline{emptymark} realaxis}\n\\]\nwhere \\( emptymark \\) satisfy \\( \\left|emptymark\\right|<1 \\) and \\( variability \\) is a constant of absolute value 1. See, for example, J. L. Walsh, Interpolation and Approximation in the Complex Domain. American Mathematical Society, Providence, R.I., 1935, pp. 281 ff."
    },
    "garbled_string": {
      "map": {
        "P": "hjgrksla",
        "z": "qzxwvtnp",
        "\\xi": "vlmkrgsa",
        "\\theta": "jhncptoa",
        "\\omega": "sdlqwmna",
        "Q": "gpkzlqra",
        "f": "zxqmplra",
        "k": "nmbtczal",
        "\\alpha": "wqdmslke",
        "\\beta": "rtzgnpse",
        "a": "prhxlvsq",
        "b": "kmqdwnrz",
        "p": "lsvqdrxm",
        "q": "xkprznmh",
        "r": "cdjhspqv",
        "s": "fzktmnlw",
        "n": "qfrvzngc",
        "\\sigma": "bhzmtwqa",
        "a_1": "gcsvrnpe",
        "a_2": "vrqmsdth",
        "a_n": "frqdlkzp",
        "b_k": "hnszqvle"
      },
      "question": "<<<\n10. Given \\( hjgrksla(qzxwvtnp)=qzxwvtnp^{2}+prhxlvsq qzxwvtnp+kmqdwnrz \\), a quadratic polynomial of the complex variable \\( qzxwvtnp \\) with complex coefficients \\( prhxlvsq, kmqdwnrz \\). Suppose that \\( |hjgrksla(qzxwvtnp)|=1 \\) for every \\( qzxwvtnp \\) such that \\( |qzxwvtnp|=1 \\). Prove that \\( prhxlvsq=kmqdwnrz=0 \\).\n>>>",
      "solution": "<<<\nFirst Solution. Let \\( hjgrksla(qzxwvtnp)=qzxwvtnp^{2}+prhxlvsq qzxwvtnp+kmqdwnrz \\). Let \\( prhxlvsq=lsvqdrxm+i xkprznmh, kmqdwnrz=cdjhspqv+i fzktmnlw \\), where \\( lsvqdrxm, xkprznmh, cdjhspqv, fzktmnlw \\) are real. We are given that \\( |hjgrksla(1)|=|hjgrksla(-1)|=|hjgrksla(i)| =|hjgrksla(-i)|=1 \\), and we find\n\\[\n\\begin{aligned}\n|hjgrksla(1)|^{2} & =|1+lsvqdrxm+i xkprznmh+cdjhspqv+i fzktmnlw|^{2} \\\\\n& =1+lsvqdrxm^{2}+xkprznmh^{2}+cdjhspqv^{2}+fzktmnlw^{2}+2 lsvqdrxm+2 cdjhspqv+2 lsvqdrxm\\,cdjhspqv+2 xkprznmh\\,fzktmnlw \\\\\n|hjgrksla(-1)|^{2} & =|1-lsvqdrxm-i xkprznmh+cdjhspqv+i fzktmnlw|^{2} \\\\\n& =1+lsvqdrxm^{2}+xkprznmh^{2}+cdjhspqv^{2}+fzktmnlw^{2}-2 lsvqdrxm+2 cdjhspqv-2 lsvqdrxm\\,cdjhspqv-2 xkprznmh\\,fzktmnlw \\\\\n|hjgrksla(i)|^{2} & =|-1+i lsvqdrxm-xkprznmh+cdjhspqv+i fzktmnlw|^{2} \\\\\n& =1+lsvqdrxm^{2}+xkprznmh^{2}+cdjhspqv^{2}+fzktmnlw^{2}+2 xkprznmh-2 cdjhspqv-2 xkprznmh\\,cdjhspqv+2 lsvqdrxm\\,fzktmnlw \\\\\n|hjgrksla(-i)|^{2} & =|-1-i lsvqdrxm+xkprznmh+cdjhspqv+i fzktmnlw|^{2} \\\\\n& =1+lsvqdrxm^{2}+xkprznmh^{2}+cdjhspqv^{2}+fzktmnlw^{2}-2 xkprznmh-2 cdjhspqv+2 xkprznmh\\,cdjhspqv-2 lsvqdrxm\\,fzktmnlw .\n\\end{aligned}\n\\]\n\nAdding these equations, we get\n\\[\n4=4+4\\left(lsvqdrxm^{2}+xkprznmh^{2}+cdjhspqv^{2}+fzktmnlw^{2}\\right)\n\\]\n\nIt follows that \\( lsvqdrxm=xkprznmh=cdjhspqv=fzktmnlw=0 \\), so \\( prhxlvsq=kmqdwnrz=0 \\).\n\nWe can equally well use a different set of roots of one. If \\( vlmkrgsa \\) is a primitive \\( qfrvzngc \\)th root of one, \\( qfrvzngc>2 \\), then\n\\[\nqfrvzngc=\\sum_{nmbtczal=1}^{qfrvzngc}\\left|hjgrksla\\left(vlmkrgsa^{nmbtczal}\\right)\\right|^{2}=qfrvzngc\\left(1+|prhxlvsq|^{2}+|kmqdwnrz|^{2}\\right),\n\\]\nand \\( prhxlvsq=kmqdwnrz=0 \\) follows. We can also replace the sum by an integral. For all real \\( jhncptoa \\), we have\n\\[\n\\begin{aligned}\n1=\\left|hjgrksla\\left(e^{i jhncptoa}\\right)\\right|^{2}= & \\left(e^{2 i jhncptoa}+prhxlvsq e^{i jhncptoa}+kmqdwnrz\\right)\\left(e^{-2 i jhncptoa}+\\overline{prhxlvsq} e^{-i jhncptoa}+\\overline{kmqdwnrz}\\right) \\\\\n= & \\overline{kmqdwnrz} e^{2 i jhncptoa}+(\\overline{prhxlvsq}+prhxlvsq \\overline{kmqdwnrz}) e^{i jhncptoa}+1+|prhxlvsq|^{2}+|kmqdwnrz|^{2} \\\\\n& +(prhxlvsq+\\overline{prhxlvsq}\\,\\overline{kmqdwnrz}) e^{-i jhncptoa}+kmqdwnrz e^{-2 i jhncptoa} .\n\\end{aligned}\n\\]\n\nIntegrating over \\( [0,2\\pi] \\) gives\n\\[\n2\\pi=2\\pi\\left(1+|prhxlvsq|^{2}+|kmqdwnrz|^{2}\\right),\n\\]\nso \\( prhxlvsq=kmqdwnrz=0 \\), as before.\n\nSecond Solution. If \\( wqdmslke, rtzgnpse, \\gamma \\) are complex numbers such that \\( |wqdmslke|=|rtzgnpse|=|\\gamma|=1 \\) and \\( wqdmslke+rtzgnpse+\\gamma=3 \\), then \\( wqdmslke=rtzgnpse=\\gamma=1 \\). Let \\( sdlqwmna=(-1+i\\sqrt{3})/2 \\), a cube root of one. Then \\( |hjgrksla(1)|=|sdlqwmna hjgrksla(sdlqwmna)|=\\left|sdlqwmna^{2} hjgrksla\\left(sdlqwmna^{2}\\right)\\right|=1 \\) and \\( hjgrksla(1)+sdlqwmna hjgrksla(sdlqwmna)+sdlqwmna^{2} hjgrksla\\left(sdlqwmna^{2}\\right)=3 \\), so\n\\[\nhjgrksla(1)=sdlqwmna hjgrksla(sdlqwmna)=sdlqwmna^{2} hjgrksla\\left(sdlqwmna^{2}\\right)=1.\n\\]\nHence \\( hjgrksla(1)=1, \\; hjgrksla(sdlqwmna)=sdlqwmna^{2}, \\; hjgrksla\\left(sdlqwmna^{2}\\right)=sdlqwmna=sdlqwmna^{4} \\). There is only one polynomial of degree less than three taking these values at these points, namely \\( hjgrksla(qzxwvtnp)=qzxwvtnp^{2} \\).\n\nThis argument also works using higher roots of one or by integration.\n\nThird Solution. Put \\( gpkzlqra(qzxwvtnp)=1+prhxlvsq qzxwvtnp+kmqdwnrz qzxwvtnp^{2} \\). For \\( |qzxwvtnp|=1 \\),\n\\[\n|gpkzlqra(qzxwvtnp)|=\\left|gpkzlqra(qzxwvtnp)qzxwvtnp^{-2}\\right|=|hjgrksla(qzxwvtnp)|=1.\n\\]\nBecause \\( gpkzlqra(0)=1 \\), the maximum modulus principle forces \\( gpkzlqra \\) to be constant; thus \\( gpkzlqra(qzxwvtnp)=1 \\) and \\( prhxlvsq=kmqdwnrz=0 \\).\n\nRemark. The proofs extend: if\n\\[\nhjgrksla(qzxwvtnp)=qzxwvtnp^{qfrvzngc}+gcsvrnpe qzxwvtnp^{qfrvzngc-1}+vrqmsdth qzxwvtnp^{qfrvzngc-2}+\\cdots+frqdlkzp\n\\]\nand \\( |hjgrksla(qzxwvtnp)|=1 \\) for all \\( |qzxwvtnp|=1 \\), then\n\\[\ngcsvrnpe=vrqmsdth=\\cdots=frqdlkzp=0.\n\\]\n\nBlaschke studied functions \\( zxqmplra \\) analytic on the open unit disc and continuous on the closed disc with \\( |zxqmplra(qzxwvtnp)|=1 \\) when \\( |qzxwvtnp|=1 \\). He showed\n\\[\nzxqmplra(qzxwvtnp)=bhzmtwqa\\prod_{nmbtczal=1}^{qfrvzngc}\\frac{qzxwvtnp-hnszqvle_{nmbtczal}}{1-\\overline{hnszqvle_{nmbtczal}}\\,qzxwvtnp},\n\\]\nwhere \\( hnszqvle_{1},hnszqvle_{2},\\ldots,hnszqvle_{qfrvzngc} \\) satisfy \\( |hnszqvle_{nmbtczal}|<1 \\) and \\( bhzmtwqa \\) has absolute value 1. (See J. L. Walsh, Interpolation and Approximation in the Complex Domain, AMS, Providence, 1935, pp. 281 ff.)\n>>>"
    },
    "kernel_variant": {
      "question": "Let $m\\ge 2$ and $n\\ge 3$ be fixed integers.  \nFor a multi–index $\\alpha=(\\alpha_{1},\\ldots,\\alpha_{m})\\in\\mathbb Z_{\\ge 0}^{\\,m}$ put  \n$|\\alpha|=\\alpha_{1}+\\cdots+\\alpha_{m}$ and $z^{\\alpha}=z_{1}^{\\alpha_{1}}\\cdots z_{m}^{\\alpha_{m}}$.\n\nConsider a non–zero polynomial in $m$ variables  \n\\[\nP(z_{1},\\ldots,z_{m})=\\sum_{|\\alpha|\\le n} a_{\\alpha}\\,z^{\\alpha},\\qquad a_{\\alpha}\\in\\mathbb C.\n\\]\n\nAssume that on the $m$–dimensional unit torus  \n\\[\n\\mathbb T^{m}= \\bigl\\{(z_{1},\\ldots,z_{m})\\in\\mathbb C^{m}\\; :\\; |z_{1}|=\\cdots=|z_{m}|=1 \\bigr\\}\n\\]\nthe modulus of $P$ is constant: there exists $K>0$ such that  \n\\[\n|P(z_{1},\\ldots,z_{m})|\\equiv K\\qquad\\text{for every }(z_{1},\\ldots,z_{m})\\in\\mathbb T^{m}. \\tag{$\\star$}\n\\]\n\n(a) Prove that there is exactly one multi–index $\\alpha^{0}$ with $|\\alpha^{0}|\\le n$ for which $a_{\\alpha^{0}}\\neq 0$.\n\n(b) Show that necessarily $|a_{\\alpha^{0}}|=K$ and that, after multiplying $P$ by a complex number of modulus $1$ and possibly permuting the variables, one obtains  \n\\[\nP(z_{1},\\ldots,z_{m})=K\\cdot z_{1}^{\\alpha^{0}_{1}}\\,z_{2}^{\\alpha^{0}_{2}}\\cdots z_{m}^{\\alpha^{0}_{m}}.\n\\]\n\n(c) Deduce that if $\\deg P=n$ then $|\\alpha^{0}|=n$; hence, among all degree–$n$ polynomials in $m$ variables, the only ones whose modulus is constant on $\\mathbb T^{m}$ are unimodular rotations of a single monomial.\n\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "solution": "Step 1 – Fourier expansion on the torus  \nWrite $z_{j}=e^{i\\theta_{j}}$ with $\\theta_{j}\\in[0,2\\pi)$, and endow $\\mathbb T^{m}$ with the normalized Haar measure  \n\\[\nd\\mu(\\theta)=\\frac{d\\theta_{1}\\cdots d\\theta_{m}}{(2\\pi)^{m}}.\n\\]\nThe characters $e^{i\\langle\\gamma,\\theta\\rangle}$, $\\gamma\\in\\mathbb Z^{m}$, form an orthonormal basis of $L^{2}(\\mathbb T^{m})$.  \nFor the finite support  \n\\[\nS=\\{\\alpha\\in\\mathbb Z_{\\ge 0}^{\\,m}\\; :\\; a_{\\alpha}\\ne 0\\}\n\\]\nwe have  \n\\[\nP(e^{i\\theta})=\\sum_{\\alpha\\in S} a_{\\alpha}\\,e^{i\\langle\\alpha,\\theta\\rangle},\n\\]\nhence  \n\\[\n|P(e^{i\\theta})|^{2}=\\sum_{\\alpha,\\beta\\in S} a_{\\alpha}\\,\\overline{a_{\\beta}}\\,\ne^{i\\langle\\alpha-\\beta,\\theta\\rangle}. \\tag{1}\n\\]\n\nStep 2 – The correlation identities  \nBy hypothesis $(\\star)$ the left–hand side of (1) equals the constant $K^{2}$, whose Fourier series has only the zero frequency.  Identifying the Fourier coefficients we obtain  \n\\[\n\\sum_{\\alpha\\in\\mathbb Z^{m}} a_{\\alpha+\\gamma}\\,\\overline{a_{\\alpha}}=0\\quad(\\gamma\\neq 0),\\tag{2}\n\\]\n\\[\n\\sum_{\\alpha\\in S}|a_{\\alpha}|^{2}=K^{2}.\\tag{3}\n\\]\n(The sums in (2)–(3) are taken over all $\\alpha\\in\\mathbb Z^{m}$, with $a_{\\alpha}=0$ when $\\alpha\\notin S$.)\n\nStep 3 – Extreme indices isolate a single summand  \nAssume, for a contradiction, that $S$ contains at least two distinct indices.  Equip $\\mathbb Z^{m}$ with the lexicographic order and set  \n\\[\n\\alpha^{-}=\\min\\nolimits_{\\text{lex}}S,\\qquad\n\\alpha^{+}=\\max\\nolimits_{\\text{lex}}S,\\qquad\n\\gamma=\\alpha^{+}-\\alpha^{-}\\ne 0.\n\\]\n\nClaim.  If $\\alpha\\in S$ and $\\alpha+\\gamma\\in S$, then necessarily $\\alpha=\\alpha^{-}$ and $\\alpha+\\gamma=\\alpha^{+}$; consequently the sum in (2) corresponding to this particular $\\gamma$ reduces to the single term $a_{\\alpha^{+}}\\,\\overline{a_{\\alpha^{-}}}$.\n\nProof.  \nSuppose $\\alpha\\in S$ with $\\alpha+\\gamma\\in S$.  Because $\\alpha^{-}$ is lexicographically minimal, $\\alpha\\ge\\alpha^{-}$.  Let $k$ be the first coordinate where $\\alpha$ and $\\alpha^{-}$ differ, so $\\alpha_{k}>(\\alpha^{-})_{k}$.  For $j<k$ we have $\\alpha_{j}=(\\alpha^{-})_{j}$, hence  \n\\[\n(\\alpha+\\gamma)_{j}=\\alpha_{j}+\\gamma_{j}\n=(\\alpha^{-})_{j}+(\\alpha^{+}-\\alpha^{-})_{j}=(\\alpha^{+})_{j}.\n\\]\nIn coordinate $k$ we compute  \n\\[\n(\\alpha+\\gamma)_{k}\n=\\alpha_{k}+\\gamma_{k}\n>(\\alpha^{-})_{k}+\\gamma_{k}\n=(\\alpha^{-})_{k}+(\\alpha^{+})_{k}-(\\alpha^{-})_{k}\n=(\\alpha^{+})_{k}.\n\\]\nHence $\\alpha+\\gamma$ is lexicographically larger than $\\alpha^{+}$ unless $\\alpha_{k}=(\\alpha^{-})_{k}$, contradicting the choice of $k$.  Therefore $k$ cannot exist, i.e.\\ $\\alpha=\\alpha^{-}$ and $\\alpha+\\gamma=\\alpha^{+}$, proving the claim.\n\nAccordingly  \n\\[\n\\sum_{\\alpha}a_{\\alpha+\\gamma}\\,\\overline{a_{\\alpha}}\n=a_{\\alpha^{+}}\\,\\overline{a_{\\alpha^{-}}}. \\tag{4}\n\\]\n\nStep 4 – Reaching the contradiction  \nEquation (2) with this $\\gamma$ forces the left–hand side of (4) to vanish, yet both $a_{\\alpha^{+}}$ and $a_{\\alpha^{-}}$ are non–zero by definition of $S$.  Thus $a_{\\alpha^{+}}\\,\\overline{a_{\\alpha^{-}}}=0$, a contradiction.  Therefore $S$ cannot contain two different indices; it consists of exactly one multi–index, which we denote $\\alpha^{0}$.  This proves part (a).\n\nStep 5 – The size of the lone coefficient  \nWith only one non–zero coefficient, (3) yields $|a_{\\alpha^{0}}|^{2}=K^{2}$, hence $|a_{\\alpha^{0}}|=K$.  Multiplying $P$ by a unimodular constant we may assume $a_{\\alpha^{0}}=K$, and obtain  \n\\[\nP(z_{1},\\ldots,z_{m})=K\\cdot z^{\\alpha^{0}}\n=K\\cdot z_{1}^{\\alpha^{0}_{1}}\\cdots z_{m}^{\\alpha^{0}_{m}}.\n\\]\nPermuting the coordinates if desired gives the form stated in (b).\n\nStep 6 – The degree condition  \nIf $\\deg P=n$, then necessarily $|\\alpha^{0}|=n$; otherwise $\\deg P<|\\alpha^{0}|$ or $|\\alpha^{0}|<n$, contradicting the definition of degree.  Hence the only degree–$n$ polynomials with constant modulus on $\\mathbb T^{m}$ are unimodular rotations of the monomial $z^{\\alpha^{0}}$, establishing part (c).\n\n%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.412903",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher‐dimensional setting:  The problem moves from one complex variable to m ≥ 2 variables; the domain is the m-torus rather than the unit circle.  \n2. Additional structures:  Multi-index notation, Fourier analysis on 𝕋^{m}, orthogonality of exponentials, and combinatorial arguments about support sets are required.  \n3. Deeper theory:  The solution uses the full Fourier expansion of |P|² and exploits its Toeplitz-type structure; understanding Haar measure and orthogonality on compact abelian groups is essential.  \n4. Non-trivial uniqueness step:  One must choose a frequency γ that yields a single contribution in the convolution sum (2)—a subtle combinatorial observation absent from the one-variable case.  \n5. More steps:  The proof must pass through Fourier coefficients, orthogonality, support analysis, contradiction, and normalization, far richer than the single-variable counterpart.\n\nHence the variant is substantially more technical and conceptually demanding than both the original quadratic problem and the current degree-n one-variable kernel variant."
      }
    },
    "original_kernel_variant": {
      "question": "Let m ≥ 2 and n ≥ 3 be fixed integers.  \nFor a multi–index α = (α₁,…,α_m) ∈ ℤ_{\\ge 0}^{\\,m} put |α|=α₁+⋯+α_m and  \nz^α = z₁^{α₁}⋯z_m^{α_m}.  \n\nConsider a non–zero polynomial in m variables\n\n  P(z₁,…,z_m)= ∑_{|α|≤ n} a_α z^α ,  a_α∈ℂ.\n\nAssume that on the m–dimensional unit torus  \n\n  𝕋^{m} = { (z₁,…,z_m) ∈ ℂ^{m} : |z₁|=⋯=|z_m|=1 }  \n\nthe modulus of P is constant: there exists K>0 such that  \n\n  |P(z₁,…,z_m)| ≡ K  for every (z₁,…,z_m) ∈ 𝕋^{m}.        (★)\n\n(a) Prove that there is exactly one multi-index α⁰ with |α⁰|≤n for which a_{α⁰}≠0.  \n\n(b) Show that necessarily |a_{α⁰}|=K and that, after multiplying P by a complex number of modulus 1 and possibly permuting the variables, one has  \n\n  P(z₁,…,z_m)=K·z₁^{α⁰₁}z₂^{α⁰₂}⋯z_m^{α⁰_m}.  \n\n(c) Deduce that if deg P=n then |α⁰|=n; hence, among all degree-n polynomials in m variables, the only ones whose modulus is constant on 𝕋^{m} are unimodular rotations of a single monomial.",
      "solution": "Step 1 – Fourier expansion on the torus  \nWrite z_j=e^{iθ_j}, θ_j∈[0,2π).  With the normalized Haar measure  \ndμ(θ)= (2π)^{−m}dθ₁⋯dθ_m, the characters e^{i⟨γ,θ⟩}(γ∈ℤ^{m}) form an\northonormal basis of L²(𝕋^{m}).  For the finite set  \nS = {α : a_α ≠ 0} we have  \n\n  P(e^{iθ}) = Σ_{α∈S} a_α e^{i⟨α,θ⟩},  \n\nso\n\n  |P(e^{iθ})|² = Σ_{α,β∈S} a_α  \\overline{a_β}\\; e^{i⟨α−β,θ⟩}.        (1)\n\nStep 2 – The correlation identities  \nBy (★) the left side of (1) equals the constant K², whose Fourier series\ncontains only the zero frequency.  Therefore, for every γ≠0,\n\n  Σ_{α∈ℤ^{m}} a_{α+γ}\\,\\overline{a_{α}} = 0,                                 (2)\n\nwhile for γ=0 we get  \n\n  Σ_{α∈S}|a_{α}|² = K².                                                        (3)\n\n(The sums in (2)–(3) may be taken over all α, extending a_α by 0 outside S.)\n\nStep 3 – Extreme indices isolate a single summand  \nAssume, for a contradiction, that S contains at least two distinct indices.\nEquip ℤ^{m} with the lexicographic order.  Let  \n\n  α⁻ = lexicographically minimal element of S,  \n  α⁺ = lexicographically maximal element of S,\n\nand set γ = α⁺ − α⁻, so γ ≠ 0.\n\nClaim.  If α∈S and α+γ∈S, then α=α⁻ and α+γ=α⁺; in particular,\nfor this γ the sum in (2) reduces to the single term\na_{α⁺}\\,\\overline{a_{α⁻}}.\n\nProof.  \nSuppose α∈S with α+γ∈S.  \nBecause α⁻ is lexicographically minimal, α ≥ α⁻ (lex).  \nWrite k for the first coordinate in which α differs from α⁻.\nThen α_k > (α⁻)_k.  For j<k we have α_j = (α⁻)_j, hence\n(α+γ)_j = α_j + γ_j = (α⁻)_j + (α⁺−α⁻)_j = (α⁺)_j.  \nIn coordinate k we get  \n\n  (α+γ)_k = α_k + γ_k  \n             > (α⁻)_k + γ_k = (α⁻)_k + (α⁺)_k − (α⁻)_k = (α⁺)_k.\n\nThus α+γ exceeds α⁺ in the first coordinate where the two differ,\nso α+γ is lexicographically larger than α⁺.  \nBut α⁺ was chosen lex-maximal in S, so α+γ∉S unless α+γ=α⁺.\nConsequently α_k = (α⁻)_k, contradicting α_k>(α⁻)_k unless k does not exist,\ni.e. α=α⁻.  Then α+γ=α⁻+γ=α⁺, proving the claim.\n\nTherefore\n\n  Σ_{α} a_{α+γ}\\,\\overline{a_{α}} = a_{α⁺}\\,\\overline{a_{α⁻}}.                (4)\n\nStep 4 – Reaching the contradiction  \nBy (2) the left side of (4) must be zero, yet both coefficients\na_{α⁺} and a_{α⁻} are non-zero by definition of S.\nHence a_{α⁺}\\,\\overline{a_{α⁻}} = 0, a contradiction.\nThus S cannot contain two different indices; it consists of a single\nmulti-index, which we denote α⁰.  This establishes (a).\n\nStep 5 – The size of the lone coefficient  \nWith only one non-zero coefficient, (3) gives |a_{α⁰}|² = K²,\nso |a_{α⁰}|=K.  Multiplying P by a unimodular constant we may suppose\na_{α⁰}=K, obtaining  \n\n  P(z₁,…,z_m)=K·z^{α⁰}.  \n\nPermuting the coordinates if desired yields the form stated in (b).\n\nStep 6 – The degree condition  \nIf deg P=n, then necessarily |α⁰|=n; otherwise deg P<|α⁰| or |α⁰|<n,\ncontradicting the definition of degree.  Hence the only degree-n\npolynomials with constant modulus on 𝕋^{m} are unimodular rotations\nof the monomial z^{α⁰}=z₁^{α⁰₁}⋯z_m^{α⁰_m}, completing (c).",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.355653",
        "was_fixed": false,
        "difficulty_analysis": "1. Higher‐dimensional setting:  The problem moves from one complex variable to m ≥ 2 variables; the domain is the m-torus rather than the unit circle.  \n2. Additional structures:  Multi-index notation, Fourier analysis on 𝕋^{m}, orthogonality of exponentials, and combinatorial arguments about support sets are required.  \n3. Deeper theory:  The solution uses the full Fourier expansion of |P|² and exploits its Toeplitz-type structure; understanding Haar measure and orthogonality on compact abelian groups is essential.  \n4. Non-trivial uniqueness step:  One must choose a frequency γ that yields a single contribution in the convolution sum (2)—a subtle combinatorial observation absent from the one-variable case.  \n5. More steps:  The proof must pass through Fourier coefficients, orthogonality, support analysis, contradiction, and normalization, far richer than the single-variable counterpart.\n\nHence the variant is substantially more technical and conceptually demanding than both the original quadratic problem and the current degree-n one-variable kernel variant."
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}