{
  "index": "2013-B-2",
  "type": "ANA",
  "tag": [
    "ANA",
    "ALG"
  ],
  "difficulty": "",
  "question": "Let $C = \\bigcup_{N=1}^\\infty C_N$, where $C_N$ denotes the set of those `cosine polynomials' of the form\n\\[\nf(x) = 1 + \\sum_{n=1}^N a_n \\cos(2 \\pi n x)\n\\]\nfor which:\n\\begin{enumerate}\n\\item[(i)]\n$f(x) \\geq 0$ for all real $x$, and\n\\item[(ii)]\n$a_n = 0$ whenever $n$ is a multiple of $3$.\n\\end{enumerate}\nDetermine the maximum value of $f(0)$ as $f$ ranges through $C$, and \nprove that this maximum is attained.",
  "solution": "We claim that the maximum value of $f(0)$ is $3$. This is attained for\n$N=2$, $a_1=\\frac{4}{3}$, $a_2=\\frac{2}{3}$: in this case $f(x) = 1+\\frac{4}{3} \\cos(2\\pi x)+\\frac{2}{3} \\cos(4\\pi x) =\n1+\\frac{4}{3} \\cos(2\\pi x)+\\frac{2}{3}(2\\cos^2(2\\pi x)-1) = \\frac{1}{3} (2\\cos(2\\pi x)+1)^2$ is always nonnegative.\n\nNow suppose that $f = 1 + \\sum_{n=1}^N a_n \\cos(2\\pi nx) \\in C$. When $n$ is an integer, $\\cos(2\\pi n/3)$ equals $0$ if $3|n$ and $-1/2$ otherwise. Thus $a_n \\cos(2\\pi n/3) = -a_n/2$ for all $n$, and\n$f(1/3) = 1-\\sum_{n=1}^N (a_n/2)$. Since $f(1/3) \\geq 0$, $\\sum_{n=1}^N a_n \\leq 2$, whence $f(0) = 1 + \\sum_{n=1}^N a_n \\leq 3$.",
  "vars": [
    "x",
    "n",
    "a_n"
  ],
  "params": [
    "C",
    "C_N",
    "N",
    "f"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "realvar",
        "n": "indexvar",
        "a_n": "coeffn",
        "C": "cosineset",
        "C_N": "cosinesub",
        "N": "maxdeg",
        "f": "cosinef"
      },
      "question": "Let $cosineset = \\bigcup_{maxdeg=1}^\\infty cosinesub$, where $cosinesub$ denotes the set of those `cosine polynomials' of the form\n\\[\ncosinef(realvar) = 1 + \\sum_{indexvar=1}^{maxdeg} coeffn \\cos(2 \\pi indexvar realvar)\n\\]\nfor which:\n\\begin{enumerate}\n\\item[(i)]\n$cosinef(realvar) \\geq 0$ for all real $realvar$, and\n\\item[(ii)]\n$coeffn = 0$ whenever $indexvar$ is a multiple of $3$.\n\\end{enumerate}\nDetermine the maximum value of $cosinef(0)$ as $cosinef$ ranges through $cosineset$, and \nprove that this maximum is attained.",
      "solution": "We claim that the maximum value of $cosinef(0)$ is $3$. This is attained for $maxdeg=2$, $coeff1=\\dfrac{4}{3}$, $coeff2=\\dfrac{2}{3}$. In this case\n\\[\ncosinef(realvar)=1+\\frac{4}{3}\\cos(2\\pi realvar)+\\frac{2}{3}\\cos(4\\pi realvar)=1+\\frac{4}{3}\\cos(2\\pi realvar)+\\frac{2}{3}\\bigl(2\\cos^{2}(2\\pi realvar)-1\\bigr)=\\frac{1}{3}\\bigl(2\\cos(2\\pi realvar)+1\\bigr)^{2},\n\\]\nwhich is always non-negative.\n\nNow suppose that\n\\[\ncosinef = 1 + \\sum_{indexvar=1}^{maxdeg} coeffn \\cos(2\\pi indexvar realvar)\\in cosineset.\n\\]\nWhen $indexvar$ is an integer, $\\cos(2\\pi indexvar/3)$ equals $0$ if $3\\mid indexvar$ and $-1/2$ otherwise. Thus $coeffn \\cos(2\\pi indexvar/3) = -\\dfrac{coeffn}{2}$ for all $indexvar$, and consequently\n\\[\ncosinef\\!\\left(\\frac13\\right)=1-\\sum_{indexvar=1}^{maxdeg}\\frac{coeffn}{2}.\n\\]\nBecause $cosinef(1/3)\\ge 0$, we have $\\sum_{indexvar=1}^{maxdeg} coeffn \\le 2$, whence\n\\[\ncosinef(0)=1+\\sum_{indexvar=1}^{maxdeg} coeffn \\le 3.\n\\]\nTherefore the maximal possible value of $cosinef(0)$ is $3$, and it is indeed achieved by the example exhibited above."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "beaverlake",
        "n": "honeybee",
        "a_n": "sandcastle",
        "C": "buttercup",
        "C_N": "buttercupgarden",
        "N": "dragonfruit",
        "f": "marshmallow"
      },
      "question": "Let $buttercup = \\bigcup_{dragonfruit=1}^\\infty buttercupgarden$, where $buttercupgarden$ denotes the set of those `cosine polynomials' of the form\n\\[\nmarshmallow(beaverlake) = 1 + \\sum_{honeybee=1}^{dragonfruit} sandcastle \\cos(2 \\pi honeybee beaverlake)\n\\]\nfor which:\n\\begin{enumerate}\n\\item[(i)]\n$marshmallow(beaverlake) \\geq 0$ for all real $beaverlake$, and\n\\item[(ii)]\n$sandcastle = 0$ whenever $honeybee$ is a multiple of $3$.\n\\end{enumerate}\nDetermine the maximum value of $marshmallow(0)$ as $marshmallow$ ranges through $buttercup$, and \nprove that this maximum is attained.",
      "solution": "We claim that the maximum value of $marshmallow(0)$ is $3$. This is attained for\n$dragonfruit=2$, $a_1=\\frac{4}{3}$, $a_2=\\frac{2}{3}$: in this case $marshmallow(beaverlake) = 1+\\frac{4}{3} \\cos(2\\pi beaverlake)+\\frac{2}{3} \\cos(4\\pi beaverlake) =\n1+\\frac{4}{3} \\cos(2\\pi beaverlake)+\\frac{2}{3}(2\\cos^2(2\\pi beaverlake)-1) = \\frac{1}{3} (2\\cos(2\\pi beaverlake)+1)^2$ is always nonnegative.\n\nNow suppose that $marshmallow = 1 + \\sum_{honeybee=1}^{dragonfruit} sandcastle \\cos(2\\pi honeybee beaverlake) \\in buttercup$. When $honeybee$ is an integer, $\\cos(2\\pi honeybee/3)$ equals $0$ if $3|honeybee$ and $-1/2$ otherwise. Thus $sandcastle \\cos(2\\pi honeybee/3) = -sandcastle/2$ for all $honeybee$, and\n$marshmallow(1/3) = 1-\\sum_{honeybee=1}^{dragonfruit} (sandcastle/2)$. Since $marshmallow(1/3) \\geq 0$, $\\sum_{honeybee=1}^{dragonfruit} sandcastle \\leq 2$, whence $marshmallow(0) = 1 + \\sum_{honeybee=1}^{dragonfruit} sandcastle \\leq 3$.  "
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "immovablepoint",
        "n": "continuumindex",
        "a_n": "ordinarymater",
        "C": "intersectioncollection",
        "C_N": "intersectionelement",
        "N": "fractionalvalue",
        "f": "unchangingscalar"
      },
      "question": "Let $intersectioncollection = \\bigcup_{fractionalvalue=1}^\\infty intersectionelement_{fractionalvalue}$, where $intersectionelement_{fractionalvalue}$ denotes the set of those `cosine polynomials' of the form\n\\[\nunchangingscalar(immovablepoint) = 1 + \\sum_{continuumindex=1}^{fractionalvalue} ordinarymater_{continuumindex} \\cos(2 \\pi continuumindex immovablepoint)\n\\]\nfor which:\n\\begin{enumerate}\n\\item[(i)]\n$unchangingscalar(immovablepoint) \\geq 0$ for all real $immovablepoint$, and\n\\item[(ii)]\n$ordinarymater_{continuumindex} = 0$ whenever $continuumindex$ is a multiple of $3$.\n\\end{enumerate}\nDetermine the maximum value of $unchangingscalar(0)$ as $unchangingscalar$ ranges through $intersectioncollection$, and \nprove that this maximum is attained.",
      "solution": "We claim that the maximum value of $unchangingscalar(0)$ is $3$. This is attained for\n$fractionalvalue=2$, $ordinarymater_1=\\frac{4}{3}$, $ordinarymater_2=\\frac{2}{3}$: in this case $unchangingscalar(immovablepoint) = 1+\\frac{4}{3} \\cos(2\\pi immovablepoint)+\\frac{2}{3} \\cos(4\\pi immovablepoint) =\n1+\\frac{4}{3} \\cos(2\\pi immovablepoint)+\\frac{2}{3}(2\\cos^2(2\\pi immovablepoint)-1) = \\frac{1}{3} (2\\cos(2\\pi immovablepoint)+1)^2$ is always nonnegative.\n\nNow suppose that $unchangingscalar = 1 + \\sum_{continuumindex=1}^{fractionalvalue} ordinarymater_{continuumindex} \\cos(2\\pi continuumindex immovablepoint) \\in intersectioncollection$. When $continuumindex$ is an integer, $\\cos(2\\pi continuumindex/3)$ equals $0$ if $3|continuumindex$ and $-1/2$ otherwise. Thus $ordinarymater_{continuumindex} \\cos(2\\pi continuumindex/3) = -ordinarymater_{continuumindex}/2$ for all $continuumindex$, and\n$unchangingscalar(1/3) = 1-\\sum_{continuumindex=1}^{fractionalvalue} (ordinarymater_{continuumindex}/2)$. Since $unchangingscalar(1/3) \\geq 0$, $\\sum_{continuumindex=1}^{fractionalvalue} ordinarymater_{continuumindex} \\leq 2$, whence $unchangingscalar(0) = 1 + \\sum_{continuumindex=1}^{fractionalvalue} ordinarymater_{continuumindex} \\leq 3$. Thus $3$ is indeed the maximum value, and it is attained as shown."
    },
    "garbled_string": {
      "map": {
        "x": "krztmphy",
        "n": "gvqslmra",
        "a_n": "jdkwrpei",
        "C": "vzahcben",
        "C_N": "xpqorjxm",
        "N": "ydfslkqt",
        "f": "rqmcztiv"
      },
      "question": "Let $vzahcben = \\bigcup_{ydfslkqt=1}^\\infty xpqorjxm$, where $xpqorjxm$ denotes the set of those `cosine polynomials' of the form\n\\[\nrqmcztiv(krztmphy) = 1 + \\sum_{gvqslmra=1}^{ydfslkqt} jdkwrpei \\cos(2 \\pi gvqslmra krztmphy)\n\\]\nfor which:\n\\begin{enumerate}\n\\item[(i)]\nrqmcztiv(krztmphy) \\geq 0 for all real krztmphy, and\n\\item[(ii)]\njdkwrpei = 0 whenever gvqslmra is a multiple of $3$.\n\\end{enumerate}\nDetermine the maximum value of $rqmcztiv(0)$ as rqmcztiv ranges through vzahcben, and \nprove that this maximum is attained.",
      "solution": "We claim that the maximum value of $rqmcztiv(0)$ is $3$. This is attained for $ydfslkqt=2$, $a_1=\\frac{4}{3}$, $a_2=\\frac{2}{3}$: in this case $rqmcztiv(krztmphy) = 1+\\frac{4}{3} \\cos(2\\pi krztmphy)+\\frac{2}{3} \\cos(4\\pi krztmphy) =\n1+\\frac{4}{3} \\cos(2\\pi krztmphy)+\\frac{2}{3}(2\\cos^2(2\\pi krztmphy)-1) = \\frac{1}{3} (2\\cos(2\\pi krztmphy)+1)^2$ is always nonnegative.\n\nNow suppose that $rqmcztiv = 1 + \\sum_{gvqslmra=1}^{ydfslkqt} jdkwrpei \\cos(2\\pi gvqslmra krztmphy) \\in vzahcben$. When $gvqslmra$ is an integer, $\\cos(2\\pi gvqslmra/3)$ equals $0$ if $3|gvqslmra$ and $-1/2$ otherwise. Thus $jdkwrpei \\cos(2\\pi gvqslmra/3) = -jdkwrpei/2$ for all gvqslmra, and\n$rqmcztiv(1/3) = 1-\\sum_{gvqslmra=1}^{ydfslkqt} (jdkwrpei/2)$. Since $rqmcztiv(1/3) \\geq 0$, $\\sum_{gvqslmra=1}^{ydfslkqt} jdkwrpei \\leq 2$, whence $rqmcztiv(0) = 1 + \\sum_{gvqslmra=1}^{ydfslkqt} jdkwrpei \\leq 3$.",
      "confidence": 0.13
    },
    "kernel_variant": {
      "question": "Fix an integer \\(s\\ge 2\\).  \nFor every \\(N\\in\\mathbf N\\) denote by \\(\\mathcal P_{N}\\) the family of all matrix–valued trigonometric polynomials  \n\n\\[\n\\mathbf F(x)=I_s+\\sum_{n=1}^{N}\\Bigl(A_n\\,e^{2\\pi i n x}+A_n^{\\!*}\\,e^{-2\\pi i n x}\\Bigr)\n\\qquad (x\\in\\mathbf R),\n\\]\n\nwhose coefficients satisfy  \n• \\(A_n^{\\!*}=A_n\\)  (Hermitian),  \n• \\(A_n\\succeq 0\\)  (positive–semidefinite),  \n• \\(\\operatorname{rank}A_n\\le 1\\),  \n• \\(A_n=0\\) for every even index \\(n\\),  \n\nand such that  \n\n(i)  \\(\\mathbf F(x)\\succeq 0\\) for every real \\(x\\).  \n\nPut \\(\\mathcal P:=\\bigcup_{N=1}^{\\infty}\\mathcal P_{N}\\).\n\n(a)  Compute  \n\n\\[\n\\sup_{\\mathbf F\\in\\mathcal P}\\;\n        \\rho\\!\\bigl(\\mathbf F(0)\\bigr),\n\\qquad\n\\rho(M):=\\max\\{\\lambda\\;:\\;\n                \\lambda\\text{ is an eigenvalue of the Hermitian matrix }M\\}.\n\\]\n\n(b)  Prove that this supremum is attained and give a complete description of all\n\\(\\mathbf F\\in\\mathcal P\\) for which  \n\\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)\\) equals the maximal value.",
      "solution": "Throughout let  \n\n\\[\nS:=\\sum_{\\substack{n\\ge 1\\\\ n\\text{ odd}}}A_n\n        \\qquad(\\text{a finite sum for every } \\mathbf F\\in\\mathcal P),\n\\qquad\n\\mathbf F(0)=I_s+2S.\n\\tag{0}\n\\]\n\nBecause every summand \\(A_n\\) is positive–semidefinite,  \n\n\\[\n0\\preceq S.\n\\tag{0′}\n\\]\n\nStep 1.  A universal scalar inequality.   \nFix a unit vector \\(v\\in\\mathbf C^{\\,s}\\) and define  \n\n\\[\nf_v(x):=v^{\\!*}\\mathbf F(x)v\n       =1+\\sum_{\\substack{n\\ge 1\\\\ n\\text{ odd}}}\n               a_n(v)\\cos(2\\pi n x),\n\\qquad a_n(v):=2\\,v^{\\!*}A_n v\\ge 0 .\n\\]\n\nEach \\(f_v\\) is a non–negative cosine polynomial.  \nEvaluating at \\(x=\\frac12\\) (every odd cosine equals \\(-1\\)) yields  \n\n\\[\n0\\le f_v\\!\\bigl(\\tfrac12\\bigr)=1-\\sum_{\\text{odd }n}a_n(v),\n\\]\nhence  \n\n\\[\n\\sum_{\\text{odd }n}a_n(v)\\le 1.\n\\tag{1}\n\\]\n\nIn particular  \n\n\\[\nv^{\\!*}Sv=\\frac12\\sum_{\\text{odd }n}a_n(v)\\le\\frac12\n          \\quad\\text{for every unit }v.\n\\]\nConsequently  \n\n\\[\n0\\preceq S\\preceq\\frac12\\,I_s,\\qquad\n\\|S\\|=\\lambda_{\\max}(S)\\le\\frac12.\n\\tag{2}\n\\]\n\nStep 2.  The optimal upper bound.  \nBy (0) and (2),\n\n\\[\n\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=1+2\\,\\lambda_{\\max}(S)\\le 1+2\\cdot\\frac12=2.\n\\tag{3}\n\\]\n\nThus for all \\(\\mathbf F\\in\\mathcal P\\) one has \\(\\rho(\\mathbf F(0))\\le 2\\).\n\nStep 3.  When is the bound attained?  \n\nLemma 1.  \nFor \\(\\mathbf F\\in\\mathcal P\\) the following are equivalent.  \n(a) \\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=2\\);  \n(b) \\(\\|S\\|=\\frac12\\).\n\nProof.  \nThe implication (b) ⇒ (a) is immediate from (3).  \nConversely, assume (a) and choose a unit eigenvector \\(w\\) of \\(\\mathbf F(0)\\)\nwith eigenvalue \\(2\\).  Then\n\\(Sw=\\tfrac12w\\), whence \\(\\|S\\|\\ge\\frac12\\);\ntogether with (2) this forces \\(\\|S\\|=\\frac12\\). ∎\n\nStep 4.  Construction of extremal polynomials (sufficiency).  \nLet \\(S\\succeq0\\) with \\(\\|S\\|=\\tfrac12\\).  \nBecause \\(S\\) is positive–semidefinite of finite rank \\(r\\le s\\),\nchoose a rank-one decomposition  \n\n\\[\nS=\\sum_{j=1}^{r}\\lambda_j\\,u_j u_j^{\\!*},\n\\qquad\nu_j\\in\\mathbf C^{\\,s}\\text{ unit},\\;\n0<\\lambda_j\\le\\tfrac12.\n\\tag{4}\n\\]\n\nPick pairwise distinct odd integers \\(n_1,\\dots ,n_r\\) and set  \n\n\\[\nA_{n_j}:=\\lambda_j\\,u_j u_j^{\\!*}\\;(1\\le j\\le r),\\qquad\nA_n:=0\\text{ for all remaining }n.\n\\tag{5}\n\\]\n\nEach \\(A_{n_j}\\) is Hermitian, rank \\(1\\), positive–semidefinite; all even\nindices vanish. Hence (5) defines \\(\\mathbf F\\in\\mathcal P\\) with the chosen\nmatrix \\(S\\).\n\nPositivity of \\(\\mathbf F\\).  \nFor arbitrary \\(v\\in\\mathbf C^{\\,s}\\),\n\n\\[\nf_v(x)=1+2\\sum_{j=1}^{r}\\lambda_j\n          |u_j^{\\!*}v|^{2}\\cos(2\\pi n_jx).\n\\]\n\nSince \\(|\\cos(2\\pi n_jx)|\\le1\\) and \\(\\sum_{j}\\lambda_j|u_j^{\\!*}v|^{2}\n      =v^{\\!*}Sv\\le\\frac12\\) by (2), we have  \n\n\\[\nf_v(x)\\ge 1-2\\cdot\\frac12=0\\qquad(\\forall x,v),\n\\]\nso \\(\\mathbf F(x)\\succeq0\\).  \nBecause \\(\\|S\\|=\\tfrac12\\), Lemma 1 yields\n\\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=2\\); hence the bound \\(2\\) is attainable.\n\nStep 5.  Completeness (necessity).  \nIf \\(\\mathbf F\\in\\mathcal P\\) attains the bound, Lemma 1 gives\n\\(\\|S\\|=\\tfrac12\\).  Conversely, any matrix \\(S\\succeq0\\) obeying\n\\(\\|S\\|=\\frac12\\) can be realised by the procedure of Step 4; therefore\n\n\\[\n\\bigl\\{\\mathbf F\\in\\mathcal P:\\rho(\\mathbf F(0))=2\\bigr\\}\n      =\\bigl\\{\\mathbf F\\in\\mathcal P:\\|S\\|=\\tfrac12\\bigr\\}.\n\\]\n\nStep 6.  Classification theorem.  \n\nTheorem.  \nA polynomial \\(\\mathbf F\\in\\mathcal P\\) is extremal,\ni.e. \\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=2\\),\niff its coefficient matrix \\(S\\) satisfies \\(\\|S\\|=\\frac12\\).\nGiven such an \\(S\\), choose a rank-one decomposition (4) and distinct odd\nfrequencies to define the coefficients by (5); this produces an extremal\n\\(\\mathbf F\\).\nConversely, every extremal \\(\\mathbf F\\) arises in this manner.\n\nExamples.  \n(1)  Pure one-frequency maximisers:  \n\\(S=\\tfrac12\\,u u^{\\!*}\\) (rank 1) realised by \\(A_1=\\tfrac12 uu^{\\!*}\\) with all\nother coefficients zero, giving  \n\n\\[\n\\mathbf F(x)=I_s+\\tfrac12\\bigl(uu^{\\!*}e^{2\\pi ix}+uu^{\\!*}e^{-2\\pi ix}\\bigr).\n\\]\n\n(2)  Multiple independent directions:  \n\\(S=\\tfrac12 e_1e_1^{\\!*}+\\tfrac12 e_2e_2^{\\!*}\\)  \ncan be decomposed with, e.g.,\n\\(A_1=\\tfrac12 e_1e_1^{\\!*},\\; A_3=\\tfrac12 e_2e_2^{\\!*}\\);\nonce again \\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=2\\).\n\nThis completes parts (a) and (b).",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.829482",
        "was_fixed": false,
        "difficulty_analysis": "1.  The original problem deals with *scalar* polynomials; the enhanced variant requires working with **matrix-valued** trigonometric polynomials.  Positive–semidefiniteness must now be verified in the operator sense, demanding familiarity with functional analysis and matrix theory.  \n\n2.  A new **rank constraint** (every Fourier coefficient has rank ≤ 1) adds a non-linear structural condition that interacts delicately with positivity and with the optimisation target (the spectral radius).  Identifying and proving that only rank-one perturbations can be extremal is impossible by elementary scalar arguments.  \n\n3.  The objective is no longer a simple value at a point but the **spectral radius**, a nonlinear function of the matrix, which forces the use of Rayleigh–Ritz and eigenvalue considerations.  \n\n4.  Showing uniqueness of extremisers requires combining one-variable extremal analysis with operator-theoretic arguments (simplicity of the top eigenvalue, reconstruction of the coefficient matrices from quadratic forms), a blend of techniques absent from both the original and the kernel variant.  \n\nThese layers of additional structure and the necessity to fuse harmonic analysis with linear-algebraic spectral theory make the enhanced variant substantially more intricate and conceptually deeper than the original problem."
      }
    },
    "original_kernel_variant": {
      "question": "Fix an integer \\(s\\ge 2\\).  \nFor every \\(N\\in\\mathbf N\\) denote by \\(\\mathcal P_{N}\\) the family of all matrix–valued trigonometric polynomials  \n\n\\[\n\\mathbf F(x)=I_s+\\sum_{n=1}^{N}\\Bigl(A_n\\,e^{2\\pi i n x}+A_n^{\\!*}\\,e^{-2\\pi i n x}\\Bigr)\n\\qquad (x\\in\\mathbf R),\n\\]\n\nwhose coefficients satisfy  \n• \\(A_n^{\\!*}=A_n\\)  (Hermitian),  \n• \\(A_n\\succeq 0\\)  (positive–semidefinite),  \n• \\(\\operatorname{rank}A_n\\le 1\\),  \n• \\(A_n=0\\) for every even index \\(n\\),  \n\nand such that  \n\n(i)  \\(\\mathbf F(x)\\succeq 0\\) for every real \\(x\\).  \n\nPut \\(\\mathcal P:=\\bigcup_{N=1}^{\\infty}\\mathcal P_{N}\\).\n\n(a)  Compute  \n\n\\[\n\\sup_{\\mathbf F\\in\\mathcal P}\\;\n        \\rho\\!\\bigl(\\mathbf F(0)\\bigr),\n\\qquad\n\\rho(M):=\\max\\{\\lambda\\;:\\;\n                \\lambda\\text{ is an eigenvalue of the Hermitian matrix }M\\}.\n\\]\n\n(b)  Prove that this supremum is attained and give a complete description of all\n\\(\\mathbf F\\in\\mathcal P\\) for which  \n\\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)\\) equals the maximal value.",
      "solution": "Throughout let  \n\n\\[\nS:=\\sum_{\\substack{n\\ge 1\\\\ n\\text{ odd}}}A_n\n        \\qquad(\\text{a finite sum for every } \\mathbf F\\in\\mathcal P),\n\\qquad\n\\mathbf F(0)=I_s+2S.\n\\tag{0}\n\\]\n\nBecause every summand \\(A_n\\) is positive–semidefinite,  \n\n\\[\n0\\preceq S.\n\\tag{0′}\n\\]\n\nStep 1.  A universal scalar inequality.   \nFix a unit vector \\(v\\in\\mathbf C^{\\,s}\\) and define  \n\n\\[\nf_v(x):=v^{\\!*}\\mathbf F(x)v\n       =1+\\sum_{\\substack{n\\ge 1\\\\ n\\text{ odd}}}\n               a_n(v)\\cos(2\\pi n x),\n\\qquad a_n(v):=2\\,v^{\\!*}A_n v\\ge 0 .\n\\]\n\nEach \\(f_v\\) is a non–negative cosine polynomial.  \nEvaluating at \\(x=\\frac12\\) (every odd cosine equals \\(-1\\)) yields  \n\n\\[\n0\\le f_v\\!\\bigl(\\tfrac12\\bigr)=1-\\sum_{\\text{odd }n}a_n(v),\n\\]\nhence  \n\n\\[\n\\sum_{\\text{odd }n}a_n(v)\\le 1.\n\\tag{1}\n\\]\n\nIn particular  \n\n\\[\nv^{\\!*}Sv=\\frac12\\sum_{\\text{odd }n}a_n(v)\\le\\frac12\n          \\quad\\text{for every unit }v.\n\\]\nConsequently  \n\n\\[\n0\\preceq S\\preceq\\frac12\\,I_s,\\qquad\n\\|S\\|=\\lambda_{\\max}(S)\\le\\frac12.\n\\tag{2}\n\\]\n\nStep 2.  The optimal upper bound.  \nBy (0) and (2),\n\n\\[\n\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=1+2\\,\\lambda_{\\max}(S)\\le 1+2\\cdot\\frac12=2.\n\\tag{3}\n\\]\n\nThus for all \\(\\mathbf F\\in\\mathcal P\\) one has \\(\\rho(\\mathbf F(0))\\le 2\\).\n\nStep 3.  When is the bound attained?  \n\nLemma 1.  \nFor \\(\\mathbf F\\in\\mathcal P\\) the following are equivalent.  \n(a) \\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=2\\);  \n(b) \\(\\|S\\|=\\frac12\\).\n\nProof.  \nThe implication (b) ⇒ (a) is immediate from (3).  \nConversely, assume (a) and choose a unit eigenvector \\(w\\) of \\(\\mathbf F(0)\\)\nwith eigenvalue \\(2\\).  Then\n\\(Sw=\\tfrac12w\\), whence \\(\\|S\\|\\ge\\frac12\\);\ntogether with (2) this forces \\(\\|S\\|=\\frac12\\). ∎\n\nStep 4.  Construction of extremal polynomials (sufficiency).  \nLet \\(S\\succeq0\\) with \\(\\|S\\|=\\tfrac12\\).  \nBecause \\(S\\) is positive–semidefinite of finite rank \\(r\\le s\\),\nchoose a rank-one decomposition  \n\n\\[\nS=\\sum_{j=1}^{r}\\lambda_j\\,u_j u_j^{\\!*},\n\\qquad\nu_j\\in\\mathbf C^{\\,s}\\text{ unit},\\;\n0<\\lambda_j\\le\\tfrac12.\n\\tag{4}\n\\]\n\nPick pairwise distinct odd integers \\(n_1,\\dots ,n_r\\) and set  \n\n\\[\nA_{n_j}:=\\lambda_j\\,u_j u_j^{\\!*}\\;(1\\le j\\le r),\\qquad\nA_n:=0\\text{ for all remaining }n.\n\\tag{5}\n\\]\n\nEach \\(A_{n_j}\\) is Hermitian, rank \\(1\\), positive–semidefinite; all even\nindices vanish. Hence (5) defines \\(\\mathbf F\\in\\mathcal P\\) with the chosen\nmatrix \\(S\\).\n\nPositivity of \\(\\mathbf F\\).  \nFor arbitrary \\(v\\in\\mathbf C^{\\,s}\\),\n\n\\[\nf_v(x)=1+2\\sum_{j=1}^{r}\\lambda_j\n          |u_j^{\\!*}v|^{2}\\cos(2\\pi n_jx).\n\\]\n\nSince \\(|\\cos(2\\pi n_jx)|\\le1\\) and \\(\\sum_{j}\\lambda_j|u_j^{\\!*}v|^{2}\n      =v^{\\!*}Sv\\le\\frac12\\) by (2), we have  \n\n\\[\nf_v(x)\\ge 1-2\\cdot\\frac12=0\\qquad(\\forall x,v),\n\\]\nso \\(\\mathbf F(x)\\succeq0\\).  \nBecause \\(\\|S\\|=\\tfrac12\\), Lemma 1 yields\n\\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=2\\); hence the bound \\(2\\) is attainable.\n\nStep 5.  Completeness (necessity).  \nIf \\(\\mathbf F\\in\\mathcal P\\) attains the bound, Lemma 1 gives\n\\(\\|S\\|=\\tfrac12\\).  Conversely, any matrix \\(S\\succeq0\\) obeying\n\\(\\|S\\|=\\frac12\\) can be realised by the procedure of Step 4; therefore\n\n\\[\n\\bigl\\{\\mathbf F\\in\\mathcal P:\\rho(\\mathbf F(0))=2\\bigr\\}\n      =\\bigl\\{\\mathbf F\\in\\mathcal P:\\|S\\|=\\tfrac12\\bigr\\}.\n\\]\n\nStep 6.  Classification theorem.  \n\nTheorem.  \nA polynomial \\(\\mathbf F\\in\\mathcal P\\) is extremal,\ni.e. \\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=2\\),\niff its coefficient matrix \\(S\\) satisfies \\(\\|S\\|=\\frac12\\).\nGiven such an \\(S\\), choose a rank-one decomposition (4) and distinct odd\nfrequencies to define the coefficients by (5); this produces an extremal\n\\(\\mathbf F\\).\nConversely, every extremal \\(\\mathbf F\\) arises in this manner.\n\nExamples.  \n(1)  Pure one-frequency maximisers:  \n\\(S=\\tfrac12\\,u u^{\\!*}\\) (rank 1) realised by \\(A_1=\\tfrac12 uu^{\\!*}\\) with all\nother coefficients zero, giving  \n\n\\[\n\\mathbf F(x)=I_s+\\tfrac12\\bigl(uu^{\\!*}e^{2\\pi ix}+uu^{\\!*}e^{-2\\pi ix}\\bigr).\n\\]\n\n(2)  Multiple independent directions:  \n\\(S=\\tfrac12 e_1e_1^{\\!*}+\\tfrac12 e_2e_2^{\\!*}\\)  \ncan be decomposed with, e.g.,\n\\(A_1=\\tfrac12 e_1e_1^{\\!*},\\; A_3=\\tfrac12 e_2e_2^{\\!*}\\);\nonce again \\(\\rho\\!\\bigl(\\mathbf F(0)\\bigr)=2\\).\n\nThis completes parts (a) and (b).",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.633578",
        "was_fixed": false,
        "difficulty_analysis": "1.  The original problem deals with *scalar* polynomials; the enhanced variant requires working with **matrix-valued** trigonometric polynomials.  Positive–semidefiniteness must now be verified in the operator sense, demanding familiarity with functional analysis and matrix theory.  \n\n2.  A new **rank constraint** (every Fourier coefficient has rank ≤ 1) adds a non-linear structural condition that interacts delicately with positivity and with the optimisation target (the spectral radius).  Identifying and proving that only rank-one perturbations can be extremal is impossible by elementary scalar arguments.  \n\n3.  The objective is no longer a simple value at a point but the **spectral radius**, a nonlinear function of the matrix, which forces the use of Rayleigh–Ritz and eigenvalue considerations.  \n\n4.  Showing uniqueness of extremisers requires combining one-variable extremal analysis with operator-theoretic arguments (simplicity of the top eigenvalue, reconstruction of the coefficient matrices from quadratic forms), a blend of techniques absent from both the original and the kernel variant.  \n\nThese layers of additional structure and the necessity to fuse harmonic analysis with linear-algebraic spectral theory make the enhanced variant substantially more intricate and conceptually deeper than the original problem."
      }
    }
  },
  "checked": true,
  "problem_type": "proof"
}