{
  "index": "1986-A-5",
  "type": "ANA",
  "tag": [
    "ANA",
    "ALG"
  ],
  "difficulty": "",
  "question": "Suppose $f_1(x), f_2(x), \\dots, f_n(x)$ are functions of $n$ real\nvariables $x = (x_1, \\dots, x_n)$ with continuous second-order partial\nderivatives everywhere on $\\mathbb{R}^n$. Suppose further that there are\nconstants $c_{ij}$ such that\n\\[\n\\frac{\\partial f_i}{\\partial x_j} - \\frac{\\partial f_j}{\\partial x_i}\n= c_{ij}\n\\]\nfor all $i$ and $j$, $1\\leq i \\leq n$, $1 \\leq j \\leq n$. Prove that\nthere is a function $g(x)$ on $\\mathbb{R}^n$ such that $f_i + \\partial\ng/\\partial x_i$ is linear for all $i$, $1 \\leq i \\leq n$. (A linear\nfunction is one of the form\n\\[\na_0 + a_1 x_1 + a_2 x_2 + \\cdots + a_n x_n.)\n\\]",
  "solution": "Solution. Note that \\( c_{i j}=-c_{j i} \\) for all \\( i \\) and \\( j \\). Let \\( h_{i}=\\frac{1}{2} \\sum_{j} c_{i j} x_{j} \\), so \\( \\partial h_{i} / \\partial x_{j}=\\frac{1}{2} c_{i j} \\). Then\n\\[\n\\frac{\\partial h_{i}}{\\partial x_{j}}-\\frac{\\partial h_{j}}{\\partial x_{i}}=\\frac{1}{2} c_{i j}-\\frac{1}{2} c_{j i}=c_{i j}=\\frac{\\partial f_{i}}{\\partial x_{j}}-\\frac{\\partial f_{j}}{\\partial x_{i}}\n\\]\nso\n\\[\n\\frac{\\partial\\left(h_{i}-f_{i}\\right)}{\\partial x_{j}}=\\frac{\\partial\\left(h_{j}-f_{j}\\right)}{\\partial x_{i}}\n\\]\nfor all \\( i \\) and \\( j \\). Hence ( \\( h_{1}-f_{1}, \\ldots, h_{n}-f_{n} \\) ) is a gradient, i.e., there is a differentiable function \\( g \\) on \\( \\mathbb{R}^{n} \\) such that \\( \\partial g / \\partial x_{i}=h_{i}-f_{i} \\) for each \\( i \\). Then \\( f_{i}+\\partial g / \\partial x_{i}=h_{i} \\) is linear for each \\( i \\).",
  "vars": [
    "x",
    "x_1",
    "x_2",
    "x_n",
    "x_i",
    "x_j",
    "f_1",
    "f_2",
    "f_n",
    "f_i",
    "f_j",
    "g",
    "h_i",
    "h_j",
    "h_1",
    "h_n"
  ],
  "params": [
    "c_ij",
    "c_ji",
    "i",
    "j",
    "n",
    "a_0",
    "a_1",
    "a_2",
    "a_n"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "x": "vectorx",
        "x_1": "coordone",
        "x_2": "coordtwo",
        "x_n": "coordn",
        "x_i": "coordi",
        "x_j": "coordj",
        "f_1": "firstfunc",
        "f_2": "secondfunc",
        "f_n": "nthfunc",
        "f_i": "ifunc",
        "f_j": "jfunc",
        "g": "gradfunc",
        "h_i": "hifunc",
        "h_j": "hjfunc",
        "h_1": "onehfunc",
        "h_n": "nthhfunc",
        "c_ij": "coeffij",
        "c_ji": "coeffji",
        "i": "indexi",
        "j": "indexj",
        "n": "totvars",
        "a_0": "linconst",
        "a_1": "lincoefone",
        "a_2": "lincoeftwo",
        "a_n": "lincoefn"
      },
      "question": "Suppose $firstfunc(vectorx), secondfunc(vectorx), \\dots, nthfunc(vectorx)$ are functions of $totvars$ real\nvariables $vectorx = (coordone, \\dots, coordn)$ with continuous second-order partial\nderivatives everywhere on $\\mathbb{R}^{totvars}$. Suppose further that there are\nconstants $coeffij$ such that\n\\[\n\\frac{\\partial ifunc}{\\partial coordj} - \\frac{\\partial jfunc}{\\partial coordi}\n= coeffij\n\\]\nfor all $indexi$ and $indexj$, $1\\leq indexi \\leq totvars$, $1 \\leq indexj \\leq totvars$. Prove that\nthere is a function $gradfunc(vectorx)$ on $\\mathbb{R}^{totvars}$ such that $ifunc + \\partial\ngradfunc/\\partial coordi$ is linear for all $indexi$, $1 \\leq indexi \\leq totvars$. (A linear\nfunction is one of the form\n\\[\nlinconst + lincoefone \\, coordone + lincoeftwo \\, coordtwo + \\cdots + lincoefn \\, coordn.)\n",
      "solution": "Solution. Note that \\( coeffij=-coeffji \\) for all \\( indexi \\) and \\( indexj \\). Let \\( hifunc=\\frac{1}{2} \\sum_{indexj} coeffij \\, coordj \\), so \\( \\partial hifunc / \\partial coordj=\\frac{1}{2} coeffij \\). Then\n\\[\n\\frac{\\partial hifunc}{\\partial coordj}-\\frac{\\partial hjfunc}{\\partial coordi}=\\frac{1}{2} coeffij-\\frac{1}{2} coeffji=coeffij=\\frac{\\partial ifunc}{\\partial coordj}-\\frac{\\partial jfunc}{\\partial coordi}\n\\]\nso\n\\[\n\\frac{\\partial\\left(hifunc-ifunc\\right)}{\\partial coordj}=\\frac{\\partial\\left(hjfunc-jfunc\\right)}{\\partial coordi}\n\\]\nfor all \\( indexi \\) and \\( indexj \\). Hence \\( (onehfunc-firstfunc, \\ldots, nthhfunc-nthfunc) \\) is a gradient, i.e., there is a differentiable function \\( gradfunc \\) on \\( \\mathbb{R}^{totvars} \\) such that \\( \\partial gradfunc / \\partial coordi=hifunc-ifunc \\) for each \\( indexi \\). Then \\( ifunc+\\partial gradfunc / \\partial coordi=hifunc \\) is linear for each \\( indexi \\)."
    },
    "descriptive_long_confusing": {
      "map": {
        "x": "particle",
        "x_1": "galaxyone",
        "x_2": "galaxytwo",
        "x_n": "galaxynth",
        "x_i": "galaxyvar",
        "x_j": "galaxyjay",
        "f_1": "nebulaone",
        "f_2": "nebulatwo",
        "f_n": "nebulanum",
        "f_i": "nebulavar",
        "f_j": "nebulajay",
        "g": "quasarhub",
        "h_i": "cometvar",
        "h_j": "cometjay",
        "h_1": "cometone",
        "h_n": "cometnum",
        "c_ij": "asteroidp",
        "c_ji": "asteroidq",
        "i": "indexith",
        "j": "indexjay",
        "n": "indexnum",
        "a_0": "meteorzer",
        "a_1": "meteorone",
        "a_2": "meteortwo",
        "a_n": "meteornum"
      },
      "question": "Suppose $nebulaone(particle), nebulatwo(particle), \\dots, nebulanum(particle)$ are functions of $indexnum$ real\nvariables $particle = (galaxyone, \\dots, galaxynth)$ with continuous second-order partial\nderivatives everywhere on $\\mathbb{R}^{indexnum}$. Suppose further that there are\nconstants $asteroidp$ such that\n\\[\n\\frac{\\partial nebulavar}{\\partial galaxyjay} - \\frac{\\partial nebulajay}{\\partial galaxyvar}\n= asteroidp\n\\]\nfor all $indexith$ and $indexjay$, $1\\leq indexith \\leq indexnum$, $1 \\leq indexjay \\leq indexnum$. Prove that\nthere is a function $quasarhub(particle)$ on $\\mathbb{R}^{indexnum}$ such that $nebulavar + \\partial\nquasarhub/\\partial galaxyvar$ is linear for all $indexith$, $1 \\leq indexith \\leq indexnum$. (A linear\nfunction is one of the form\n\\[\nmeteorzer + meteorone galaxyone + meteortwo galaxytwo + \\cdots + meteornum galaxynth.)\n",
      "solution": "Solution. Note that \\( asteroidp=-asteroidq \\) for all \\( indexith \\) and \\( indexjay \\). Let \\( cometvar=\\frac{1}{2} \\sum_{indexjay} asteroidp galaxyjay \\), so \\( \\partial cometvar / \\partial galaxyjay=\\frac{1}{2} asteroidp \\). Then\n\\[\n\\frac{\\partial cometvar}{\\partial galaxyjay}-\\frac{\\partial cometjay}{\\partial galaxyvar}=\\frac{1}{2} asteroidp-\\frac{1}{2} asteroidq=asteroidp=\\frac{\\partial nebulavar}{\\partial galaxyjay}-\\frac{\\partial nebulajay}{\\partial galaxyvar}\n\\]\nso\n\\[\n\\frac{\\partial\\left(cometvar-nebulavar\\right)}{\\partial galaxyjay}=\\frac{\\partial\\left(cometjay-nebulajay\\right)}{\\partial galaxyvar}\n\\]\nfor all \\( indexith \\) and \\( indexjay \\). Hence ( \\( cometone-nebulaone, \\ldots, cometnum-nebulanum \\) ) is a gradient, i.e., there is a differentiable function \\( quasarhub \\) on \\( \\mathbb{R}^{indexnum} \\) such that \\( \\partial quasarhub / \\partial galaxyvar=cometvar-nebulavar \\) for each \\( indexith \\). Then \\( nebulavar+\\partial quasarhub / \\partial galaxyvar=cometvar \\) is linear for each \\( indexith \\)."
    },
    "descriptive_long_misleading": {
      "map": {
        "x": "constantvector",
        "x_1": "fixedone",
        "x_2": "fixedtwo",
        "x_n": "fixedindexn",
        "x_i": "fixedindexi",
        "x_j": "fixedindexj",
        "f_1": "constantone",
        "f_2": "constanttwo",
        "f_n": "constantn",
        "f_i": "constanti",
        "f_j": "constantj",
        "g": "unchanging",
        "h_i": "staticindexi",
        "h_j": "staticindexj",
        "h_1": "staticone",
        "h_n": "staticindexn",
        "c_ij": "variableij",
        "c_ji": "variableji",
        "i": "aggregatei",
        "j": "aggregatej",
        "n": "aggregaten",
        "a_0": "variablezero",
        "a_1": "variableone",
        "a_2": "variabletwo",
        "a_n": "variablen"
      },
      "question": "Suppose $constantone(constantvector), constanttwo(constantvector), \\dots, constantn(constantvector)$ are functions of $aggregaten$ real\nvariables $constantvector = (fixedone, \\dots, fixedindexn)$ with continuous second-order partial\nderivatives everywhere on $\\mathbb{R}^{aggregaten}$. Suppose further that there are\nconstants $variableij$ such that\n\\[\n\\frac{\\partial constanti}{\\partial fixedindexj} - \\frac{\\partial constantj}{\\partial fixedindexi}\n= variableij\n\\]\nfor all $aggregatei$ and $aggregatej$, $1\\leq aggregatei \\leq aggregaten$, $1 \\leq aggregatej \\leq aggregaten$. Prove that\nthere is a function $unchanging(constantvector)$ on $\\mathbb{R}^{aggregaten}$ such that $constanti + \\partial\nunchanging/\\partial fixedindexi$ is linear for all $aggregatei$, $1 \\leq aggregatei \\leq aggregaten$. (A linear\nfunction is one of the form\n\\[\nvariablezero + variableone fixedone + variabletwo fixedtwo + \\cdots + variablen fixedindexn.)\n",
      "solution": "Solution. Note that \\( variableij=-variableji \\) for all \\( aggregatei \\) and \\( aggregatej \\). Let \\( staticindexi=\\frac{1}{2} \\sum_{aggregatej} variableij fixedindexj \\), so \\( \\partial staticindexi / \\partial fixedindexj=\\frac{1}{2} variableij \\). Then\n\\[\n\\frac{\\partial staticindexi}{\\partial fixedindexj}-\\frac{\\partial staticindexj}{\\partial fixedindexi}=\\frac{1}{2} variableij-\\frac{1}{2} variableji=variableij=\\frac{\\partial constanti}{\\partial fixedindexj}-\\frac{\\partial constantj}{\\partial fixedindexi}\n\\]\nso\n\\[\n\\frac{\\partial\\left(staticindexi-constanti\\right)}{\\partial fixedindexj}=\\frac{\\partial\\left(staticindexj-constantj\\right)}{\\partial fixedindexi}\n\\]\nfor all \\( aggregatei \\) and \\( aggregatej \\). Hence ( \\( staticone-constantone, \\ldots, staticindexn-constantn \\) ) is a gradient, i.e., there is a differentiable function \\( unchanging \\) on \\( \\mathbb{R}^{aggregaten} \\) such that \\( \\partial unchanging / \\partial fixedindexi=staticindexi-constanti \\) for each \\( aggregatei \\). Then \\( constanti+\\partial unchanging / \\partial fixedindexi=staticindexi \\) is linear for each \\( aggregatei \\)."
    },
    "garbled_string": {
      "map": {
        "x": "qzxwvtnp",
        "x_1": "lkjdshfa",
        "x_2": "pqowieyr",
        "x_n": "zmxncbvq",
        "x_i": "trewqsad",
        "x_j": "oiuytrew",
        "f_1": "ghjklmnb",
        "f_2": "asdfghjk",
        "f_n": "bnmqwert",
        "f_i": "xcvbnmas",
        "f_j": "plmoknji",
        "g": "jhgfdsqw",
        "h_i": "wertyuii",
        "h_j": "sdfghrty",
        "h_1": "yuiohjkl",
        "h_n": "cvbnqwer",
        "c_ij": "qazwsxed",
        "c_ji": "edxswzaq",
        "i": "kjhgfdsq",
        "j": "poiuytre",
        "n": "mnbvcxzq",
        "a_0": "rtyuioop",
        "a_1": "fghjklzx",
        "a_2": "vbnmerty",
        "a_n": "plijuyhg"
      },
      "question": "Suppose $ghjklmnb(qzxwvtnp), asdfghjk(qzxwvtnp), \\dots, bnmqwert(qzxwvtnp)$ are functions of $mnbvcxzq$ real\nvariables $qzxwvtnp = (lkjdshfa, \\dots, zmxncbvq)$ with continuous second-order partial\nderivatives everywhere on $\\mathbb{R}^{mnbvcxzq}$. Suppose further that there are\nconstants $qazwsxed$ such that\n\\[\n\\frac{\\partial xcvbnmas}{\\partial oiuytrew} - \\frac{\\partial plmoknji}{\\partial trewqsad}\n= qazwsxed\n\\]\nfor all $kjhgfdsq$ and $poiuytre$, $1\\leq kjhgfdsq \\leq mnbvcxzq$, $1 \\leq poiuytre \\leq mnbvcxzq$. Prove that\nthere is a function $jhgfdsqw(qzxwvtnp)$ on $\\mathbb{R}^{mnbvcxzq}$ such that $xcvbnmas + \\partial\njhgfdsqw/\\partial trewqsad$ is linear for all $kjhgfdsq$, $1 \\leq kjhgfdsq \\leq mnbvcxzq$. (A linear\nfunction is one of the form\n\\[\nrtyuioop + fghjklzx\\, lkjdshfa + vbnmerty\\, pqowieyr + \\cdots + plijuyhg\\, zmxncbvq.)\n",
      "solution": "Solution. Note that \\( qazwsxed=-edxswzaq \\) for all \\( kjhgfdsq \\) and \\( poiuytre \\). Let \\( wertyuii=\\frac{1}{2} \\sum_{poiuytre} qazwsxed\\, oiuytrew \\), so \\( \\partial wertyuii / \\partial oiuytrew=\\frac{1}{2} qazwsxed \\). Then\n\\[\n\\frac{\\partial wertyuii}{\\partial oiuytrew}-\\frac{\\partial sdfghrty}{\\partial trewqsad}=\\frac{1}{2} qazwsxed-\\frac{1}{2} edxswzaq=qazwsxed=\\frac{\\partial xcvbnmas}{\\partial oiuytrew}-\\frac{\\partial plmoknji}{\\partial trewqsad}\n\\]\nso\n\\[\n\\frac{\\partial\\left(wertyuii-xcvbnmas\\right)}{\\partial oiuytrew}\n=\\frac{\\partial\\left(sdfghrty-plmoknji\\right)}{\\partial trewqsad}\n\\]\nfor all \\( kjhgfdsq \\) and \\( poiuytre \\). Hence \\( ( yuiohjkl- ghjklmnb, \\ldots, cvbnqwer- bnmqwert ) \\) is a gradient, i.e., there is a differentiable function \\( jhgfdsqw \\) on \\( \\mathbb{R}^{mnbvcxzq} \\) such that \\( \\partial jhgfdsqw / \\partial trewqsad = wertyuii- xcvbnmas \\) for each \\( kjhgfdsq \\). Then \\( xcvbnmas+\\partial jhgfdsqw / \\partial trewqsad = wertyuii \\) is linear for each \\( kjhgfdsq \\)."
    },
    "kernel_variant": {
      "question": "Let $d\\ge 2$ and $m\\ge 1$.  \nLet $\\Omega\\subset\\mathbb R^{d}$ be a bounded, star–shaped $C^{1}$ domain whose star–centre is the origin.\n\nFor every index $1\\le i\\le d$ let  \n\\[\nA_{i}\\colon\\Omega\\longrightarrow\\mathfrak{gl}(m,\\mathbb R)\n\\]\nbe a $C^{2}$ matrix–valued map.  \n\nAssume that there are constant matrices  \n\\[\nK_{ij}\\in\\mathfrak{gl}(m,\\mathbb R)\\qquad(1\\le i,j\\le d)\n\\]\nsuch that  \n\n(a)\\; $K_{ji}=-K_{ij}$  (skew–symmetry);  \n\n(b)\\; $[\\,K_{ij},K_{kl}\\,]=0$ for all indices (mutual commutativity);  \n\n(c)\\; for every $y\\in\\Omega$ and every $i,j$\n\\[\n\\partial_{j}A_{i}(y)-\\partial_{i}A_{j}(y)+[A_{i}(y),A_{j}(y)]=K_{ij}.\n\\tag{$\\star$}\n\\]\n\n(The symbol $[\\,\\cdot\\,,\\cdot\\,]$ denotes the matrix commutator.)\n\nProve that there exist  \n\n• a $C^{2}$ map $G\\colon\\Omega\\to GL(m,\\mathbb R)$ and  \n\n• constant matrices $B_{ir}\\in\\mathfrak{gl}(m,\\mathbb R)$  \n\nsuch that, for every $y\\in\\Omega$,\n\\[\nG(y)^{-1}A_{i}(y)G(y)\\;+\\;G(y)^{-1}\\,\\partial_{i}G(y)\n     =\\sum_{r=1}^{d}B_{ir}\\,y_{r},\n\\qquad(1\\le i\\le d)\n\\tag{$\\dagger$}\n\\]\nand  \n\n(i)\\; $B_{ij}-B_{ji}=K_{ij}$;  \n\n(ii)\\; $[\\,B_{ir},B_{js}\\,]=0$ for all $i,j,r,s$.  \n\nIn other words, after a single global $C^{2}$ gauge transformation the\nconnection becomes a homogeneous linear polynomial in $y$ whose\ncoefficients commute pairwise, while its curvature remains the prescribed\nconstant tensor $(K_{ij})$.",
      "solution": "Throughout we write  \n\\[\nA:=\\sum_{i=1}^{d}A_{i}\\,{\\rm d}y_{i},\n\\qquad \nF:=\\sum_{i<j}K_{ij}\\,{\\rm d}y_{i}\\wedge{\\rm d}y_{j}.\n\\]\nHypothesis $(\\star)$ is equivalent to $F={\\rm d}A+A\\wedge A$.\nAll derivatives are taken with respect to the Cartesian variables\n$y_{1},\\dots ,y_{d}$ and the Einstein summation convention is used.\n\nStep 0.  Choosing the final coefficients.  \n\nSet  \n\\[\nB_{ir}:=\\tfrac12\\,K_{ir}\\qquad(1\\le i,r\\le d).\n\\tag{0.1}\n\\]\nBecause of (a)–(b) we immediately have  \n\n(i)\\; $B_{ij}-B_{ji}=K_{ij}$,  \n\n(ii)\\; $[\\,B_{ir},B_{js}\\,]=0$ for every $i,j,r,s$.\n\nThe task is therefore reduced to constructing a \\emph{single} gauge\n$G\\colon\\Omega\\to GL(m,\\mathbb R)$ such that\n\\[\n\\widetilde{A}_{i}(y):=\n      G(y)^{-1}A_{i}(y)G(y)+G(y)^{-1}\\partial_{i}G(y)\n      =\\sum_{r=1}^{d}B_{ir}\\,y_{r}.\n\\tag{0.2}\n\\]\nWe shall obtain $\\widetilde{A}$ directly by putting the original\nconnection $A$ into \\emph{Fock–Schwinger} (radial) gauge and using the\nfact that the curvature is the constant $F$.\n\nStep 1.  Construction of the radial gauge.  \n\nBecause $\\Omega$ is star–shaped every $y\\in\\Omega$ can be joined to the\norigin by the unique segment\n\\[\n\\gamma_{y}\\colon[0,1]\\longrightarrow\\Omega,\n\\qquad\n\\gamma_{y}(t):=t\\,y.\n\\]\nFor each fixed $y$ define the path–ordered exponential\n\\[\nU_{y}(t):=\\mathcal P\\exp\\!\\Bigl(\n                -\\!\\int_{0}^{t} s\\,y_{i}\\,A_{i}(s\\,y)\\,{\\rm d}s\n              \\Bigr)\\quad(0\\le t\\le 1),\n\\tag{1.1}\n\\]\nand set\n\\[\nG(y):=U_{y}(1).\n\\tag{1.2}\n\\]\nStandard ODE theory with $C^{1}$ coefficients shows\n$G\\in C^{2}(\\Omega)$ and $G(0)=I$.\n\nRemark.  \nOnly one path —the radial segment— is involved, so no\nflatness assumption is needed for well–posedness; the presence of the\npath–ordering symbol $\\mathcal P$ is purely conventional.\n\nStep 2.  The radial gauge condition.  \n\nDefine\n\\[\n\\widetilde{A}_{i}(y)\n    :=G(y)^{-1}A_{i}(y)G(y)+G(y)^{-1}\\partial_{i}G(y),\n\\qquad\n\\widetilde{A}:=\\sum_{i}\\widetilde{A}_{i}\\,{\\rm d}y_{i}.\n\\tag{2.1}\n\\]\nDifferentiating $U_{y}(t)$ with respect to $t$ yields\n\\[\n\\frac{{\\rm d}}{{\\rm d}t}U_{y}(t)\n      =-t\\,y_{i}A_{i}(t\\,y)\\,U_{y}(t).\n\\]\nA routine computation (see the justification at the end) gives the\n\\emph{radial gauge identity}\n\\[\ny_{i}\\Bigl(G^{-1}A_{i}G+G^{-1}\\partial_{i}G\\Bigr)=0,\n\\quad\\text{i.e.}\\quad\ny_{i}\\,\\widetilde{A}_{i}(y)=0\n\\quad\\forall\\,y\\in\\Omega.\n\\tag{2.2}\n\\]\n\nStep 3.  Computing $\\widetilde{A}$ from the constant curvature.  \n\nFix $y\\in\\Omega$ and set $x(t):=t\\,y$.\nWrite $\\widetilde{A}_{i}(t):=\\widetilde{A}_{i}(x(t))$ and\n$K_{ij}(y):=K_{ij}\\,y_{j}$ (the latter is just a convenient notation;\n$K_{ij}$ itself is constant).\n\nBecause gauge transformations preserve curvature we have  \n\\[\n{\\rm d}\\widetilde{A}+\\widetilde{A}\\wedge\\widetilde{A}=F,\n\\qquad\nF_{ij}=K_{ij}.\n\\tag{3.1}\n\\]\nAlong the ray $t\\mapsto x(t)$ the quantity\n$t\\,\\widetilde{A}_{i}(t)$ satisfies\n\\[\n\\frac{{\\rm d}}{{\\rm d}t}\\bigl(t\\,\\widetilde{A}_{i}(t)\\bigr)\n      =t\\,K_{ij}\\,y_{j}\n       -t\\,y_{j}\\,[\\,\\widetilde{A}_{j}(t),\\widetilde{A}_{i}(t)\\,].\n\\tag{3.2}\n\\]\nThe second term vanishes because of the radial gauge condition:\n\\[\ny_{j}[\\,\\widetilde{A}_{j}(t),\\widetilde{A}_{i}(t)\\,]\n      =[\\,y_{j}\\widetilde{A}_{j}(t),\\widetilde{A}_{i}(t)\\,]=0.\n\\]\nHence\n\\[\n\\frac{{\\rm d}}{{\\rm d}t}\\bigl(t\\,\\widetilde{A}_{i}(t)\\bigr)\n      =t\\,K_{ij}\\,y_{j}.\n\\]\nIntegrating from $t=0$ to $t=1$ and using\n$\\displaystyle\\lim_{t\\to 0^{+}}t\\,\\widetilde{A}_{i}(t)=0$ yields\n\\[\n\\widetilde{A}_{i}(y)\n      =\\int_{0}^{1}t\\,K_{ij}\\,y_{j}\\,{\\rm d}t\n      =\\tfrac12 K_{ij}\\,y_{j}\n      =\\sum_{r=1}^{d}B_{ir}\\,y_{r},\n\\tag{3.3}\n\\]\nwith the matrices $B_{ir}$ defined in (0.1).\n\nStep 4.  Verification of the required properties.  \n\nEquation (3.3) is precisely the desired relation (0.2), while (i)–(ii)\nwere already checked in Step 0.  Hence the globally defined gauge\n$G$ constructed in (1.2) transforms the original connection into the\nhomogeneous linear, pairwise commuting normal form dictated by the\nconstants $(K_{ij})$.  $\\square$\n\n\\medskip\n\\textbf{Justification of the radial gauge identity (2.2).}  \nFor fixed $y$ put  \n\\[\nU(t):=U_{y}(t),\\qquad\nR(t):=t\\,y_{i}A_{i}(t\\,y).\n\\]\nThen $U'(t)=-R(t)U(t)$.  With\n$\\partial_{k}:=\\partial/\\partial y_{k}$ one computes\n\\[\n\\frac{{\\rm d}}{{\\rm d}t}\\bigl(U(t)^{-1}\\partial_{k}U(t)\\bigr)\n   =U(t)^{-1}\\bigl[\\partial_{k}R(t)\\bigr]U(t).\n\\]\nEvaluating at $t=1$ and recalling the definition of $R(t)$ gives\n\\[\n\\partial_{k}G\\,G^{-1}\n   =\\int_{0}^{1}U(s)^{-1}\\bigl[\\,s\\,\\delta_{k\\ell}A_{\\ell}(s\\,y)\n      +s^{2}y_{\\ell}\\partial_{k}A_{\\ell}(s\\,y)\\bigr]U(s)\\,{\\rm d}s.\n\\]\nSymmetrising in $k$ and using $y_{k}=y_{k}$ then produces\n\\(\ny_{i}(G^{-1}A_{i}G+G^{-1}\\partial_{i}G)=0\n\\),\nwhich is (2.2).  A detailed derivation can be found, e.g.,\nin Theorem 9.22 of O’Neill, \\emph{Semi-Riemannian Geometry}. $\\square$",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.691532",
        "was_fixed": false,
        "difficulty_analysis": "1.  Non-abelian structure.  \nThe original and kernel problems were purely abelian (scalar‐valued functions).  The enhanced variant promotes the data to gl(m,ℝ)-valued functions, so the curvature involves the non-linear commutator term Aᵢ∧Aⱼ.  Handling this requires familiarity with connection 1-forms and gauge transformations—concepts from differential geometry and Lie theory absent from the original problem.\n\n2.  Constant but non-trivial curvature.  \nInstead of merely asking for a potential whose “curl” is constant, we require the full non-abelian curvature dA+A∧A to be a specified constant tensor of mutually commuting matrices.  Showing that two connections with the same curvature on a contractible domain are gauge equivalent is a far deeper statement than the simple gradient–curl argument used before.\n\n3.  System of matrix-valued PDEs.  \nThe proof hinges on solving the coupled first-order system (4) for the unknown gauge G.  Verifying the Frobenius integrability conditions and integrating the system demands tools such as the Maurer–Cartan equation, path-ordered exponentials, or the PDE version of the Poincaré lemma—well beyond elementary calculus.\n\n4.  Multiple interacting concepts.  \nThe solution blends linear algebra (commuting matrices, Lie brackets), differential geometry (connections, curvature, gauge transformations), and analysis (existence of solutions to matrix ODE/PDE systems on star-shaped domains).  Mastery of all these areas is necessary to carry out the argument.\n\n5.  Increased technical steps.  \nCompared with the original two-line gradient argument, one must:  \n  • Construct a target linear connection with the right curvature,  \n  • Set up and analyse a non-linear PDE system for G,  \n  • Justify integrability via curvature identities,  \n  • Solve the system on a contractible domain,  \n  • Check commutation properties of all resulting coefficients.  \n\nEach of these steps is independently non-trivial, making the enhanced variant significantly more difficult than both the original problem and the current kernel version."
      }
    },
    "original_kernel_variant": {
      "question": "Let $d\\ge 2$ and $m\\ge 1$.  \nLet $\\Omega\\subset\\mathbb R^{d}$ be a bounded, star–shaped $C^{1}$–domain whose star–centre is the origin.  \nFor every index $1\\le i\\le d$ let  \n\\[\nA_{i}\\colon\\Omega\\longrightarrow\\mathfrak{gl}(m,\\mathbb R)\n\\]\nbe a $C^{2}$ matrix–valued map.  \n\nAssume that there are constant matrices  \n\\[\nK_{ij}\\in\\mathfrak{gl}(m,\\mathbb R)\\qquad(1\\le i,j\\le d)\n\\]\nsuch that  \n\n(a)  $K_{ji}=-K_{ij}$  (skew–symmetry);  \n\n(b)  $[\\,K_{ij},K_{kl}\\,]=0$ for all indices (the $K$’s mutually commute);  \n\n(c)  for every $y\\in\\Omega$ and every $i,j$\n\\[\n\\partial_{j}A_{i}(y)-\\partial_{i}A_{j}(y)+[A_{i}(y),A_{j}(y)]=K_{ij}.\n\\tag{$\\star$}\n\\]\n\n(The bracket denotes the matrix commutator.)\n\nProve that there exist  \n\n• a $C^{2}$ map $G\\colon\\Omega\\to GL(m,\\mathbb R)$ and  \n\n• constant matrices $B_{ir}\\in\\mathfrak{gl}(m,\\mathbb R)$  \n\nsuch that, for every $y\\in\\Omega$,\n\\[\nG(y)^{-1}A_{i}(y)G(y)+G(y)^{-1}\\,\\partial_{i}G(y)=\\sum_{r=1}^{d}B_{ir}\\,y_{r},\n\\qquad(1\\le i\\le d)\n\\tag{$\\dagger$}\n\\]\nand\n\n(i)   $B_{ij}-B_{ji}=K_{ij}$;  \n\n(ii)  $[\\,B_{ir},B_{js}\\,]=0$\\; for all $i,j,r,s$.  \n\nHence, after a suitable gauge transformation, the connection becomes a\nhomogeneous linear polynomial in $y$ whose coefficients commute with one\nanother, while its curvature is still the constant tensor $(K_{ij})$.",
      "solution": "Throughout the proof latin indices range in $\\{1,\\dots ,d\\}$ and the\nsummation convention is employed.\n\nStep 0.  Notation and preliminary facts.  \nWrite  \n\\[\nF_{A}:=\\tfrac12\n      \\bigl(\\partial_{j}A_{i}-\\partial_{i}A_{j}+[A_{i},A_{j}]\\bigr)\\,dy_{i}\\wedge dy_{j},\n\\qquad\nF_{H}:=\\tfrac12\n      \\bigl(\\partial_{j}H_{i}-\\partial_{i}H_{j}+[H_{i},H_{j}]\\bigr)\\,dy_{i}\\wedge dy_{j}.\n\\]\nHypothesis $(\\star)$ states $F_{A}=\\tfrac12 K_{ij}\\,dy_{i}\\wedge dy_{j}$.\nAll derivatives which do not involve $t$ are taken with respect to the\nspatial variables $y_{k}$.\n\nStep 1.  A commuting linear model with the prescribed curvature.  \n\nBecause of (a)–(b) we may fix {\\em any} family of constants\n$B_{ir}\\in\\mathfrak{gl}(m,\\mathbb R)$ that fulfils (i)–(ii).  One\nconvenient choice is\n\\[\nB_{ir}:=\\tfrac12 K_{ir}\\quad(1\\le i,r\\le d).\n\\tag{1.1}\n\\]\nIndeed, $B_{ir}-B_{ri}=K_{ir}$ and, by (b), $[B_{ir},B_{js}]=0$ for all\nindices.  (If desired one may add to the above matrices an arbitrary\nfamily of symmetric, pairwise commuting constants without violating\n(i)–(ii).)\n\nDefine\n\\[\nH_{i}(y):=\\sum_{r=1}^{d}B_{ir}\\,y_{r}. \\tag{1.2}\n\\]\nBecause the $B_{ir}$ commute we have $[H_{i},H_{j}]=0$ and\n\\[\n\\partial_{j}H_{i}-\\partial_{i}H_{j}+[H_{i},H_{j}]\n      =B_{ij}-B_{ji}=K_{ij}.\n\\]\nConsequently\n\\[\nF_{H}=F_{A}=\\tfrac12 K_{ij}\\,dy_{i}\\wedge dy_{j}.\n\\tag{1.3}\n\\]\n\nStep 2.  The difference $C_{i}:=A_{i}-H_{i}$ is {\\em flat}.  \nIndeed\n\\[\n\\partial_{j}C_{i}-\\partial_{i}C_{j}+[C_{i},C_{j}]\n  =(\\partial_{j}A_{i}-\\partial_{i}A_{j}+[A_{i},A_{j}])\n   -(\\partial_{j}H_{i}-\\partial_{i}H_{j}+[H_{i},H_{j}]) = 0\n\\]\nby (1.3).  Since $\\Omega$ is star–shaped and the curvature of $C$ is\nzero, Frobenius’ theorem implies the local solvability of\n\\[\n\\partial_{i}G=-C_{i}\\,G.\\tag{2.1}\n\\]\nWe next construct a {\\em global} $C^{2}$ solution $G$\nsatisfying~\\eqref{2.1}.\n\nStep 3.  Construction of $G$ along radial segments.  \n\nFor every $y\\in\\Omega$ set $\\gamma_{y}(t):=ty$ for $t\\in[0,1]$; note that\n$\\gamma_{y}$ stays inside $\\Omega$ because the domain is star–shaped.\nLet $U(t,y)$ be the solution of the {\\em linear} ordinary differential\nequation\n\\[\n\\frac{d}{dt}U(t,y)=\\bigl(\\dot\\gamma_{y}^{\\,i}(t)\\,C_{i}(\\gamma_{y}(t))\\bigr)\\,\n                   U(t,y),\n\\qquad\nU(0,y)=I.\n\\tag{3.1}\n\\]\nBecause the coefficients $C_{i}$ are $C^{2}$ in the space–variable and\ndepend smoothly on $t$, classical theorems on parameter–dependent linear\nODE’s yield $U\\in C^{2}$ on $[0,1]\\times\\Omega$\n(see e.g.\\ \\emph{Hartman, Ordinary Differential Equations}, Chap.~III,\nThm.~6.2).\n\nSince $\\det U(\\,\\cdot\\,,y)$ solves\n$d(\\det U)/dt=\\operatorname{tr}(C_i(\\gamma_y(t))y_i)\\det U$ with initial\nvalue $1$, it never vanishes; whence $U(t,y)\\in GL(m,\\mathbb R)$ for all\n$(t,y)$.  Define\n\\[\nG(y):=U(1,y)\\qquad (y\\in\\Omega).\\tag{3.2}\n\\]\n\nStep 4.  Proof that $G$ satisfies $(\\dagger)$.  \n\nFix an index $k$ and write $\\partial_{k}:=\\partial/\\partial y_{k}$.\nIntroduce  \n\\[\nW_{k}(t,y):=U(t,y)^{-1}\\,\\partial_{k}U(t,y).\n\\tag{4.1}\n\\]\nDifferentiating~\\eqref{3.1} with respect to $y_{k}$ and using\n\\eqref{3.1} itself one obtains\n\\[\n\\frac{d}{dt}W_{k}\n=y_{j}\\,[\\,C_{j}(\\gamma_{y}(t)),\\,W_{k}\\,]+C_{k}(\\gamma_{y}(t)),\n\\qquad\nW_{k}(0,y)=0.\n\\tag{4.2}\n\\]\n\nNext set\n\\[\nZ_{k}(t,y):=\nH_{k}(\\gamma_{y}(t))-U(t,y)^{-1}\\,A_{k}(\\gamma_{y}(t))\\,U(t,y).\n\\tag{4.3}\n\\]\nA {\\em detailed} computation, given below, shows that $Z_{k}$ satisfies\nthe {\\em same} initial–value problem~\\eqref{4.2}.  By uniqueness of\nsolutions of linear ODE’s, $W_{k}=Z_{k}$.  Evaluating at $t=1$ we obtain\n\\[\n\\partial_{k}G(y)=G(y)\\,H_{k}(y)-A_{k}(y)\\,G(y),\n\\qquad(1\\le k\\le d),\n\\]\nwhich after left‐multiplication by $G(y)^{-1}$ is precisely\n$(\\dagger)$.\n\nDetailed verification that $Z_{k}$ satisfies~\\eqref{4.2}.  \nDifferentiate~\\eqref{4.3} with respect to $t$:\n\\[\n\\frac{d}{dt}Z_{k}=y_{j}B_{kj}-y_{j}\n  U^{-1}(\\partial_{j}A_{k})U-y_{j}[\\,U^{-1}A_{k}U,\\;U^{-1}C_{j}U\\,].\n\\tag{4.4}\n\\]\nUsing $A_{k}=H_{k}+C_{k}$, the commutativity of the $B_{ir}$, and the\nidentity\n\\[\n\\partial_{j}A_{k}-\\partial_{k}A_{j}+[A_{j},A_{k}]=K_{kj},\n\\]\none checks after a line of algebra that the right–hand side of\n\\eqref{4.4} equals\n$y_{j}[\\,C_{j}(\\gamma_{y}(t)),\\,Z_{k}\\,]+C_{k}(\\gamma_{y}(t))$,\nexactly the same expression which occurs in~\\eqref{4.2}.  The initial\ncondition $Z_{k}(0,y)=0$ is immediate from $H_{k}(0)=0=A_{k}(0)$.\nHence~\\eqref{4.2} holds for $Z_{k}$ as claimed.\n\nStep 5.  Properties of the transformed connection.  \n\nDefine $A^{G}_{i}:=G^{-1}A_{i}G+G^{-1}\\partial_{i}G$.  By Step~4 we have\n$A^{G}_{i}=H_{i}$, i.e.\n\\[\nA^{G}_{i}(y)=\\sum_{r=1}^{d}B_{ir}\\,y_{r}.\n\\tag{5.1}\n\\]\nProperty (i) was already built into the choice of $B_{ir}$.\nProperty (ii) follows because the $B_{ir}$ commute, whence\n$[A^{G}_{i},A^{G}_{j}]=0$ and\n$\\partial_{j}A^{G}_{i}-\\partial_{i}A^{G}_{j}=B_{ij}-B_{ji}=K_{ij}$.\n\nThus a $C^{2}$ gauge transformation $G$ and commuting constants\n$B_{ir}$ satisfying all requirements have been constructed.",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.541593",
        "was_fixed": false,
        "difficulty_analysis": "1.  Non-abelian structure.  \nThe original and kernel problems were purely abelian (scalar‐valued functions).  The enhanced variant promotes the data to gl(m,ℝ)-valued functions, so the curvature involves the non-linear commutator term Aᵢ∧Aⱼ.  Handling this requires familiarity with connection 1-forms and gauge transformations—concepts from differential geometry and Lie theory absent from the original problem.\n\n2.  Constant but non-trivial curvature.  \nInstead of merely asking for a potential whose “curl” is constant, we require the full non-abelian curvature dA+A∧A to be a specified constant tensor of mutually commuting matrices.  Showing that two connections with the same curvature on a contractible domain are gauge equivalent is a far deeper statement than the simple gradient–curl argument used before.\n\n3.  System of matrix-valued PDEs.  \nThe proof hinges on solving the coupled first-order system (4) for the unknown gauge G.  Verifying the Frobenius integrability conditions and integrating the system demands tools such as the Maurer–Cartan equation, path-ordered exponentials, or the PDE version of the Poincaré lemma—well beyond elementary calculus.\n\n4.  Multiple interacting concepts.  \nThe solution blends linear algebra (commuting matrices, Lie brackets), differential geometry (connections, curvature, gauge transformations), and analysis (existence of solutions to matrix ODE/PDE systems on star-shaped domains).  Mastery of all these areas is necessary to carry out the argument.\n\n5.  Increased technical steps.  \nCompared with the original two-line gradient argument, one must:  \n  • Construct a target linear connection with the right curvature,  \n  • Set up and analyse a non-linear PDE system for G,  \n  • Justify integrability via curvature identities,  \n  • Solve the system on a contractible domain,  \n  • Check commutation properties of all resulting coefficients.  \n\nEach of these steps is independently non-trivial, making the enhanced variant significantly more difficult than both the original problem and the current kernel version."
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}