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  "body": "\n## Statement\n\nLet $M$ be a real-analytic manifold and let $\\mathcal{I}$ be an [[exterior differential system|901CCKQHVY]] on $M$ generated by real-[[analytic|081WKEDK6C]] differential forms.\nLet $p \\in M$ and let $E \\subseteq T_pM$ be an $n$-dimensional ordinary integral element of $\\mathcal{I}$, in the sense of [[Cartan's test|WV4S47X6QC]].\nSo $E$ has a flag $\\set{0} = E_0 \\subset E_1 \\subset \\dots \\subset E_n = E$ of integral elements with $\\dim E_k = k$ and $E_k$ regular for $k \u003c n$.\nThen there is a real-analytic $n$-dimensional integral manifold $N \\subseteq M$ of $\\mathcal{I}$, a [[submanifold|VKFW27WXEZ]] with $p \\in N$ and $T_pN = E$.\n\n## Remarks\n\nThe proof builds $N$ one dimension at a time.\nStart from a point.\nAn integral manifold of dimension $k$, tangent to $E_k$, is extended to one of dimension $k + 1$, tangent to $E_{k+1}$.\nThis step solves an initial value problem with the [[Cauchy–Kovalevskaya theorem|97HMV9BT2W]].\nRegularity of $E_k$ ensures that the equations to solve are in Cauchy–Kovalevskaya form, and closure of $\\mathcal{I}$ under $d$ ensures that the new manifold is again integral.\nAt each step there is freedom in the choice of initial data, and counting it gives the size of the space of solutions.\n\nExample: let $\\mathcal{I}$ be generated by $1$-forms $\\theta^1, \\dots, \\theta^s$, independent at each point of an $m$-dimensional manifold.\nIf $d\\theta^a \\equiv 0$ modulo $\\theta^1, \\dots, \\theta^s$, then every $(m - s)$-plane in the kernel of the $\\theta^a$ is ordinary.\nThe theorem then gives integral manifolds of dimension $m - s$, as does the [[Frobenius theorem|5Q4BHBFXJY]], which holds without analyticity.\n\nAnalyticity is needed in general.\nThe method rests on the Cauchy–Kovalevskaya theorem, which has no smooth version.\nLewy's example gives a linear partial differential equation with polynomial coefficients and a smooth right-hand side with no solution near any point.\nWriting such an equation as an exterior differential system shows that the smooth analogue of the theorem fails.\n\n## Explanations\n\n### L1 human\n\nThe theorem gives solutions to systems of partial differential equations written in the language of differential forms.\nIt says that if the algebraic test of Cartan is passed at a point, then a genuine solution passes through that point with the predicted tangent plane.\nIt needs all data to be real-analytic, that is, given locally by convergent power series.\n\n### L2 human\n\nThe theorem is the main existence result of the theory of exterior differential systems.\nIt contains the Cauchy–Kovalevskaya theorem and, in the analytic case, the Frobenius theorem.\nIt was used, for instance, to show that every real-analytic Riemannian manifold of dimension $n$ embeds locally isometrically into Euclidean space of dimension $n(n+1)/2$, the Cartan–Janet theorem.\nIn practice one first checks Cartan's test, prolonging the system if the test fails, and then applies the theorem.\n",
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