# Cauchy–Kowalevski theorem

Cauchy–Kowalevski theorem Differential equations Navier–Stokes differential equations used to simulate airflow around an obstruction Scope show Fields Classification show Types show Relation to processes Solution show Existence and uniqueness show General topics show Solution methods People show List vte In mathematics, the Cauchy–Kovalevskaya theorem (also written as the Cauchy–Kowalevski theorem) is the main local existence and uniqueness theorem for analytic partial differential equations associated with Cauchy initial value problems. A special case was proven by Augustin Cauchy (1842), and the full result by Sophie Kovalevskaya (1875).

Contents 1 First order Cauchy–Kovalevskaya theorem 2 Proof by analytic majorization 3 Higher-order Cauchy–Kovalevskaya theorem 3.1 Example 4 Cauchy–Kovalevskaya–Kashiwara theorem 4.1 Example 5 References 6 External links First order Cauchy–Kovalevskaya theorem This theorem is about the existence of solutions to a system of m differential equations in n dimensions when the coefficients are analytic functions. The theorem and its proof are valid for analytic functions of either real or complex variables.

Let K denote either the fields of real or complex numbers, and let V = Km and W = Kn. Let A1, ..., An−1 be analytic functions defined on some neighbourhood of (0, 0) in W × V and taking values in the m × m matrices, and let b be an analytic function with values in V defined on the same neighbourhood. Then there is a neighbourhood of 0 in W on which the quasilinear Cauchy problem {displaystyle partial _{x_{n}}f=A_{1}(x,f)partial _{x_{1}}f+cdots +A_{n-1}(x,f)partial _{x_{n-1}}f+b(x,f)} with initial condition {displaystyle f(x)=0} on the hypersurface {displaystyle x_{n}=0} has a unique analytic solution ƒ : W → V near 0.

Lewy's example shows that the theorem is not more generally valid for all smooth functions.

The theorem can also be stated in abstract (real or complex) vector spaces. Let V and W be finite-dimensional real or complex vector spaces, with n = dim W. Let A1, ..., An−1 be analytic functions with values in End (V) and b an analytic function with values in V, defined on some neighbourhood of (0, 0) in W × V. In this case, the same result holds.

Proof by analytic majorization Both sides of the partial differential equation can be expanded as formal power series and give recurrence relations for the coefficients of the formal power series for f that uniquely determine the coefficients. The Taylor series coefficients of the Ai's and b are majorized in matrix and vector norm by a simple scalar rational analytic function. The corresponding scalar Cauchy problem involving this function instead of the Ai's and b has an explicit local analytic solution. The absolute values of its coefficients majorize the norms of those of the original problem; so the formal power series solution must converge where the scalar solution converges.

Higher-order Cauchy–Kovalevskaya theorem If F and fj are analytic functions near 0, then the non-linear Cauchy problem {displaystyle partial _{t}^{k}h=Fleft(x,t,partial _{t}^{j},partial _{x}^{alpha }hright),{text{ where }}j

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