In mathematics, the Artin approximation theorem is a fundamental result of Michael Artin (1969) in deformation theory which implies that formal power series with coefficients in a field k are well-approximated by the algebraic functions on k.
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More precisely, Artin proved two such theorems: one, in 1968, on approximation of complex analytic solutions by formal solutions (in the case k = C); and an algebraic version of this theorem in 1969.
Statement of the theorem
Let
x = x1, …, xndenote a collection of n indeterminates,
k[[x]] the ring of formal power series with indeterminates x over a field k, and
y = y1, …, yma different set of indeterminates. Let
f(x, y) = 0be a system of polynomial equations in k[x, y], and c a positive integer. Then given a formal power series solution ŷ(x) ∈ k[[x]] there is an algebraic solution y(x) consisting of algebraic functions (more precisely, algebraic power series) such that
ŷ(x) ≡ y(x) mod (x)c.Discussion
Given any desired positive integer c, this theorem shows that one can find an algebraic solution approximating a formal power series solution up to the degree specified by c. This leads to theorems that deduce the existence of certain formal moduli spaces of deformations as schemes.