Artin approximation theorem
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.
Statement of the theorem 
- x = x1, …, xn
denote a collection of n indeterminates,
k[[x]] the ring of formal power series with indeterminates x over a field k, and
- y = y1, …, ym
a different set of indeterminates. Let
- f(x, y) = 0
be 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 such that
- ŷ(x) ≡ y(x) mod (x)c.
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.
- Artin, Michael (1969), "Algebraic approximation of structures over complete local rings", Publications Mathématiques de l'IHÉS (36): 23–58, MR 0268188
- Artin, Michael. Algebraic Spaces. Yale University Press, 1971.