In contrast to the Banach space case, in which the invertibility of the derivative at a point is sufficient for a map to be locally invertible, the Nash–Moser theorem requires the derivative to be invertible in a neighborhood. The theorem is widely used to prove local existence for non-linear partial differential equations in spaces of smooth functions. It is particularly useful when the inverse to the derivative "loses" derivatives, and therefore the Banach space implicit function theorem cannot be used.
While Nash (1956) originated the theorem as a step in his proof of the Nash embedding theorem, Moser (1966a, 1966b) showed that Nash's methods could be successfully applied to solve problems on periodic orbits in celestial mechanics.
The formal statement of the theorem is as follows:
- Let and be tame Frechet spaces and let be a smooth tame map. Suppose that the equation for the derivative has a unique solution for all and all , and that the family of inverses is a smooth tame map. Then P is locally invertible, and each local inverse is a smooth tame map.
- Moser, Jürgen (1966a), "A rapidly convergent iteration method and non-linear partial differential equations. I", Ann. Scuola Norm. Sup. Pisa (3), 20: 265–315, MR 0199523
- Moser, Jürgen (1966b), "A rapidly convergent iteration method and non-linear partial differential equations. II", Ann. Scuola Norm. Sup. Pisa (3), 20: 499–535, MR 0206461
- Nash, John (1956), "The imbedding problem for Riemannian manifolds", Annals of Mathematics, 63 (1): 20–63, JSTOR 1969989, MR 0075639, doi:10.2307/1969989.