In mathematics, and more precisely, in functional analysis and PDEs, the Schauder estimates are a collection of results due to Juliusz Schauder (1934, 1937) concerning the regularity of solutions to linear, uniformly elliptic partial differential equations. The estimates say that when the equation has appropriately smooth terms and appropriately smooth solutions, then the Hölder norm of the solution can be controlled in terms of the Hölder norms for the coefficient and source terms. Since these estimates assume by hypothesis the existence of a solution, they are called a priori estimates. There is both an interior result, giving a Hölder condition for the solution in interior domains away from the boundary, and a boundary result, giving the Hölder condition for the solution in the entire domain. The former bound depends only on the spatial dimension, the equation, and the distance to the boundary; the latter depends on the smoothness of the boundary as well. The Schauder estimates are a necessary precondition to using the method of continuity to prove the existence and regularity of solutions to the Dirichlet problem for elliptic PDEs. This result says that when the coefficients of the equation and the nature of the boundary conditions are sufficiently smooth, there is a smooth classical solution to the PDE.
Notation The Schauder estimates are given in terms of weighted Hölder norms; the notation will follow that given in the text of D. Gilbarg and Neil Trudinger (1983). The supremum norm of a continuous function f ∈ C ( Ω ) {\displaystyle f\in C(\Omega )} is given by
| f | 0 ; Ω = sup x ∈ Ω | f ( x ) | {\displaystyle |f|_{0;\Omega }=\sup _{x\in \Omega }|f(x)|}
For a function which is Hölder continuous with exponent α {\displaystyle \alpha } , that is to say f ∈ C 0 , α ( Ω ) {\displaystyle f\in C^{0,\alpha }(\Omega )} , the usual Hölder seminorm is given by
[ f ] 0 , α ; Ω = sup x , y ∈ Ω | f ( x ) − f ( y ) | | x − y | α . {\displaystyle [f]_{0,\alpha ;\Omega }=\sup _{x,y\in \Omega }{\frac {|f(x)-f(y)|}{|x-y|^{\alpha }}}.}
The sum of the two is the full Hölder norm of f
| f | 0 , α ; Ω = | f | 0 ; Ω + [ f ] 0 , α ; Ω = sup x ∈ Ω | f ( x ) | + sup x , y ∈ Ω | f ( x ) − f ( y ) | | x − y | α . {\displaystyle |f|_{0,\alpha ;\Omega }=|f|_{0;\Omega }+[f]_{0,\alpha ;\Omega }=\sup _{x\in \Omega }|f(x)|+\sup _{x,y\in \Omega }{\frac {|f(x)-f(y)|}{|x-y|^{\alpha }}}.}
For differentiable functions u, it is necessary to consider the higher order norms, involving derivatives. The norm in the space of functions with k continuous derivatives, C k ( Ω ) {\displaystyle C^{k}(\Omega )} , is given by
| u | k ; Ω = ∑ | β | ≤ k sup x ∈ Ω | D β u ( x ) | {\displaystyle |u|_{k;\Omega }=\sum _{|\beta |\leq k}\sup _{x\in \Omega }|D^{\beta }u(x)|}
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