Céa's lemma

Céa's lemma is a result in the field of functional analysis and calculus of variations, particularly in the context of optimal control problems. The lemma is often used to derive estimates for the behavior of solutions to variational problems. In a general sense, Céa's lemma states that under certain conditions, the error in the approximation of a functional can be controlled in terms of the norm of a corresponding linear functional applied to the error of the function.