What is the smallest possible constant in Céa's lemma?
We consider finite element approximations of a second order elliptic problem on a bounded polytopic domain in with . The constant appearing in Céa’s lemma and coming from its standard proof can be very large when the coefficients of an elliptic operator attain considerably different values. We restrict ourselves to regular families of uniform partitions and linear simplicial elements. Using a lower bound of the interpolation error and the supercloseness between the finite element solution and...