When you have fine-scale features (e.g. boundary layers) in the solution, its FEM approximation on coarse meshes converge at strange apparent rates. Looking at Cea's lemma, is this behaviour because of:
- the best approximation error behaving strangely?
- or the constant changing with h?
- or should I actually look at Strang's first lemma? (errors in numerical integration of very fine-scale data)