Now，I want to calculate the error on the skeleton of mesh， namely ，the L^2 error of mass flux jump [v_h] \cdot n #tex
[tex] \sum_{F \in \mathcal{F}{h}} \frac{1}{h{F}}\left| \llbracket {v}{h} \rrbracket \cdot {n}{F}|{L^{2}(F)}^{2}[/tex]
#\mathcal{F}{h} stand for all facets of mesh

The simplest way to compute this quantity is by defining a bilinear form a so that a(u,u) coincides with your (sum of) integral(s). Then you Apply the bilinear form to your discrete solution u and take the inner product of the result with u again.

Thank you very much for your reply. Sorry，I may not fully understand what you mean. This is my code for Stokes . I don’t know how to calculate in my code. In addition，I also wonder， is there other way to calculate？

Alternatives? I think this is the most straight-forward way to do it. As of now there is no “Integrate” equivalent that would allow you to integrate the skeleton-terms directly. However, I think the above approach is also very natural.