Solution
= Solution
If $\Delta f=0$, the $h^4$ term vanishes, so the local residual is $O(h^6)$ and the global error is $O(h^4)$.
= Solution
If $\Delta f=0$, the $h^4$ term vanishes, so the local residual is $O(h^6)$ and the global error is $O(h^4)$.