I calculated a one-dimensional KdV equation. Through grid refinement, i.e. the interval [- 40,40] was divided into 80，160，320，400 parts. However, the L2 error between the four numerical solutions obtained after refinement and the exact solution remained unchanged.

Hello, thank you for your answer. I did not use varf, but directly used solve .In fact, with the refinement of the spatial grid, the error between numerical solution and exact solution is increasing.