# "A posteriori" estimates for finite difference methods

Suppose I have a PDE in a rectangular domain that I am solving numerically via a finite difference method. How do I answer the question, "How fine do I need to make the grid to so that my solution is within $$\epsilon$$ of the true solution?" We can assume some basic interpolation scheme is used to fill in the points outside of the grid.

For a concrete example, we might consider solving a 2-d heat equation with Crank-Nicolson. But I'd like methods that apply more broadly.

My understanding is that for finite element methods, there is a well-developed theory of "a posteriori" estimates that answer my question. Does a theory providing rigorous error bounds exist for finite difference methods?

• It may be useful for this to check out the Richardson extrapolation idea, en.wikipedia.org/wiki/Richardson_extrapolation May 24 at 23:03
• @MaximUmansky I may be mistaken, but my undestanding is that Richardson extrapolation will not offer full rigorous error bounds, because the error estimates it provides rely on unknown constants in the a priori bounds for the FD method. (I suppose one could in principle follow the proofs to find those constants, however, but I have never seen this done.) May 25 at 20:25

If the grid is fine enough, the numerical solution should depend on the grid spacing ℎ as $$𝑓(ℎ)=𝑓_{𝑒𝑥𝑎𝑐𝑡}+𝐴ℎ^𝑛$$ where there are three unknown parameters $$𝑓_{𝑒𝑥𝑎𝑐𝑡}$$, $$𝐴$$, $$𝑛$$ (the last one may be known apriori from the numerical algorithms used in the code). But even with three unknown parameters, with numerical solutions on three different grids, say with grid spacing $$h$$, $$2h$$, $$4h$$, you'll have three equations, that's enough to determine those three unknown parameters. After that, you'll know how small ℎ has to be to make the error $$\epsilon=𝐴ℎ^𝑛$$ smaller than the tolerance.
• I think this is essentially Richardson extrapolation? I agree for practical purposes this is excellent. However I think it is not enough for fully rigorous control, because really we have $f(h) = f_{\mathrm{exact}} + A h^n + B h^m + \dots$ where $m > n$, so the unknown $B$ is still there an uncontrolled. It can only be a serious problem if it huge, but I do not know of a proof that it is not huge... May 25 at 21:35
• Consider the following estimate, which I found in Sergey Repin's "A Posteriori Estimates for Partial Differential Equations," chapter 3. Consider Poisson's equation $\Delta u + f = 0$ on some domain $\Omega$ with boundary condition identically $0$. For a Galerkin approximation $u_h$ to the true solution $u$, we have $\| \nabla( u - u_h) \| \le C \| \Delta u_h + f \|$. Here $C$ is a constant from Friedirchs inequality for $\Omega$, which is easy to determine explicitly. Hence we obtain a rigorous error estimate with no uncontrolled constants. May 29 at 5:18