When considering variable coefficients, Eq. (8.38) becomes

and the commutator between and needs to be uniformly bounded for all resolutions in order to obtain an energy estimate.Estimates for the term involving the commutator have been given in [317, 319, 318, 407]. In order to discuss them, we first notice that the SBP property of with respect to and the symmetry of imply that the operator is symmetric with respect to ,

for all grid functions and . Therefore, its norm is equal to its spectral radius and we have the estimate for all grid functions . Hence, the problem is reduced to finding an upper bound for the spectral radius of , which is independent of the scalar product .Next, in order to find such an upper bound, we write the FD operator as

where the ’s are the coefficient of a banded matrix; that is, there exists , which is independent of such that for . Then, we have from which it follows, under the assumption that is continuously differentiable and that its derivative is bounded, , , where . Now we can easily estimate the spectral radius of , based on the simple observation that for any norm on the space of grid functions . Choosing the 1-norm we find from which it follows that The important point to notice here is that for each fixed , the sum in the expression for involves at most non-vanishing terms, since for . For the SBP operators and scalar products used in practice, namely those for which the latter has the structure given by Eq. (8.29), can be bounded by a constant, which is independent of resolution. Since the spectral radius of is equal to that of its transposed, we may interchange the roles of and in the definition of the constant , and with these observations we arrive at the following result:Lemma 7 (Discrete commutator estimate). Consider a FD operator of the form (8.42), which satisfies the SBP property with respect to a scalar product , and let be symmetric. Then, the following commutator estimate holds:

for all grid functions and resolutions , where withRemarks

- A key ingredient used above to uniformly bound the norm of is that the SBP scalar products used in practice have the form (8.29). In those cases, both the boundary width and the boundary stencil size (defined below Example 52) associated with the corresponding difference operators are independent of . Therefore, the constants and can also be bounded independently of .
- For the operator defined in Example 51, for instance, Eq. (8.48) gives , , and we obtain the optimal estimate corresponding to the one in the continuum limit,
- For the operator defined in Example 52, in turn, Eq. (8.48) gives and .
- For spectral methods the constants and typically grow with as the coefficients do not form a banded matrix anymore. This leads to difficulties when estimating the commutator; see [407] for a discussion on this point.
- It is also possible to avoid the estimate on the commutator between and altogether through skew-symmetric differencing [260], in which the problem is discretized according to A straightforward energy estimate shows that this leads to strict stability, after the imposition of appropriate boundary conditions.
- SBP by itself is not enough to obtain an energy estimate since the boundary conditions still need to be imposed, and in a way such that the boundary terms in the estimate after SBP are under control. This is the topic of Section 10.

Living Rev. Relativity 15, (2012), 9
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