The pseudoinverse of the Laplacian matrix: Asymptotic behavior of its trace

In this paper we are concerned with the asymptotic behavior of

$${\rm{tr}}({\cal L}_{{\rm{sq}}}^ + ) = {1 \over 4}\sum\limits_{\matrix{{j,k = 0} \cr {(j,k) \ne (0,0)} \cr } }^{n - 1} {{1 \over {1 - {1 \over 2}(\cos {{2\pi j} \over n} + \cos {{2\pi k} \over n})}},} $$

the trace of the pseudoinverse of the Laplacian matrix related with the square lattice, as n → ∞. The method we developed for such sums in former papers depends on the use of Taylor approximations for the summands. It was shown that the error term depends on whether the Taylor polynomial used is of degree two or higher. Here we carry this out for the square lattice with a fourth degree Taylor polynomial and thereby obtain a result with an improved error term which is perhaps the most precise one can hope for.