# Specific entry of inverse of large matrix

Given a large (say 10x10) sparse Hermitian matrix with symbolic entries, I need four specific entries of the inverse matrix. It seems wasteful to me to compute 100 entries of which I only need a few in the end.

Is there a way to compute a small number of specific entries of the inverse matrix that is significantly more efficient than finding the inverse matrix and then extracting the elements?

Here's an example. N is half the dimension of the matrix. For N = 8, inverting takes 51 seconds on my university's computer, and we would ideally like to reach much larger N.

A = Table[If[Abs[i - j] == 1, (j - i) a, 0], {i, #}, {j, #}] &;
B = Table[If[Abs[i - j] == 1, b, 0], {i, #}, {j, #}] &;

\[Sigma]1[N_] := B@N + Table[
If[Abs[i - j] == 0, 2 \[Mu], 0] + If[i == j == 1, -x, 0] +
If[i == j == N, -x, 0]
, {i, N}, {j, N}];
\[Sigma]2[N_] := -B@N + Table[
+If[i == j == 1, -y, 0] + If[i == j == N, -y, 0]
, {i, N}, {j, N}];

(* Elements of M *)
el[N_, i_, j_] := Which[i <= N && j <= N,
\[Sigma]1[N][[i, j]], i > N && j > N,
\[Sigma]2[N][[i - N, j - N]], i > N && j <= N,
A[N][[i - N, j]], i <= N && j > N,
-A[N][[i, j - N]]]

M[N_] := Table[el[N, i, j], {i, 2 N}, {j, 2 N}];


Daniel Lichtblau's idea LinearSolve[M@8,UnitVector[16,8]] takes 4.1 seconds, so it is certainly much faster than inverting the entire matrix, even if it has to be done seperately for each entry.

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If you have ACM access, search for Lin Lin et al SelInv paper. –  rasher Mar 24 at 21:22
As it is stated, the Q is off-topic. –  Sektor Mar 24 at 21:43
This question appears to be off-topic because it is not about Mathematica. –  bobthechemist Mar 24 at 22:21
@bobthechemist I have to disagree here. It is clear that you can do an Inverse matrix calculation in Mathematica, and checking to see whether a fast version of Inverse[m][[{3,4},{2,1}]] exists is not unreasonable. It can be very advantageous. Many functions in Mathematica can be 'cut-off' to stop early and give a restricted set of solutions. Take, for instance, Ordering or Position. –  Sjoerd C. de Vries Mar 24 at 22:28
LinearSolve[mat,ej] will give the jth column only of the inverse, when ej is the jth coordinate axis unit vector. I would expect that to be 2-3 times faster than using Inverse. –  Daniel Lichtblau Mar 24 at 22:37

Cofactor[mat_,i_,j_]/;(And@@Thread[{i,j}<=Dimensions[mat]]):=(-1)^(i+j)*Det[Drop[#,{j}]&/@Drop[mat,{i}]];