. nonzeroindices = set([])
. for i in range(n):
.. for j in range(n):
... if A(i,j) == 1:
.... nonzeroindices.union(set([i]))
.... nonzeroindices.union(set([j]))
. nonzeroindex = sorted(nonzeroindices)
. newmatrix = []
. for i in range(n):
.. newmatrix.append([])
.. for j in range(n):
... newmatrix[i].append(A(nonzeroindex[i],nonzeroindex[j]))

Then compute the eigenvalues of the around 2log(n)-by-2log(n) newmatrix.

. nonzeroindices = set([])
. for i in range(n):
.. for j in range(n):
... if A(i,j) == 1:
.... nonzeroindices.union(set([i]))
. nonzeroindex = sorted(nonzeroindices)
. newmatrix = []
. for i in range(n):
.. newmatrix.append([])
.. for j in range(n):
... newmatrix[i].append(A(nonzeroindex[i],nonzeroindex[j]))

Then compute the eigenvalues of the around 2log(n)-by-2log(n) newmatrix.