L = LOGM(A) is the matrix logarithm of A.
Complex results are produced if A has negative eigenvalues.
A warning message is printed if the computed expm(L) is not close to A.
[L,esterr] = logm(A) does not print any warning message, but
returns an estimate of the relative residual, norm(expm(L)-A)/norm(A).
If A is real symmetric or complex Hermitian, then so is LOGM(A).
Some matrices, like A = [0 1; 0 0], do not have any logarithms,
real or complex, and LOGM cannot be expected to produce one.
See also EXPM1871LU2, SQRTM , FUNM .