Random sparse matrices.
R = SPRANDN(S) has the same sparsity structure as S, but normally
distributed random entries.
R = SPRANDN(m,n,density) is a random, m-by-n, sparse matrix with
approximately density*m*n normally distributed nonzero entries.
R = SPRANDN(m,n,density,rc) also has reciprocal condition number
approximately equal to rc. R is constructed from a sum of
matrices of rank one.
If rc is a vector of length lr <= min(m,n), then R has
rc as its first lr singular values, all others are zero.
In this case, R is generated by random plane rotations
applied to a diagonal matrix with the given singular values
It has a great deal of topological and algebraic structure.
See also SPRANDSYM .