1D clustering vs.
2D "clustering"
V = {Vge} = G x E expression matrix (possibly a
subset of all genes in the experiment)
1D clustering:  fit V ~  C X
(C: each row contains one entry of 1 and the rest 0's;
the rows of X are the cluster prototypes; relax to
one nonzero entry in each row of C)
PCA: the rows of X are orthogonal
SCA: each row of C may contain up to p nonzero
entries, where p is specified by the user