prod.index
return the procuct of choosen elements of a matrix.
"prod"(X, Id)
xsimplex
.sum.simplex
to be applied to all the
elements given by xsimplex
.
Performs the product : X[1,Id[1]]*X[2,Id[2]]*...*X[p,Id[p]] if all the
elements of Id are positive. Otherwise, just return 0.