It must be one of a certain number of choices given in Section 3 . It must be one of a certain number of choices given in Section 4 . N is the order of the linear system.
P is an integer vector.
If no permutation is to be applied to the matrix, then P is unused and may be dimensioned to be of length one. If the matrix is to be permuted into another form, such as red-black, P is a coloring vector of length N indicating the integer coloring number of each equation. On output, P is then the permutation vector corresponding to the multi-color ordering. If no permutation is to be applied to the matrix, then IP is unused and may be dimensioned to be of length one. If the matrix is to be permuted, IP is an integer workspace vector of length N on input and the inverse permutation vector corresponding to P on output.