A completely positive matrix is a real n×n square matrix A=(a_(ij)) that can be factorized as
A=BB^(T),
where B^(T) stands for the transpose of B and B is any (not necessarily square) n×m matrix with nonnegative elements. The least possible number of columns (m) of B is called the factorization index (or the cp-rank) of A. The study of complete positivity originated in inequality theory and quadratic forms (Diananda 1962, Hall and Newman 1963).
There are two basic problems concerning complete positivity.
1. When is a given n×n real matrix A completely positive?
2. How can the cp-rank of A be calculated?
These two fundamental problems still remains open. The applications of completely positive matrices can be found in block designs (Hall and Newman 1963) and economic modelling (Gray and Wilson 1980).