J.H. Drew et al. [Linear and Multilinear Algebra 37 (1994) 304] conjectured that for n⩾4, the completely positive (CP) rank of every n×n completely positive matrix is at most [n2/4]. In this paper we prove that the CP rank of a 5×5 completely positive matrix which has at least one zero entry is at most 6, thus providing new supporting evidence for the conjecture.
Linear Algebra and Its Applications 363, pp.161-176