Abstract for the talk on 11.11.2021 (14:00 h)Seminar on Nonlinear Algebra
Max Pfeffer (TU Chemnitz + MPI MIS, Leipzig)
The cone of 5 by 5 completely positive matrices
In a joint work with Jose Alejandro Samper, we study the cone of completely positive (cp) matrices for the first interesting case n=5. This is a semialgebraic set, which means that the polynomial equalities and inequlities that define its boundary can be derived. We characterize the different loci of this boundary and we examine the two open sets with cp-rank 5 or 6. A numerical algorithm is presented that is fast and able to compute the cp-factorization even for matrices in the boundary. With our results, many new example cases can be produced and several insightful numerical experiments are performed that illustrate the difficulty of the cp-factorization problem.