An algorithmic test for diagonalizability of finite-dimensional PT-invariant systems

A non-Hermitian operator does not necessarily have a complete set of eigenstates, contrary to a Hermitian one. An algorithm is presented which allows one to decide whether the eigenstates of a given PT-invariant operator on a finite-dimensional space are complete or not. In other words, the algorithm checks whether a given PT-symmetric matrix is diagonalizable. The procedure neither requires to calculate any single eigenvalue nor any numerical approximation.