  A real square matrix $A$ is called a P-matrix if all its principal minors are
positive. Using the sign non-reversal property of matrices, the notion of
P-matrix has been recently extended by Kannan and Sivakumar to
infinite-dimensional Banach spaces relative to a given Schauder basis.
Motivated by their work, we discuss P-operators on separable real Hilbert
spaces. We also investigate P-operators relative to various orthonormal bases.
