Let H+n(R) be the cone of all positive semidefinite n x n real matrices.
Two of the best known partial orders that were mostly studied on subsets of
square complex matrices are the L?wner and the minus partial orders.
Motivated by applications in statistics we study these partial orders on H+n(R). We describe the form of all surjective maps on H+ n (R), n > 1, that
preserve the L?wner partial order in both directions. We present an
equivalent definition of the minus partial order on H+n(R) and also
characterize all surjective, additive maps on H+ n (R), n ? 3, that preserve
the minus partial order in both directions.