Optimal normal projections in Krein spaces

Given a pseudo-regular subspace SS of a Krein space HH with fundamental symmetry J  , if its isotropic part S∘S∘ is not trivial the set QSQS of J  -normal projections onto SS has infinitely many elements. The aim of this work is to distinguish a projection Q0∈QSQ0∈QS according to a suitable criterion. When HH is finite-dimensional, Q0Q0 turns out to be the unique minimal norm projection in QSQS with respect to the Schatten p  -norm for 1≤p<∞1≤p<∞.