Normal projections in Krein spaces

Abstract

Given a complex Krein space H with fundamental symmetry J, the aim of this note is to characterize the set of J-normal projections Q={Q∈L(H):Q2=QandQ#Q=QQ#}. The ranges of the projections in QQ are exactly those subspaces of HH which are pseudo-regular. For a fixed pseudo-regular subspace S , there are infinitely many J-normal projections onto it, unless SS is regular. Therefore, most of the material herein is devoted to parametrizing the set of J-normal projections onto a fixed pseudo-regular subspace S.