Rearrangement Invariant Orthogonal Sums in Kreĭn Spaces. I
摘要
The regular subspaces of a Kreĭn space have well-known properties analogous to the closed subspaces of a Hilbert space. A boundedness condition on sums of projection operators extends this analogy to a natural notion of infinite orthogonal direct sums. The boundedness condition is shown to have several equivalent forms and consequences for Cartesian products. Moore-Smith convergence and a theorem of T. H. Hildebrandt play key roles in the formulation of results and proofs.