2018-11-16 · MAXIMAL i?-SETS, GRASSMANN SPACES, AND STIEFEL SPACES OF A HILBERT SPACE BY ERHARD LUFT 1. Introduction. The ordinary Fredholm theory of Hubert spaces was general-ized in [4] and [5] to apply to a wider range of problems in analysis. The ideal of compact linear maps, which plays a fundamental role in the ordinary theory, was