Robert Sheckley’s Answerer for two orthogonal projections

Albrecht Böttcher, Ilya M. Spitkovsky

Research output: Chapter in Book/Report/Conference proceedingChapter


The meta theorem of this paper is that Halmos’ two projections theorem is something like Robert Sheckley’s Answerer: no question about the W*- and C*-algebras generated by two orthogonal projections will go unanswered, provided the question is not foolish. An alternative approach to questions about two orthogonal projections makes use of the supersymmetry equality introduced by Avron, Seiler, and Simon. A noteworthy insight of the paper reveals that the supersymmetric approach is nothing but Halmos in different language and hence an equivalent Answerer.

Original languageEnglish (US)
Title of host publicationOperator Theory
Subtitle of host publicationAdvances and Applications
PublisherSpringer International Publishing
Number of pages14
StatePublished - 2018

Publication series

NameOperator Theory: Advances and Applications
ISSN (Print)0255-0156
ISSN (Electronic)2296-4878


  • C-algebra
  • Drazin inverse
  • Fredholm operator
  • Orthogonal projection
  • Trace-class operator
  • W-algebra

ASJC Scopus subject areas

  • Analysis


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