had taught, for the past fifty years, a course called “Invariant Subspaces” which was noted for its monotony and virtually absolute unintelligibility, as well as for the fact that the final exam, as long as anyone could remember, had consisted of the same single yes-or-no question. The question was three pages long but the answer was always “Yes.” That was all you needed to know to pass Invariant Subspaces.
