Sumario: | Let A be a ring with 1 and denote by L (resp. R) the set of left (resp. right) invertible elements of A. If A has an involution *, there is a natural bijection between L and R. In general, it seems that there is no such bijection; if A is a Banach algebra, L and R are open subsets of A, and they have the same cardinality. More generally, we prove that the spaces Uk(An) of n X k-left-invertible matrices and kU(An) of k X n-right-invertible matrices are homotopically equivalent. As a corollary, we answer negatively two questions of Rieffel. © 1986 American Mathematical Society.
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