Open Sets of Conservative Matrices1

In this paper we present two principal results. In Theorem 1 we show that if r¡ denotes the open set of (conservative) matrices which map some subspace of c of infinite deficiency isomorphically onto c and if A denotes the closed set of matrices which sum some bounded divergent sequence, then [r']~=A. In Theorem 2 we produce a class of triangular matrices with the property that no triangular matrix in a neighborhood of one of these matrices has a range, as an operator on m, whose closure includes c. We acknowledge valuable discussions with Dr. B. Johnson of Exeter University, formerly of Yale University. We first introduce some notation, most of which is quite standard. We denote by m the Banach space of bounded sequences of complex numbers, where ||x|| = sup„ | x(n) \. We denote by c the subspace of m consisting of convergent sequences, by c0 the subspace of c consisting of sequences with limit 0, and by E°° the (nonclosed) subspace of Co consisting of sequences with only a finite number of nonzero terms. We denote by V the Banach algebra of conservative matrices (^4 is called conservative if xGc=>^4xGc), and by A the Banach algebra of conservative matrices with zeros above the principal diagonal. We denote by Ca the vector space of sequences (including unbounded sequences) which are summed by A, that is, transformed into convergent sequences by A. If CjiCAm^c we say ^4GA. When no confusion seems likely to arise we do not differentiate, for A ET, between A as a transformation from c to c and A as a transformation from m to m. In this regard we note that if A ET is considered as an operator from m to m, from c to c, or from c0 to c, the norm is the same; indeed ||^4|| = sup» ^,|oty|. Furthermore, we recall that, for A ET, if A~x exists for A as an operator on m, then A~x exists for A as an operator on c and, hence, since A~x is a matrix, A~XET as was shown by A. Wilansky and K. Zeller [3] and by M. R. Parameswaran [5] or as can be seen from our Lemma 1. We first present the following characterization of A.