Perturbations of semi-Fredholm operators in locally convex topological vector spaces

Abstract

<p>It is a well-known result of I.C. Gohberg, M.G. Krein and T. Kato that if T is a semi-Fredholm operator between Banach spaces and P a bounded operator of norm small enough, or a compact operator, then T+P is a semi-Fredholm operator with the same index as T.</p> <p>This thesis is concerned with extensions of this result to more general locally made of suitably defined small bounded or precompact perturbations or Φ₊ and Φ₋ -operators. The results obtained apply in particular to Frechet spaces and effectively extend the theorems of I.C. Gohberg, M.G. Krein and T. Kata as well as several or Ju.M. Vladimirski.</p> <p>Duality is shown to be a convenient tool to prove many or these results. Some applications are also given.</p>