Type: Article
Publication Date: 2006-01-01
Citations: 862
DOI: https://doi.org/10.1590/s0101-82052006000200002
This paper is a reprint of the original work by A. P. Calderón published by the Brazilian Mathematical Society (SBM) in ATAS of SBM (Rio de Janeiro), pp. 65-73, 1980. The original paper had no abstract, so this reprint to be truthful to the original work is published with no abstract.