About fifty years aga S. G. Mikhlin, in solving the regularization problem for two-dimensional singular integral operators 56], assigned to each such operator a func- tion which he called a symbol, and showed that regularization is possible if the infimum of the modulus of the symbol is positive. Later, the notion of a symbol was extended to multidimensional singular integral operators (of arbitrary dimension) 57, 58, 21, 22]. Subsequently, the synthesis of singular integral, and differential operators 2, 8, 9]led to the theory of pseudodifferential operators 17, 35] (see also ...
About fifty years aga S. G. Mikhlin, in solving the regularization problem for two-dimensional singular integral operators 56], assigned to each such...
