G. S. Romashchenko
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Methods Funct. Anal. Topology 12 (2006), no. 3, 286-295
Necessary and sufficient conditions for a convolution Volterra operator to be similar in a Sobolev space to the operator $J^\alpha$ are obtained. A criterion of similarity is obtained as well.
Methods Funct. Anal. Topology 10 (2004), no. 1, 63-74