On the norms of singular integral operators along certain curves with intersections

It is well known K] that the norm and essential norm of singular integral operator for contours composed by two rays depend on the angle between the rays. The goal of the paper is both to show that for the contour that is an intersection oftwo straight lines the norm and essential norm of singular integral operatordo not depend on the angle between the lines, andto calculate these norms. Some generalizations for the case when is an intersection of several simple contours are also considered.