ИСТИНА

This theory originates from the known treatise by F.Tricomi (1923), in which normal form for such an equation was proposed near a typical point of type change line, at which the characteristic direction is not tangent to the line. The correct proof for this form was given a decade later by M.Cibrario. In general, the creation of the theory of normal forms for generic linear second order mixed type partial equations on the plane was completed in the late twentieth century. These results and recent achievements of this theory are discussed in the talk.