Author: Zonalʹnoe soveshchanie-seminar zavedui͡ushchikh kafedrami i vedushchikh prepodavateleĭ matematiki vuzov Belorusskoĭ, Latviĭskoĭ, Litovskoĭ, Ėstonskoĭ SSR i Kaliningradskoĭ oblasti RSFSR (6th 1987 Tallinn, Estonia)