Dumanyan, V.Zh. “ON BOUNDEDNESS OF A CLASS OF FIRST ORDER LINEAR DIFFERENTIAL OPERATORS IN THE SPACE OF $(n-1)$-DIMENSIONALLY CONTINUOUS FUNCTIONS”. Proceedings of the YSU A: Physical and Mathematical Sciences, vol. 47, no. 2 (231), June 2013, pp. 8-14, doi:10.46991/PYSU:A/2013.47.2.008.