M. Karakhanyan and H. Kamalyan, “SOME REMARKS ON PROPERTIES OF ELEMENTS FROM COMPLEX BANACH ALGEBRAS”, Proc. YSU A: Phys. Math. Sci., vol. 47, no. 2 (231), pp. 15–21, Jun. 2013, doi: 10.46991/PYSUA.2013.47.2.015.