[2] KUCZKOWSKI J. E.:
On гadicals in ceгtain class of semigroups. Mat. čas. 20, 1970, 278-280.
MR 0304518
[3] LAL H.:
Racicals in quasi-commutative semigroups. Mat. čas. 25, 1975, 287-288.
MR 0396808
[4] SCHWARZ Š.:
Prime ideals and maximal ideals in semigгoups. Czech. Math. J. 19, 1969, 72-79.
MR 0237680
[5] ŠULKA R.:
O нильпoтeнтныx элeмeнтax, идeaлax и paдикaлax пoлyгpyпп. Mat. fyz. čas. 13, 1963, 209-222.
MR 0199297