[2] KUCZKOWSKI J. E.: 
On radicals in certain class of semigroups. Mat. časop. SAV 20, 1970, 278-280. 
MR 0304518[4] ŠULKA R.: 
On nilpotent elements, ideals and гadicals of a semigroup. Mat. fyz. časop. SAV 13, 1963, 209-222 (Russian). 
MR 0199297[5] ŠULKA R.: 
The maximal semilattice decomposition of a semigroup. Mat. časop. SAV 21, 1971, 269-276. 
MR 0304525 | 
Zbl 0228.20048[6] КMEŤ, R: 
On гadicals in semigгoups. Math. Slovaca 32, 1982, 183-188. 
MR 0658253