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2. A. Ballester-Bolinches and L. M. Ezquerro, Classes of Finite Groups (Springer, New York, Dordrecht, 2006).
3. K. Doerk and T. Hawkes, Finite Soluble Groups, (De Gruyter, Berlin, 1992).
4. A. I. Mal’tsev, Algebraic Systems (Nauka, Moscow, 1970; Springer, Berlin, 2011).
5. R. Baer, “Der Kern, eine charakteristische Untergruppe,” Compos. Math. 1, 254–283 (1935).