[1] Akız, H.F., Alemdar, N., Mucuk, O., and Şahan, T., Coverings of internal groupoids and crossed modules in the category of groups with operations, Georgian Math. J. 20(2) (2013), 223-238.
[2] Brown, R., Higgins, P.J., and Sivera, R., “Nonabelian Algebraic Topology: Filtered Spaces, Crossed Complexes, Cubical Homotopy Groupoids”, EMS series of lectures in mathematics, European Mathematical Society, 2011.
[3] Brown, R. and Huebschmann, J., “Identities among relations”, in: Low-Dimensional Topology, Cambridge University Press, 1982.
[4] Brown, R. and Mucuk, O., Covering groups of non-connected topological groups revisited, Math. Proc. Camb. Phil. Soc. 115(1), (1994), 97-110.
[5] Brown, R. and Spencer, C.B., G-groupoids, crossed modules and the fundamental groupoid of a topological group, Indagat. Math. 79(4) (1976), 296-302.
[6] Datuashvili, T., Central series for groups with action and leibniz algebras, Georgian Math. J. 9(4) (2002), 671-682.
[7] Datuashvili, T., Witt’s theorem for groups with action and free leibniz algebras, Georgian Math. J. 11(4) (2004), 691-712.
[8] Huebschmann, J., Crossed n-folds extensions of groups and cohomology, Comment. Math. Helv. 55 (1980), 302-313.
[9] Karakaş , S.D. and Mucuk, O., Liftings and covering morphisms of crossed modules in group-groupoids, Turk. J. Math. 45(3) (2021), 1407-1417.
[10] Loday, J.L., Cohomologie et groupe de Steinberg relatifs, J. Algebra 54(1) (1978), 178-202.
[11] Loday, J.L., Spaces with finitely many non-trivial homotopy groups, J. Pure. Appl. Algebra 24(2) (1982), 179-202.
[12] Lue, A.S.T., Semi-complete crossed modules and holomorphs of groups, B. Lond. Math. Soc. 11(1) (1979), 8-16.
[13] Mucuk, O. and Akız, H.F., Covering morphisms of topological internal groupoids, Hacet. J. Math. Stat. 49 (2020), 1020-1029.
[14] Mucuk, O. and Şahan, T., Coverings and crossed modules of topological groups with operations, Turk. J. Math. 38(5) (2014), 833-845.
[15] Mucuk, O. and Şahan, T., Group-groupoid actions and liftings of crossed modules, Georgian Math. J. 26(3), (2019), 437-447.
[16] Mucuk, O., Şahan, T. and Alemdar, N., Normality and quotients in crossed modules and group-groupoids, Appl. Categor. Struct. 23(3) (2015), 415-428.
[17] Orzech, G., Obstruction theory in algebraic categories, I, J. Pure. Appl. Algebra 2(4) (1972), 287-314.
[18] Orzech, G., Obstruction theory in algebraic categories, II, J. Pure. Appl. Algebra 2(4) (1972), 315-340.
[19] Porter, T., Extensions, crossed modules and internal categories in categories of groups with operations, P. Edinburgh Math. Soc. 30(3) (1987), 373-381.
[20] Şahan, T., Further remarks on liftings of crossed modules, Hacet. J. Math. Stat. 48(3) (2019), 743-752.
[21] Temel, S., Crossed semimodules and cat1-monoids, Korean J. Math. 27(2) (2019), 535-545.
[22] Temel, S., Crossed squares, crossed modules over groupoids and cat1-2-groupoids, Categ. Gen. Algebr. Struct. Appl. 13(1) (2020), 125-142.
[23] Temel, S., Şahan, T. and Mucuk, O., Crossed modules, double group-groupoids and crossed squares, Filomat 34(6) (2020), 1755-1769.
[24] Temel, S., Some notes on crossed semimodules, Turk. J. Math. 46 (2022), 768-784.
[25] Whitehead, J.H.C., Note on a previous paper entitled “On adding relations to homotopy groups”, Ann. Math. 47(4) (1946), 806-810.
[26] Whitehead, J.H.C., Combinatorial homotopy. II, Bull. Amer. Math. Soc. 55(5) (1949), 453-496.
[27] Yavari, M. and Salemkar, A., The category of generalized crossed modules, Categ. Gen. Algebr. Struct. Appl. 10(1) (2019), 157-171.