Marczewski emphasized the importance of free algebras, leading to the publication of more than 50 papers on the algebraic theory of free algebras by Marczewski himself, together with Jan Mycielski, Władysław Narkiewicz, Witold Nitka, J. Płonka, S. Świerczkowski, K. Urbanik, and others. Starting with William Lawvere's thesis in 1963, techniques from category theory have become important in universal algebra. See also Graph algebra Term algebra Clone Universal algebraic geometry Simple universal algebra Footnotes References Bergman, George M., 1998. An Invitation to General Algebra and Universal Constructions (pub. Henry Helson, 15 the Crescent, Berkeley CA, 94708) 398 pp. . Birkhoff, Garrett, 1946. Universal algebra. Comptes Rendus du Premier Congrès Canadien de Mathématiques, University of Toronto Press, Toronto, pp. 310–326. Burris, Stanley N., and H.P. Sankappanavar, 1981. A Course in Universal Algebra Springer-Verlag. Free online edition. Cohn, Paul Moritz, 1981. Universal Algebra. Dordrecht, Netherlands: D.Reidel Publishing. (First published in 1965 by Harper & Row) Freese, Ralph, and Ralph McKenzie, 1987. Commutator Theory for Congruence Modular Varieties, 1st ed. London Mathematical Society Lecture Note Series, 125. Cambridge Univ. Press. . Free online second edition. Grätzer, George, 1968. Universal Algebra D. Van Nostrand Company, Inc. Higgins, P. J. Groups with multiple operators. Proc. London Math. Soc. (3) 6 (1956), 366–416. Higgins, P.J., Algebras with a scheme of operators. Mathematische Nachrichten (27) (1963) 115–132. Hobby, David, and Ralph McKenzie, 1988. The Structure of Finite Algebras American Mathematical Society. . Free online edition. Jipsen, Peter, and Henry Rose, 1992. Varieties of Lattices, Lecture Notes in Mathematics 1533. Springer Verlag. . Free online edition. Pigozzi, Don. General Theory of Algebras. Free online edition. Smith, J.D.H., 1976. Mal'cev Varieties, Springer-Verlag. Whitehead, Alfred North, 1898. A Treatise on Universal Algebra, Cambridge. (Mainly of historical interest.) External links Algebra Universalis—a journal dedicated to Universal Algebra.