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The 2-category VAR of finitary varieties is not varietal over CAT. We introduce the concept of an algebraically exact category and prove that the 2-category ALG of all algebraically exact categories is an equational hull of VAR w.r.t. all operations with rank. Every algebraically exact category $\cal K$ is complete, exact, and has filtered colimits which (a) commute with finite limits and (b) distribute over products; besides (c) regular epimorphisms in $\cal K$ are product-stable. It is not known whether (a) - (c) characterize algebraic exactness. An equational hull of VAR w.r.t. all operations is also discussed.
@article{TAC_2001_8_a8, author = {J. Adamek and F. W. Lawvere and J. Rosicky}, title = {How algebraic is algebra?}, journal = {Theory and applications of categories}, pages = {253--283}, publisher = {mathdoc}, volume = {8}, year = {2001}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2001_8_a8/} }
J. Adamek; F. W. Lawvere; J. Rosicky. How algebraic is algebra?. Theory and applications of categories, Tome 8 (2001), pp. 253-283. http://geodesic.mathdoc.fr/item/TAC_2001_8_a8/