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Birkhoff's variety theorem from universal algebra characterises equational subcategories of varieties. We give an analogue of Birkhoff's theorem in the setting of enrichment in categories. For a suitable notion of an equational subcategory we characterise these subcategories by their closure properties in the ambient algebraic category.
@article{TAC_2016_31_a3, author = {Matej Dostal}, title = {A two-dimensional {Birkhoff's} theorem}, journal = {Theory and applications of categories}, pages = {73--100}, publisher = {mathdoc}, volume = {31}, year = {2016}, language = {en}, url = {http://geodesic.mathdoc.fr/item/TAC_2016_31_a3/} }
Matej Dostal. A two-dimensional Birkhoff's theorem. Theory and applications of categories, Tome 31 (2016), pp. 73-100. http://geodesic.mathdoc.fr/item/TAC_2016_31_a3/