@article{TAC_2005_14_a17,
     author = {Jiri Adamek and Vera Trnkova},
     title = {Birkhoff's variety theorem with and without free algebras},
     journal = {Theory and applications of categories},
     pages = {424--450},
     publisher = {mathdoc},
     volume = {14},
     year = {2005},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TAC_2005_14_a17/}
}