Two examples in varieties of monoids
John R. Isbell · Mathematical Proceedings of the Cambridge Philosophical Society · 1970
This note exhibits an infinite independent set of laws for monoids (viz. (xpyp)2 = (ypxp)2, p prime) and a variety of groups defined by one law (which is x2y2 = y2x2) such that the smallest containing variety of monoids is not finitely definable.