There are many examples of nonfinitely based varieties. In a finite signature, is there an example of such variety with a known explicit set of identities?

You can look at "Bases for Equational Theories of Semigroups" by P Perkins, J Algebra 11, 298314 (1968). Theorem 2: the identities $xyzw=xzyw$ and $yx^ky=xyx^{k2}yx$ for $k=2,3,\dots$ define a nonfinitely based variety of semigroups. 


Pick a similarity type $\sigma$ that is "nice": finite with no zeroary operations will do. Consider the set B of all terms in the variables a and b, and for every such term $t(a,b)$ consider the identity that says this term is associative, or $t(a,t(b,c)) \approx t(t(a,b),c)$. If $t$ is $u(a)$, this is realized as $u(a) \approx u(u(a))$. Let $A_\sigma$ be the set of identities so produced. In work which I distributed, I showed $A_\sigma$ is finitely based when $\sigma$ consists of a single binary operation. It remains finitely based if the type consists of only unary operations. What is also true (and should be published) is that for all other nice types, in particular the type with two binary operations, $A_\sigma$ is not finitely based. More interesting facts about hyperidentities (such as hyperassociativity above) being represented by identities hold, but I have not kept up with the literature. I am confident one can manufacture many such examples of non finitely based varieties this way. Gerhard "Should Get Back To Writing" Paseman, 2014.01.16 

