THE TYPE SET OF A VARIETY IS NOT COMPUTABLE
Ralph McKenzie, Japheth Wood · International Journal of Algebra and Computation · 2001
Several problems involving the local structure of finite algebras are shown to be unsolvable by interpreting the halting problem of Turing machines. Specifically, these problems are to decide, given a finite algebra A, whether the type-set of the variety generated by A, which is a subset of {1,2,3,4,5}, omits 2, or 3, or 4, or 5, respectively.