Adolfo Piperno, and Enrico Tronci. “Regular Systems of Equations in λ-calculus.” In *Ictcs*. Mantova - Italy, 1989. DOI: 10.1142/S0129054190000230.

Many problems arising in equational theories like Lambda-calculus and Combinatory Logic can be expressed by combinatory equations or systems of equations. However, the solvability problem for an arbitrarily given class of systems is in general undecidable. In this paper we shall focus our attention on a decidable class of systems, which will be called regular systems, and we shall analyse some classical problems and well-known properties of Lambda-calculus that can be described and solved by means of regular systems. The significance of such class will be emphasized showing that for slight extensions of it the solvability problem turns out to be undecidable.