Primitivity Testing in Free Group Algebras via Duality

Let F be a free group and K a field. The free group algebra K[F] bears a strong resemblance to F, making it an excellent tool in the study of free groups. For example, by a theorem due to Cohn and Lewin, one-sided ideals in K[F] are free as K[F]-modules, analogously to the Nielsen-Schreier theorem. I will discuss this resemblance, along with other motivations for our interest in K[F] arising from the theory of word measures. I will then present a new algorithm for deciding if a given element is part of some basis of a given ideal, similarly to what Whitehead’s algorithm performs in free groups. Based on joint work with Danielle Ernst-West and Doron Puder.