Kaplansky's zero divisor and unit conjectures on elements with supports of size $3$

Abstract

Kaplansky's zero divisor conjecture (unit conjecture, respectively) states that for a torsion-free group $G$ and a field $\mathbb{F}$, the group ring $\mathbb{F}[G]$ has no zero divisors (has no unit with support of size greater than $1$). In this paper, we study possible zero divisors and units in $\mathbb{F}[G]$ whose supports have size $3$. For any field $\mathbb{F}$ and all torsion-free groups $G$, we prove that if $\alpha \beta =0$ for some non-zero $\alpha ,\beta \in \mathbb{F}[G]$ such that $|supp(\alpha )|=3$, then $|supp(\beta )|\ge 10$. If $\mathbb{F}={\mathbb{F}}_2$ is the field with 2 elements, the latter result can be improved so that $|supp(\beta )|\ge 20$. This improves a result in [J. Group Theory, 16 (2013), no. 5, 667-693]. Concerning the unit conjecture, we prove that if $\alpha \beta =1$ for some $\alpha ,\beta \in \mathbb{F}[G]$ such that $|supp(\alpha )|=3$, then $|supp(\beta )|\ge 9$. The latter improves a part of a result in [Exp. Math., 24 (2015), 326-338] to arbitrary fields.