The $R_{\infty}$ property for Houghton's groups
  We study twisted conjugacy classes of a family of groups which are called Houghton's groups $\mathcal{H}_n$ ($n \in\mathbb{N}$), the group of translations of $n$ rays of discrete points at infinity. We prove that the Houghton's groups $\mathcal{H}_n$ have the $R_{\infty}$ property for all $n\in \mathbb{N}$. 
Jang Hyun Jo, Jong Bum Lee, and Sang Rae Lee
math.GR