Ramanujan subshifts

Ievgen Bondarenko, Rostislav Grigorchuk, Alina Vdovina

A finite, connected, $(d+1)$-regular graph $G$ is called Ramanujan if every its eigenvalue $\hat{I}»$ satisfies either $\hat{I}»=\pm (d+1)$ or $|\hat{I}»|\le 2\sqrt{d}$. The Ramanujan condition cor…