Ramanujan subshifts

Ievgen Bondarenko, Rostislav Grigorchuk, Alina Vdovina

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