A note on mixing times of planar random walks

Abstract

We present an infinite family of finite planar graphs $\{X_n\}$ with degree at most five and such that for some constant $c > 0$, $$ \lambda_1(X_n) \geq c(\frac{\log \diam(X_n)}{\diam(X_n)})^2\,, $$ where $\lambda_1$ denotes the smallest non-zero eigenvalue of the graph Laplacian. This significantly simplifies a construction of Louder and Souto. We also remark that such a lower bound cannot hold when the diameter is replaced by the average squared distance: There exists a constant $c > 0$ such that for any family $\{X_n\}$ of planar graphs we have $$ \lambda_1(X_n) \leq c (\frac{1}{|X_n|^2} \sum_{x,y \in X_n} d(x,y)^2)^{-1}\,, $$ where $d$ denotes the path metric on $X_n$.