Chung, Fan, and S.-T. Yau. “Coverings, Heat Kernels and Spanning Trees”. The Electronic Journal of Combinatorics 6, no. 1 (December 12, 1998): R12. Accessed March 28, 2026. https://www.combinatorics.org/ojs/index.php/eljc/article/view/v6i1r12.