Caughman, J. S., and J. J. P. Veerman. “Kernels of Directed Graph Laplacians”. The Electronic Journal of Combinatorics 13, no. 1 (April 11, 2006): #R39. Accessed May 21, 2026. https://www.combinatorics.org/ojs/index.php/eljc/article/view/v13i1r39.