Jirimutu, Xu, Xirong, Feng, Wei, and Bao, Yulan. “Proof of a Conjecture on the Gracefulness of a Digraph”. Utilitas Mathematica 81 (May 9, 2010). Accessed May 8, 2026. https://utilitasmathematica.com/index.php/Index/article/view/724.