Foata, Dominique, and Guo-Niu Han. “The $(t,q)$-Analogs of Secant and Tangent Numbers”. The Electronic Journal of Combinatorics 18, no. 2 (May 13, 2011): P7. Accessed March 15, 2026. https://www.combinatorics.org/ojs/index.php/eljc/article/view/v18i2p7.