Bóna, Miklós. “A Combinatorial Proof of the Log-Concavity of a Famous Sequence Counting Permutations”. The Electronic Journal of Combinatorics 11, no. 2 (January 24, 2005): #N2. Accessed May 3, 2026. https://www.combinatorics.org/ojs/index.php/eljc/article/view/v11i2n2.