Du, Daniel K., and Qing-Hu Hou. “Partially Ordinal Sums and $P$-Partitions”. The Electronic Journal of Combinatorics 19, no. 4 (November 22, 2012): #P29. Accessed May 15, 2026. https://www.combinatorics.org/ojs/index.php/eljc/article/view/v19i4p29.