scienceFeb 22, 202510:18pending
OEIS A000139: Two-stack sortable permutations and friends
About this episode
In this OEIS Deep Dive, we explore A000139, a remarkably interconnected sequence that counts two-stack-sortable permutations, rooted non-separable planar maps, ways to dissect a square into smaller squares with n+1 vertices, and left ternary trees. We discuss why these diverse interpretations arise, outline generating-function forms and the asymptotic growth, and highlight links to Stirling numbers and other OEIS sequences. We also touch on potential applications and open questions that invite further exploration.
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