This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A273754 #14 Feb 16 2025 08:33:35 %S A273754 1,1,1,2,3,8,18,70,180,770,2610,14570,54720,330960,1562814,11946080, %T A273754 57126780,429542960,2465050968,22517159760 %N A273754 Number of binary heaps on [n] that give a heap when the first element is removed. %H A273754 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Heap.html">Heap</a> %H A273754 Wikipedia, <a href="https://en.wikipedia.org/wiki/Binary_heap">Binary heap</a> %e A273754 There are 3 (min) heaps for n=4: 1234, 1243, 1324. When the first element is removed only 2 heaps remain: 234, 243. Thus a(4) = 2. %e A273754 a(5) = 3: 12345, 12354, 12435. %e A273754 a(6) = 8: 123456, 123465, 123546, 123564, 123645, 123654, 124356, 124365. %Y A273754 Cf. A056971, A273755. %K A273754 nonn,more %O A273754 1,4 %A A273754 _Alois P. Heinz_, May 29 2016