A273754 Number of binary heaps on [n] that give a heap when the first element is removed.

1, 1, 1, 2, 3, 8, 18, 70, 180, 770, 2610, 14570, 54720, 330960, 1562814, 11946080, 57126780, 429542960, 2465050968, 22517159760

Offset: 1

Author

Alois P. Heinz, May 29 2016

Keywords

• nonn
• more

Examples

			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.
a(5) = 3: 12345, 12354, 12435.
a(6) = 8: 123456, 123465, 123546, 123564, 123645, 123654, 124356, 124365.
		

Links

• Eric Weisstein's World of Mathematics, Heap
• Wikipedia, Binary heap

Crossrefs

Cf. A056971, A273755.