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

1,4

Links

Table of n, a(n) for n=1..20.

Eric Weisstein's World of Mathematics, Heap

Wikipedia, Binary heap

Example

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.

Crossrefs

Cf. A056971, A273755.

Sequence in context: A113183 A157015 A240645 * A242099 A041205 A002356

Adjacent sequences: A273751 A273752 A273753 * A273755 A273756 A273757

Keyword

nonn,more

Author

Alois P. Heinz, May 29 2016

Status

approved