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A273754
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Number of binary heaps on [n] that give a heap when the first element is removed.
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1
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1, 1, 1, 2, 3, 8, 18, 70, 180, 770, 2610, 14570, 54720, 330960, 1562814, 11946080, 57126780, 429542960, 2465050968, 22517159760
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OFFSET
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1,4
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LINKS
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Eric Weisstein's World of Mathematics, Heap
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EXAMPLE
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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.
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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STATUS
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approved
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