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A327091
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Number of chiral pairs of length n words with integer entries that cover an initial interval of positive integers.
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2
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0, 1, 5, 36, 264, 2335, 23609, 272880, 3543360, 51123511, 811313945, 14045781456, 263429150544, 5320671461575, 115141595216009, 2657827340717760, 65185383511024320, 1692767331624879031, 46400793659613081785, 1338843898122140977776, 40562412499251225624624
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OFFSET
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1,3
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COMMENTS
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If the word is achiral, i.e., the same as its reverse, we ignore it. If different from its reverse, we count it and its reverse as a chiral pair.
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LINKS
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FORMULA
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a(n) = Sum_{k=1..n} (k!/2) * (Stirling2(n, k) - Stirling2(ceiling(n/2), k)).
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EXAMPLE
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a(3) = 5 because there are the following chiral pairs of words:
112/211, 122/221,
123/321, 132/231, 213/312.
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PROG
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(PARI) a(n) = {sum(k=1, n, k! * (stirling(n, k, 2) - stirling((n+1)\2, k, 2)) / 2)}
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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