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A338418
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a(n) is the number of length-n ballot sequences whose reversals are also ballot sequences.
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2
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1, 1, 1, 2, 3, 5, 11, 23, 50, 118, 283, 731, 1921, 5370, 14884, 43118, 130509, 397490, 1266010, 4077494, 13435445, 45687806, 157483627, 551205779, 1979511990
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OFFSET
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0,4
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COMMENTS
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Ballot sequences B have positive terms, and for any finite prefix P of B and any k > 0, the number of occurrences of k in P is greater than or equal to the number of occurrences of k+1 in P.
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LINKS
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FORMULA
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EXAMPLE
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For n = 5:
- the reversals of the following length-5 ballot sequences are also ballot sequences:
(1, 1, 1, 1, 1)
(1, 1, 1, 2, 1)
(1, 1, 2, 1, 1)
(1, 2, 1, 1, 1)
(1, 2, 1, 2, 1)
- so a(5) = 5.
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PROG
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(C++) See Links section.
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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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