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A281912
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Number of sequences of balls colored with at most n colors such that exactly one ball is of a color seen earlier in the sequence.
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3
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1, 8, 57, 424, 3425, 30336, 294553, 3123632, 36003969, 448816600, 6022033721, 86587079448, 1328753602657, 21683227579664, 375013198304025, 6853321766162656, 131976208783240193, 2671430511854158632, 56709161712552286009, 1259836187316759240200
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OFFSET
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1,2
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COMMENTS
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Note that any such sequence has at least 2 balls, and at most n+1
Number of sequences of balls colored with at most n colors such that exactly two balls are the same color as some other ball in the sequence (necessarily each other). - Jeremy Dover, Sep 26 2017
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LINKS
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FORMULA
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a(n) = n! * Sum_{k=2..n+1} binomial(k,2)/(n+1-k)!.
a(n) = n if n < 2, a(n) = n*((n+2)/(n-1)*a(n-1) - a(n-2)) for n >= 2. - Alois P. Heinz, Feb 02 2017
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EXAMPLE
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n=1 => AA -> a(1) = 1.
n=2 => AA,BB,AAB,ABA,BAA,BBA,BAB,ABB -> a(2) = 8.
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MAPLE
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a:= proc(n) option remember;
`if`(n<2, 1, a(n-1)*(n+2)/(n-1)-a(n-2))*n
end:
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MATHEMATICA
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Table[n!*Sum[Binomial[k, 2]/(n + 1 - k)!, {k, 2, n + 1}], {n, 20}] (* Michael De Vlieger, Feb 02 2017 *)
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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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