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A292930
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Triangle read by rows: T(n,k) (n>=1, 3<=k<=n+2) is the number of k-sequences of balls colored with at most n colors such that exactly three balls are the same color as some other ball in the sequence
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1
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1, 2, 8, 3, 24, 60, 4, 48, 240, 480, 5, 80, 600, 2400, 4200, 6, 120, 1200, 7200, 25200, 40320, 7, 168, 2100, 16800, 88200, 282240, 423360, 8, 224, 3360, 33600, 235200, 1128960, 3386880, 4838400, 9, 288, 5040, 60480, 529200, 3386880, 15240960, 43545600, 59875200, 10, 360, 7200, 100800, 1058400, 8467200, 50803200, 217728000, 598752000, 798336000
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OFFSET
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1,2
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COMMENTS
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Note that the three matching balls are necessarily the same color.
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LINKS
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FORMULA
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T(n, k) = binomial(k,3)*n!/(n+2-k)!.
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EXAMPLE
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n=1 => AAA -> T(1,3)=1;
n=2 => AAA,BBB -> T(2,3)=2;
AAAB,AABA,ABAA,BAAA,BBBA,BBAB,BABB,ABBB -> T(2,4)=8.
Triangle begins:
1;
2, 8;
3, 24, 60;
4, 48, 240, 480;
5, 80, 600, 2400, 4200;
...
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PROG
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(PARI) T(n, k) = binomial(k, 3)*n!/(n+2-k)!;
tabl(nn) = for (n=1, nn, for (k=3, n+2, print1(T(n, k), ", ")); print()); \\ Michel Marcus, Sep 29 2017
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CROSSREFS
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Other sequences in table: T(n,n+2) = A005990(n+1).
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KEYWORD
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AUTHOR
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STATUS
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approved
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