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A089975
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Array read by rows: T(n,k) is the number of n-letter words from a k-letter alphabet such that no letter appears more than twice.
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2
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1, 0, 1, 0, 1, 1, 0, 1, 2, 1, 0, 0, 4, 3, 1, 0, 0, 6, 9, 4, 1, 0, 0, 6, 24, 16, 5, 1, 0, 0, 0, 54, 60, 25, 6, 1, 0, 0, 0, 90, 204, 120, 36, 7, 1, 0, 0, 0, 90, 600, 540, 210, 49, 8, 1, 0, 0, 0, 0, 1440, 2220, 1170, 336, 64, 9, 1, 0, 0, 0, 0, 2520, 8100, 6120, 2226, 504, 81, 10, 1
(list; table; graph; refs; listen; history; internal format)
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OFFSET
| 0,9
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FORMULA
| T(n, k)=T(n, k-1)+n*T(n-1, k-1)+binomial(n, 2)*T(n-2, k-1) for n >= 2 and k >= 1.
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CROSSREFS
| T(1, k)=A001477(k) T(2, k)=A000290(k) T(3, k)=A007531(k) T(n, n)=A012244(n) T(n, n+1)=A036774(n) T(n, n+2)=A003692(n+1) T(2*n, n)=A000680(n) sum(T(n, k), n=0..2*k)=A003011(k) sum(T(r, n-r), r=0..n)=A089976(n).
Sequence in context: A113953 A110509 A204040 * A034366 A121465 A192396
Adjacent sequences: A089972 A089973 A089974 * A089976 A089977 A089978
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KEYWORD
| easy,nonn,tabl
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AUTHOR
| Paul Boddington (psb(AT)maths.warwick.ac.uk), Nov 17 2003
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