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A055270
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a(n)=7a(n-1)+(-1^n)*binomial(2,2-n); a(-1)=0.
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2
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1, 5, 36, 252, 1764, 12348, 86436, 605052, 4235364, 29647548, 207532836, 1452729852, 10169108964, 71183762748, 498286339236, 3488004374652, 24416030622564, 170912214357948, 1196385500505636, 8374698503539452
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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COMMENTS
| For n>=2, a(n) is equal to the number of functions f:{1,2,...,n}->{1,2,3,4,5,6,7} such that for fixed, different x_1, x_2 in {1,2,...,n} and fixed y_1, y_2 in {1,2,3,4,5,6,7} we have f(x_1)<>y_1 and f(x_2)<> y_2. - Milan R. Janjic (agnus(AT)blic.net), Apr 19 2007
a(n) is the number of generalized compositions of n when there are 6*i-1 different types of i, (i=1,2,...). [From Milan R. Janjic (agnus(AT)blic.net), Aug 26 2010]
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REFERENCES
| A. H. Beiler, Recreations in the Theory of Numbers, Dover, N.Y., 1964, pps. 122-125, 194-196.
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LINKS
| Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
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FORMULA
| a(n)=(6^2)*(7^(n-2)), n >= 2; a(0)=1, a(1)=5. G.f.(x)=(1-x)^2/(1-7x).
a(n) = Sum_{k, 0<=k<=n} A201780(n,k)*5^k. - DELEHAM Philippe, Dec 05 2011
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CROSSREFS
| Cf. A052268 and A011557. Second differences of A000420.
Sequence in context: A015547 A067376 A098305 * A164110 A201351 A188899
Adjacent sequences: A055267 A055268 A055269 * A055271 A055272 A055273
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KEYWORD
| easy,nonn
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AUTHOR
| Barry E. Williams, May 10 2000
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EXTENSIONS
| More terms from James A. Sellers (sellersj(AT)math.psu.edu), May 22 2000
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