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Search: id:A055270
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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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+0
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; listen)
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OFFSET
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0,2
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COMMENT
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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
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REFERENCES
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A. H. Beiler, Recreations in the Theory of Numbers, Dover, N.Y., 1964, pps. 122-125, 194-196.
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LINKS
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Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets
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FORMULA
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a(n)=(6^2)*(7^(n-2)), n >= 2; a(0)=1, a(1)=5. G.f.(x)=(1-x)^2/(1-7x).
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CROSSREFS
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Cf. A052268 and A011557. Second differences of A000420.
Sequence in context: A015547 A067376 A098305 this_sequence A164110 A052203 A027331
Adjacent sequences: A055267 A055268 A055269 this_sequence A055271 A055272 A055273
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KEYWORD
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easy,nonn
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AUTHOR
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Barry E. Williams, May 10 2000
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EXTENSIONS
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More terms from James A. Sellers (sellersj(AT)math.psu.edu), May 22 2000
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