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A055608
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Arrays of dumbbells.
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8
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1, 13, 92, 473, 1982, 7191, 23431, 70234, 196941, 522939, 1327002, 3240917, 7660538, 17602967, 39466363, 86593478, 186399956, 394478234, 822229746, 1690521204, 3433033150, 6893852746, 13702694284, 26982983126, 52680389239
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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REFERENCES
| I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, Wiley, N.Y., 1983,(2.3.14).
R. C. Grimson, Exact formulas for 2 x n arrays of dumbbells, J. Math. Phys., 15 (1974), 214-216.
R. B. McQuistan and S. J. Lichtman, Exact recursion relation for 2 x N arrays of dumbbells, J. Math. Phys., 11 (1970), 3095-3099.
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FORMULA
| G.f.: (1+x)^4/(1-x-x^2)^5/(1-x)^4.
a(n)=2*a(n-1)-a(n-3)+A002889(n)+A002889(n-1)
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CROSSREFS
| Cf. A002940, A002941, A002889, A046741.
Cf. A062123-A062127.
Sequence in context: A090020 A092469 A027670 * A038742 A005414 A044264
Adjacent sequences: A055605 A055606 A055607 * A055609 A055610 A055611
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KEYWORD
| easy,nonn
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AUTHOR
| Henry Bottomley (se16(AT)btinternet.com), Jun 02 2000
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