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A059948
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Number of 7-block bicoverings of an n-set.
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3
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0, 0, 0, 0, 40, 3306, 131876, 3961356, 103290096, 2488179582, 57162274972, 1274774473632, 27887396866472, 602352276704178, 12899161619186388, 274612697648135028, 5822592730060070368, 123107330974129584294
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,5
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REFERENCES
| I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.
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LINKS
| Vincenzo Librandi, Table of n, a(n) for n = 1..200
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FORMULA
| a(n)=(1/7!) * (21^n -7*15^n -21*11^n +42*10^n +105*7^n -140*6^n +105*5^n -420*4^n +35*3^n +1050*2^n -1050).
The number of m-block bicoverings of an n-set is [x^m*y^n] 1/n!*exp(-x-1/2*x^2*(exp(y)-1)) * sum(i>=0, x^i/i! * exp(binomial(i, 2)*y) ) where [x^m*y^n] extracts the coefficient of x^m*y^n, see Goulden/Jackson p.203.
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CROSSREFS
| Cf. A002718, A059443, A003462, A059945-A059947, A059949-A059951.
Sequence in context: A049215 A188154 A178721 * A045502 A123810 A146198
Adjacent sequences: A059945 A059946 A059947 * A059949 A059950 A059951
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KEYWORD
| easy,nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Feb 14 2001
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