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Search: id:A060485
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| A060485 |
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Number of 7-block tricoverings of an n-set. |
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+0
4
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| 43, 4520, 244035, 10418070, 401861943, 14778678180, 530817413155, 18837147108890, 664260814445943, 23345018969140440, 818942064306004275, 28699514624047140510, 1005201938765467579543, 35196266296400319440300
(list; graph; listen)
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OFFSET
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4,1
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COMMENT
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A covering of a set is a tricovering if every element of the set is covered by exactly three blocks of the covering.
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FORMULA
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a(n)=(1/7!)*(35^n-7*20^n-21*15^n+42*10^n+105*8^n+105*7^n+70*5^n-945*4^n-525*3^n+2450*2^n-1470); E.g.f. for k-block tricoverings of an n-set is exp(-x+x^2/2+(exp(y)-1)*x^3/3)*Sum_{k=0..inf}x^k/k!*exp(-1/2*x^2*exp(k*y))*exp(binomial(k, 3)*y).
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CROSSREFS
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Cf. A006095, A060483, A060484, A060486, A060487, A060090-A060095, A060069, A060070, A060051-A060053, A002718, A059443, A003462, A059945-A059951.
Sequence in context: A015323 A145315 A110704 this_sequence A081795 A091748 A147522
Adjacent sequences: A060482 A060483 A060484 this_sequence A060486 A060487 A060488
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KEYWORD
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nonn
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AUTHOR
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Vladeta Jovovic (vladeta(AT)Eunet.yu), Mar 20 2001
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