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A060484
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Number of 6-block tricoverings of an n-set.
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2
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1, 95, 3107, 75835, 1653771, 34384875, 700030507, 14116715435, 283432939691, 5679127043755, 113683003777707, 2274630646577835, 45502044971338411, 910133025632152235, 18203564201836161707, 364080180268471397035
(list; graph; refs; listen; history; internal format)
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OFFSET
| 3,2
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COMMENTS
| 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
| a(n)=(1/6!)*(20^n-6*10^n-15*8^n+135*4^n-310*2^n+240); 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
| Cf. A006095, A060483, A060485-A060487, A060090-A060095, A060069, A060070, A060051-A060053, A002718, A059443, A003462, A059945-A059951.
Sequence in context: A020322 A055829 A093295 * A017811 A017758 A203342
Adjacent sequences: A060481 A060482 A060483 * A060485 A060486 A060487
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KEYWORD
| nonn
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AUTHOR
| Vladeta Jovovic (vladeta(AT)eunet.rs), Mar 20 2001
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