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 A059530 Triangle T(n,k) of k-block T_0-tricoverings of an n-set. 2
 0, 0, 0, 0, 1, 3, 1, 0, 0, 0, 0, 1, 39, 89, 43, 3, 0, 0, 0, 0, 0, 252, 2192, 4090, 2435, 445, 12, 0, 0, 0, 0, 0, 1260, 37080, 179890, 289170, 188540, 50645, 4710, 70, 0, 0, 0, 0, 0, 5040, 536760, 6052730, 20660055, 29432319, 19826737, 6481160, 964495, 52430 (list; graph; refs; listen; history; text; internal format)
 OFFSET 3,6 COMMENTS A covering of a set is a tricovering if every element of the set is covered by exactly three blocks of the covering. A covering of a set is a T_0-covering if for every two distinct elements of the set there exists a block of the covering containing one but not the other element. REFERENCES I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983. LINKS FORMULA E.g.f. for k-block T_0-tricoverings of an n-set is exp(-x+1/2*x^2+1/3*x^3*y)*Sum_{i=0..inf}(1+y)^binomial(i, 3)*exp(-1/2*x^2*(1+y)^i)*x^i/i!. EXAMPLE [0, 0, 0, 0, 1, 3, 1], [0, 0, 0, 0, 1, 39, 89, 43, 3], [0, 0, 0, 0, 0, 252, 2192, 4090, 2435, 445, 12], [0, 0, 0, 0, 0, 1260, 37080, 179890, 289170, 188540, 50645, 4710, 70], ...; there are 5=1+3+1 T_0-tricoverings of a 3-set and 175=1+39+89+43+3 T_0-tricoverings of a 4-set, cf. A060070. CROSSREFS Cf. (column sums) A060069, (row sums) A060070, A060051-A060053, A002718, A059443, A003462, A059945-059951. Sequence in context: A078529 A180017 A243827 * A193525 A049828 A286131 Adjacent sequences:  A059527 A059528 A059529 * A059531 A059532 A059533 KEYWORD nonn,tabf AUTHOR Vladeta Jovovic, Feb 22 2001 STATUS approved

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Last modified February 22 02:13 EST 2019. Contains 320381 sequences. (Running on oeis4.)