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A060485 Number of 7-block tricoverings of an n-set. 5
43, 4520, 244035, 10418070, 401861943, 14778678180, 530817413155, 18837147108890, 664260814445943, 23345018969140440, 818942064306004275, 28699514624047140510, 1005201938765467579543, 35196266296400319440300 (list; graph; refs; listen; history; text; internal format)
OFFSET

4,1

COMMENTS

A covering of a set is a tricovering if every element of the set is covered by exactly three blocks of the covering.

LINKS

Andrew Howroyd, Table of n, a(n) for n = 4..200

Index entries for linear recurrences with constant coefficients, signature (110, -4991, 124120, -1887459, 18470550, -118758569, 501056740, -1355000500, 2223560000, -1973160000, 705600000).

FORMULA

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).

G.f.: x^4*(27300000*x^7 +9288000*x^6 -17908650*x^5 +6008735*x^4 -796380*x^3 +38552*x^2 +210*x -43) / ((x -1)*(2*x -1)*(3*x -1)*(4*x -1)*(5*x -1)*(7*x -1)*(8*x -1)*(10*x -1)*(15*x -1)*(20*x -1)*(35*x -1)). - Colin Barker, Jan 12 2013

CROSSREFS

Column k=7 of A060487.

Cf. A006095, A060483, A060484, A060486, A060090-A060095, A060069, A060070, A060051-A060053, A002718, A059443, A003462, A059945-A059951.

Sequence in context: A015323 A145315 A110704 * A081795 A108837 A091748

Adjacent sequences:  A060482 A060483 A060484 * A060486 A060487 A060488

KEYWORD

nonn,easy

AUTHOR

Vladeta Jovovic, Mar 20 2001

STATUS

approved

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Last modified April 22 04:29 EDT 2019. Contains 322329 sequences. (Running on oeis4.)