OFFSET
1,4
REFERENCES
I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..200
Index entries for linear recurrences with constant coefficients, signature (26,-255,1210,-2924,3384,-1440).
FORMULA
a(n) = (1/5!)*(10^n - 5*6^n - 10*4^n + 20*3^n + 30*2^n - 60).
E.g.f. for m-block bicoverings of an n-set is exp(-x-1/2*x^2*(exp(y)-1))*Sum_{i>=0} (x^i/i!)*exp(binomial(i, 2)*y).
G.f.: x^4*(288*x^2-178*x+25) / ((x-1)*(2*x-1)*(3*x-1)*(4*x-1)*(6*x-1)*(10*x-1)). - Colin Barker, Jan 11 2013
MATHEMATICA
With[{c=(1/5!)}, Table[c(10^n-5 6^n-10 4^n+20 3^n+30 2^n-60), {n, 20}]] (* Harvey P. Dale, Apr 21 2011 *)
PROG
(PARI) a(n) = {(1/5!)*(10^n - 5*6^n - 10*4^n + 20*3^n + 30*2^n - 60)} \\ Andrew Howroyd, Jan 29 2020
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Vladeta Jovovic, Feb 14 2001
EXTENSIONS
More terms from Colin Barker, Jan 11 2013
STATUS
approved