OFFSET
1,2
REFERENCES
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..1000
T. R. S. Walsh and A. B. Lehman, Counting rooted maps by genus. III: Nonseparable maps, J. Combinatorial Theory Ser. B 18 (1975), 222-259.
Index entries for linear recurrences with constant coefficients, signature (6,-15,20,-15,6,-1).
FORMULA
a(n) seems to be divisible by n+1. - Ralf Stephan, Sep 01 2003
Conjecture (for n > 1): a(n) = n*(n+1)*(n^3+6*n^2+11*n-42) / 24. - Sean A. Irvine, Apr 10 2017
The above conjectures are true. - Andrew Howroyd, Apr 03 2021
From Chai Wah Wu, Aug 08 2022: (Start)
a(n) = 6*a(n-1) - 15*a(n-2) + 20*a(n-3) - 15*a(n-4) + 6*a(n-5) - a(n-6) for n > 7.
G.f.: x^2*(2*x^5 - 12*x^4 + 30*x^3 - 36*x^2 + 18*x + 3)/(x - 1)^6. (End)
MATHEMATICA
A006428[n_] := If[n == 1, 0, n*(n + 1)*(n*(n*(n + 6) + 11) - 42)/24];
PROG
(PARI) a(n) = if(n<2, 0, n*(n+1)*(n^3+6*n^2+11*n-42) / 24) \\ Andrew Howroyd, Apr 03 2021
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
Title improved by Sean A. Irvine, Apr 10 2017
Terms a(13) and beyond from Andrew Howroyd, Apr 03 2021
STATUS
approved