OFFSET
0,2
COMMENTS
Also number of (2*n)-step closed paths (from origin to origin) in 2-dimensional lattice, using steps (t_1,t_2) (|t_1| + |t_2| = 3).
*
|
*-- --*
| | |
*-- -- -- --*
| | | | |
*-- -- --P-- -- --*
| | | | |
*-- -- -- --*
| | |
*-- --*
|
*
Point P move to any position of * in the next step.
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..400 (terms 0..185 from Vaclav Kotesovec)
Vaclav Kotesovec, Recurrence of order 4 (conjectured)
FORMULA
Conjecture: a(n) ~ 3 * 144^n / (19*Pi*n). - Vaclav Kotesovec, Nov 04 2019
PROG
(PARI) {a(n) = polcoef(polcoef(((x^3+x+1/x+1/x^3)*(y^3+y+1/y+1/y^3)-(x+1/x)*(y+1/y))^(2*n), 0), 0)}
(PARI) {a(n) = polcoef(polcoef((sum(k=0, 3, (x^k+1/x^k)*(y^(3-k)+1/y^(3-k)))-x^3-1/x^3-y^3-1/y^3)^(2*n), 0), 0)}
(PARI) f(n) = (x^(2*n+2)-1/x^(2*n+2))/(x-1/x);
a(n) = sum(k=0, 2*n, (-1)^k*binomial(2*n, k)*polcoef(f(1)^k*f(0)^(2*n-k), 0)^2)
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Nov 02 2019
STATUS
approved