OFFSET
1,3
LINKS
Z. Cinkir, Effective Resistances, Kirchhoff index and Admissible Invariants of Ladder Graphs, arXiv preprint arXiv:1503.06353 [math.CO], 2015.
FORMULA
a(n) = denominator of k(n) = (1/3)*n^2*(n-sqrt(3)/tanh(n*log(2-sqrt(3)))). k(n) is also equal to n*(n^2-1)/3 + n*sum(k=0, n-1, 1/(1 + 2*sin(Pi*k/(2*n))^2)). - Altug Alkan, Dec 02 2015
EXAMPLE
1, 5, 71/5, 214/7, 11725/209, 6031/65, 415177/2911, 140972/679, ...
MATHEMATICA
f[n_] := Denominator@ Simplify[(1/3)*n^2*(n - Sqrt[3]/Tanh[n*Log[2 - Sqrt[3]]])]; Array[f, 45] (* Robert G. Wilson v, Dec 02 2015 *)
CROSSREFS
KEYWORD
nonn,frac
AUTHOR
N. J. A. Sloane, Dec 02 2015
EXTENSIONS
a(5) corrected by Altug Alkan, Dec 02 2015
More terms from Robert G. Wilson v, Dec 02 2015
STATUS
approved