A005396 Number of 2n-step polygons on honeycomb.

1, 0, 0, 6, 0, 30, 24, 168, 288, 1170, 2760, 9504, 25776, 84006, 244104, 779970, 2356800, 7487310, 23167656, 73639326, 231348000, 737718534, 2341772136, 7498417956, 23982456384, 77119604400, 248088123192, 800978437674, 2588744704560, 8388692108016, 27217694697000

Offset: 2

Author

N. J. A. Sloane

Keywords

• nonn

References

• N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

• M. E. Fisher and M. F. Sykes, Excluded-volume problem and the Ising model of ferromagnetism, Phys. Rev. 114 (1959), 45-58.

Crossrefs

Cf. A001335.

Formula

a(n) = 2 * n * A006774(n), n > 1. - Sean A. Irvine, Aug 06 2020

Extensions

a(11)-a(21) from Sean A. Irvine, Jun 12 2016

a(0)-a(1) inserted and more terms from Sean A. Irvine, Aug 06 2020