A081623 Number of ways in which the points on an n X n square lattice can be equally occupied with spin "up" and spin "down" particles. If n is odd, we arbitrarily take the lattice to contain one more spin "up" particle than the number of spin "down" particles.

1, 1, 6, 126, 12870, 5200300, 9075135300, 63205303218876, 1832624140942590534, 212392290424395860814420, 100891344545564193334812497256, 191645966716130525165099506263706416, 1480212998448786189993816895482588794876100

Offset

0,3

Links

Alois P. Heinz, Table of n, a(n) for n = 0..57

Brian Hayes, The World in a Spin, American Scientist 88:5 (September-October 2000), pp. 384-388. [alternate link]

James Grime, Maths Problem: Complete Noughts and Crosses (Burnside's Lemma)

Formula

a(n) = C(n^2, (n^2+1)/2) if n is odd and C(n^2, n^2/2) if n is even.

a(n) = binomial(n^2,floor(n^2/2)). - Alois P. Heinz, Jul 21 2017

Example

a(2) = C(4,2) = 6.
a(3) = C(9,5) = 126.

Maple

a:= n-> (s-> binomial(s, floor(s/2)))(n^2):
seq(a(n), n=0..15);  # Alois P. Heinz, Jul 21 2017

Prog

(Mathcad or Microsoft Excel): f(n)=combin(n^2, trunc((n^2+1)/2))
(PARI) a(n)=binomial(n^2, n^2\2) \\ Charles R Greathouse IV, May 09 2013

Crossrefs

A082963 is the equivalent sequence up to reflection and rotation.

Sequence in context: A237428 A255900 A133792 * A223210 A324093 A177756

Adjacent sequences: A081620 A081621 A081622 * A081624 A081625 A081626

Keyword

easy,nonn

Author

A. Timothy Royappa, Apr 22 2003

Extensions

a(0)=1 prepended by Alois P. Heinz, Jul 21 2017

Status

approved