0,2

For n >= 1 a(n) is the number of n X n (0,1) matrices.

1/2^(n^2) is the Hankel transform of C(n,n/2)(1+(-1)^n)/(2*2^n), or C(2n,n)/4^n with interpolated zeros. - Paul Barry (pbarry(AT)wit.ie), Sep 27 2007

Hankel transform of A064062 . - Philippe DELEHAM (kolotoko(AT)wanadoo.fr), Nov 19 2007

a(n) is also the order of the semigroup (monoid) of all binary relations on an n-set. [From A. Umar (aumarh(AT)squ.edu.om), Sep 14 2008]

F. Harary and R. W. Robinson, Labeled bipartite blocks, Canad. J. Math., 31 (1979), 60-68.

Kent E. Morrison, Integer Sequences and Matrices Over Finite Fields, Journal of Integer Sequences, Vol. 9 (2006), Article 06.2.1.

Howie, J. M. Fundamentals of semigroup theory. Oxford: Clarendon Press, (1995). [From A. Umar (aumarh(AT)squ.edu.om), Sep 14 2008]

P. J. Cameron, Sequences realized by oligomorphic permutation groups, J. Integ. Seqs. Vol. 3 (2000), #00.1.5.

T. Eisenkoelbl, 2-Enumerations of halved alternating sign matrices.

T. Eisenk\"olbl, 2-Enumerations of halved alternating sign matrices

Daniele A. Gewurz and Francesca Merola, Sequences realized as Parker vectors ..., J. Integer Seqs., Vol. 6, 2003.

G. Pfeiffer, Counting Transitive Relations, Journal of Integer Sequences, Vol. 7 (2004), Article 04.3.2.

Eric Weisstein's World of Mathematics, 01-Matrix

with(finance):seq(mul(futurevalue(2, 3, k), k=0..n), n=-1..12); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jun 30 2008

a[n_]:=2^(n^2); [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Dec 13 2008]

(PARI) a(n)=polresultant((x-1)^n, (x+1)^n, x) (from R. Stephan)

Bisection of A060656.

Sequence in context: A012393 A063387 A063391 this_sequence A013028 A136632 A012919

Adjacent sequences: A002413 A002414 A002415 this_sequence A002417 A002418 A002419

nonn,easy

N. J. A. Sloane (njas(AT)research.att.com).