%I A057170
%S A057170 1,2,12,288,26880,10035200,14836039680,87734404251648,2064716402685640704,
%T A057170 194361783607326689722368,72958995691997968023051829248,109548594452892660460226753134067712,
%U A057170 656593430123179564638165745256190909087744,15741504841171021653720624575980053578961033101312
%N A057170 2-enumeration of 2n X 2n half-turn symmetric alternating-sign matrices.
%H A057170 G. Kuperberg, <a href="http://arXiv.org/abs/math.CO/0008184">Symmetry 
               classes of alternating-sign matrices under one roof, arXiv math.CO/
               0008184</a> [Th. 3]
%p A057170 A057170 := proc(n) local i, j, t1; t1 := 2^(n^2); for i to n do for j 
               to n do if j-i mod 2 <> 0 then t1 := t1*(2*j - 2*i + 1)/(2*j - 2*i) 
               end if end do end do; t1 end proc;
%Y A057170 Sequence in context: A012754 A083568 A003121 this_sequence A008338 A000178 
               A108395
%Y A057170 Adjacent sequences: A057167 A057168 A057169 this_sequence A057171 A057172 
               A057173
%K A057170 nonn,easy
%O A057170 0,2
%A A057170 N. J. A. Sloane (njas(AT)research.att.com), Feb 04 2001