%I A002047 M1688 N0666
%S A002047 1,2,6,28,244,2544,35600,659632,15106128,425802176
%N A002047 Number of 3 X (2n+1) zero-sum arrays.
%C A002047 This can be interpreted as the number of ways to choose 2n+1 cells in 
               a hexagonal grid of side n+1 such that no two are in the same row 
               or left diagonal or right diagonal. - Alex Fink (a00(AT)shaw.ca), 
               Mar 16 2005
%D A002047 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A002047 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 
               (includes this sequence).
%D A002047 B. T. Bennett and R. B. Potts, Arrays and brooks, J. Austral. Math. Soc., 
               7 (1967), 23-31.
%D A002047 A. Kotzig and P. J. Laufer, When are permutations additive?, Amer. Math. 
               Monthly, 85 (1978), 364-365.
%H A002047 C. Bebeacua, T. Mansour, A. Postnikov and S. Severini, <a href="http:/
               /arXiv.org/abs/math.CO/0506334">On the x-rays of permutations</a>
%e A002047 a(2) = 6 corresponds to
%e A002047 ..O.X.X.......X.X.O.......O.X.X.......X.O.X.......X.O.X.......X.X.O
%e A002047 .X.X.O.X.....X.O.X.X.....X.X.X.O.....X.X.X.O.....O.X.X.X.....O.X.X.X
%e A002047 X.X.X.X.O...O.X.X.X.X...X.O.X.X.X...O.X.X.X.X...X.X.X.X.O...X.X.X.O.X
%e A002047 .O.X.X.X.....X.X.X.O.....X.X.X.O.....X.O.X.X.....X.X.O.X.....O.X.X.X
%e A002047 ..X.O.X.......X.O.X.......O.X.X.......X.X.O.......O.X.X.......X.X.O
%Y A002047 Sequence in context: A006117 A118025 A119966 this_sequence A126340 A136639 
               A027109
%Y A002047 Adjacent sequences: A002044 A002045 A002046 this_sequence A002048 A002049 
               A002050
%K A002047 nonn,nice
%O A002047 0,2
%A A002047 N. J. A. Sloane (njas(AT)research.att.com).
%E A002047 More terms from Alex Fink (a00(AT)shaw.ca), Mar 16 2005