0,2

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

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

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

B. T. Bennett and R. B. Potts, Arrays and brooks, J. Austral. Math. Soc., 7 (1967), 23-31.

A. Kotzig and P. J. Laufer, When are permutations additive?, Amer. Math. Monthly, 85 (1978), 364-365.

C. Bebeacua, T. Mansour, A. Postnikov and S. Severini, On the x-rays of permutations

a(2) = 6 corresponds to

..O.X.X.......X.X.O.......O.X.X.......X.O.X.......X.O.X.......X.X.O

.X.X.O.X.....X.O.X.X.....X.X.X.O.....X.X.X.O.....O.X.X.X.....O.X.X.X

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

.O.X.X.X.....X.X.X.O.....X.X.X.O.....X.O.X.X.....X.X.O.X.....O.X.X.X

..X.O.X.......X.O.X.......O.X.X.......X.X.O.......O.X.X.......X.X.O

Sequence in context: A006117 A118025 A119966 this_sequence A126340 A136639 A027109

Adjacent sequences: A002044 A002045 A002046 this_sequence A002048 A002049 A002050

nonn,nice

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

More terms from Alex Fink (a00(AT)shaw.ca), Mar 16 2005