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A140789 We consider how many ways there are of coloring a square grid, n by n, using just two colors, black & white say. If the resulting grid has TWO lines of symmetry, then the number of different grids is given by this sequence. None of these counted are the images of any of the others under a rotation. If one wishes to count these as different, then each of these numbers can be multiplied by 2. 0
0, 1, 8, 32, 448, 2240, 64512, 556032, 33521664, 553615360, 68717379584, 2233380896768 (list; graph; refs; listen; history; internal format)
OFFSET

1,3

FORMULA

a(2m) = 2^(m^2 - 1)*(2^m + 1) - 2^(m*(m + 1)/2) for n even a(2m+1)= 2^(m^2 + 2*m + 1) - 2^((m^2 + 3*m + 2)/2) for n odd

CROSSREFS

Cf. A140650.

Sequence in context: A143557 A034193 A159277 * A120781 A139286 A139306

Adjacent sequences:  A140786 A140787 A140788 * A140790 A140791 A140792

KEYWORD

nonn

AUTHOR

Anthony C Robin (anthony_robin(AT)hotmail.com), Jul 14 2008

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Last modified February 14 20:38 EST 2012. Contains 205663 sequences.