

A060521


Number of 3 X n grids of black and white cells, no 3 of same color vertically or horizontally contiguous.


5



6, 36, 102, 378, 1260, 4374, 14946, 51384, 176238, 605022, 2076288, 7126302, 24457806, 83942100, 288096942, 988778082, 3393583068, 11647114446, 39974047290, 137194888728, 470866430838, 1616060190870, 5546478488640
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OFFSET

1,1


COMMENTS

The conjectured recursion is correct: For each n count the solutions separately where the last two rows differ in 0, 1, 2, or 3 places; a linear recursion is then readily found. The corresponding matrix has characteristic polynomial x^4  2 x^3  5 x^2 + 1, matching the recursion recursion a(n+4) = 2a(n+3) + 5a(n+2)  a(n). [From Hagen von Eitzen, Oct 21 2009]


LINKS

Table of n, a(n) for n=1..23.


FORMULA

Almost surely satisfies a(n+4) = 2a(n+3) + 5a(n+2)  a(n).


CROSSREFS

Sequence in context: A207704 A207495 A207249 * A036141 A270205 A207443
Adjacent sequences: A060518 A060519 A060520 * A060522 A060523 A060524


KEYWORD

nonn


AUTHOR

Tom Womack (tom(AT)womack.net)


EXTENSIONS

More terms from Hagen von Eitzen, Oct 21 2009


STATUS

approved



