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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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified August 2 06:52 EDT 2021. Contains 346411 sequences. (Running on oeis4.)