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A060550
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a(n) is the number of distinct patterns (modulo geometric D_3-operations) with no other than strict 120-degree rotational symmetry which can be formed by an equilateral triangular arrangement of closely packed black and white cells satisfying the local matching rule of Pascal's triangle modulo 2, where n is the number of cells in each edge of the arrangement.
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2
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0, 0, 0, 1, 0, 1, 2, 1, 2, 6, 2, 6, 12, 6, 12, 28, 12, 28, 56, 28, 56, 120, 56, 120, 240, 120, 240, 496, 240, 496, 992, 496, 992, 2016, 992, 2016, 4032, 2016, 4032, 8128, 4032, 8128, 16256, 8128, 16256, 32640, 16256, 32640, 65280, 32640
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OFFSET
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1,7
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COMMENTS
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The matching rule is such that any elementary top-down triangle of three neighboring cells in the arrangement contains either one or three white cells.
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LINKS
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FORMULA
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a(n) = 2^(floor(n/3) + (n mod 3) mod 2 - 1) - 2^(floor((n+3)/6) + d(n)-1), with d(n)=1 if n mod 6=1, otherwise d(n)=0.
Empirical g.f.: x^4*(x^2 - x + 1)*(x^2 + x + 1) / ((2*x^3-1)*(2*x^6-1)). - Colin Barker, Aug 29 2013
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PROG
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(PARI) { for (n=1, 500, write("b060550.txt", n, " ", 2^(floor(n/3) + (n%3)%2 - 1) - 2^(floor((n + 3)/6) + (n%6==1) - 1)); ) } \\ Harry J. Smith, Jul 07 2009
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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André Barbé (Andre.Barbe(AT)esat.kuleuven.ac.be), Apr 03 2001
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STATUS
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approved
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