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Search: id:A118649
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| 1, 2, 5, 12, 30, 74, 184, 456, 1132, 2808, 6968, 17288, 42896, 106432, 264080, 655232, 1625760, 4033824, 10008704, 24833536, 61616832, 152883328, 379333248, 941199488, 2335298816, 5794330112, 14376858880, 35671780352, 88508618240
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OFFSET
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0,2
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COMMENT
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For n>=2, a(n)= Number of "stable LEGO walls" (i.e., walls in which seams don't match up from one level to the next) of width 7 and height n-1 when using bricks of length 2, 3, and 4. For example, there are a(2)=5 stable LEGO walls of height 1 and they are {[2,2,3],[2,3,2],[3,2,2],[3,4],[4,3]}. [From Emilie Hogan (eahogan(AT)math.rutgers.edu), Oct 16 2009]
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FORMULA
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Conjecture: a(n)=2*a(n-1)+2*a(n-2)-2*a(n-3) with g.f. (1-x^2)/(1-2*x-2*x^2+2*x^3). [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Nov 10 2009]
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CROSSREFS
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Sequence in context: A157748 A046170 A062423 this_sequence A033482 A054341 A000106
Adjacent sequences: A118646 A118647 A118648 this_sequence A118650 A118651 A118652
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KEYWORD
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nonn,new
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AUTHOR
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Joshua Zucker (joshua.zucker(AT)stanfordalumni.org), May 10 2006
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