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Search: id:A027053
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| 1, 4, 9, 18, 35, 66, 123, 228, 421, 776, 1429, 2630, 4839, 8902, 16375, 30120, 55401, 101900, 187425, 344730, 634059, 1166218, 2145011, 3945292, 7256525, 13346832, 24548653, 45152014, 83047503, 152748174, 280947695, 516743376
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OFFSET
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2,2
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COMMENT
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a(n) is also the number of fixed polyominoes with n cells of height (or width) 2. [From David Bevan (dbevan(AT)emtex.com), Sep 09 2009]
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FORMULA
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G.f.: [x^2(1+x)^2]/[(1-x)(1-x-x^2-x^3)].
a(n) = a(n-1) + a(n-2) + a(n-3) + 4. [From David Bevan (dbevan(AT)emtex.com), Sep 09 2009]
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CROSSREFS
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Second differences of (A027026(n)-1)/2. Pairwise sums of A089068.
a(n) = A001590(n+3) - 2. [From David Bevan (dbevan(AT)emtex.com), Sep 09 2009]
Sequence in context: A019454 A019453 A038089 this_sequence A122039 A083706 A074896
Adjacent sequences: A027050 A027051 A027052 this_sequence A027054 A027055 A027056
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KEYWORD
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nonn
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AUTHOR
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Clark Kimberling (ck6(AT)evansville.edu)
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