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 A027053 T(n,n+2), T given by A027052. 2
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,2 COMMENTS Second differences of (A027026(n)-1)/2. Pairwise sums of A089068. a(n) is also the number of fixed polyominoes with n cells of height (or width) 2. [From David Bevan, Sep 09 2009] LINKS Doron Zeilberger, The generating functions and series expansions for 2D lattice-animals of globally bounded width. Index entries for linear recurrences with constant coefficients, signature (2,0,0,-1). FORMULA G.f.: x^2*(1+x)^2/((1-x)*(1-x-x^2-x^3)). a(n) = A089068(n-1)+A089068(n). a(n) = a(n-1) + a(n-2) + a(n-3) + 4. [From David Bevan, Sep 09 2009] a(n) = A001590(n+3) - 2. [From David Bevan, Sep 09 2009] a(n+1)-a(n) = A000213(n+1). - R. J. Mathar, Aug 04 2013 CROSSREFS Sequence in context: A019454 A019453 A038089 * A122039 A083706 A229072 Adjacent sequences:  A027050 A027051 A027052 * A027054 A027055 A027056 KEYWORD nonn AUTHOR STATUS approved

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