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 A228705 Expansion of (1-2*x+4*x^2-2*x^3+x^4)/((1-x)^4*(1+x^2)^2). 4
 1, 2, 4, 10, 19, 28, 40, 60, 85, 110, 140, 182, 231, 280, 336, 408, 489, 570, 660, 770, 891, 1012, 1144, 1300, 1469, 1638, 1820, 2030, 2255, 2480, 2720, 2992, 3281, 3570, 3876, 4218, 4579, 4940, 5320, 5740, 6181, 6622, 7084, 7590, 8119, 8648, 9200, 9800, 10425 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Number of n-element subsets of [n+3] having an even sum. a(3) = 10: {1,2,3}, {1,2,5}, {1,3,4}, {1,3,6}, {1,4,5}, {1,5,6}, {2,3,5}, {2,4,6}, {3,4,5}, {3,5,6}. - Alois P. Heinz, Feb 04 2017 A159914, which is half the number of (n-3)-element subsets of {1..n} having an odd sum, satisfies the same recurrence relation. However, a simple relation between a(n) and A159914(n) is not obvious. - M. F. Hasler, Jun 22 2018 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 E. Kirkman, J. Kuzmanovich and J. J. Zhang, Invariants of (-1)-Skew Polynomial Rings under Permutation Representations, arXiv preprint arXiv:1305.3973, 2013. See Example 5.6. Index entries for linear recurrences with constant coefficients, signature (4, -8, 12, -14, 12, -8, 4, -1). FORMULA a(n) = (n+2)*(2*(n+1)*(n+3)+3*(1+(-1)^n)*i^(n*(n+1)))/24, where i=sqrt(-1). [Bruno Berselli, Sep 07 2013] a(0)=1, a(1)=2, a(2)=4, a(3)=10, a(4)=19, a(5)=28, a(6)=40, a(7)=60, a(n)=4*a(n-1)-8*a(n-2)+12*a(n-3)-14*a(n-4)+12*a(n-5)-8*a(n-6)+ 4*a(n-7)- a(n-8). - Harvey P. Dale, Apr 10 2014 MATHEMATICA CoefficientList[Series[(1 - 2 x + 4 x^2 - 2 x^3 + x^4) / ((1 - x)^4 (1 + x^2)^2), {x, 0, 50}], x] (* Vincenzo Librandi, Sep 07 2013 *) LinearRecurrence[{4, -8, 12, -14, 12, -8, 4, -1}, {1, 2, 4, 10, 19, 28, 40, 60}, 50] (* Harvey P. Dale, Apr 10 2014 *) PROG (Magma) m:=50; R:=PowerSeriesRing(Integers(), m); Coefficients(R!(1-2*x+4*x^2-2*x^3+x^4)/((1-x)^4*(1+x^2)^2)); // Vincenzo Librandi, Sep 07 2013 CROSSREFS Third lower diagonal of A282011. Sequence in context: A079162 A257593 A197926 * A253772 A043330 A295961 Adjacent sequences: A228702 A228703 A228704 * A228706 A228707 A228708 KEYWORD nonn,easy AUTHOR N. J. A. Sloane, Sep 06 2013 STATUS approved

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Last modified September 22 11:51 EDT 2023. Contains 365531 sequences. (Running on oeis4.)