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 A052528 Expansion of (1 - x)/(1 - 2*x - 2*x^2 + 2*x^3). 7
 1, 1, 4, 8, 22, 52, 132, 324, 808, 2000, 4968, 12320, 30576, 75856, 188224, 467008, 1158752, 2875072, 7133632, 17699904, 43916928, 108966400, 270366848, 670832640, 1664466176, 4129863936, 10246994944, 25424785408, 63083832832 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Form the graph with matrix A = [1,1,1,1; 1,0,0,0; 1,0,0,0; 1,0,0,1]. Then a(n) counts closed walks of length n at the degree 5 vertex. - Paul Barry, Oct 02 2004 Equals the INVERT transform of (1, 3, 1, 1, 1, ...). - Gary W. Adamson, Apr 27 2009 a(n) is also the number of vertex-transitive cover graphs of lattice quotients of essential lattice congruences of the weak order on the symmetric group S_{n+1}. See Table 1 in the Hoang/Mütze reference in the Links section. - Torsten Muetze, Nov 28 2019 LINKS G. C. Greubel, Table of n, a(n) for n = 0..1000 Hung Phuc Hoang, Torsten Mütze, Combinatorial generation via permutation languages. II. Lattice congruences, arXiv:1911.12078 [math.CO], 2019. INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 455 Index entries for linear recurrences with constant coefficients, signature (2,2,-2). FORMULA G.f.: (1 - x)/(1 - 2*x - 2*x^2 + 2*x^3). Recurrence: a(1) = 1, a(0) = 1, a(2) = 4, 2*a(n) - 2*a(n+1) - 2*a(n+2) + a(n+3) = 0. a(n) = Sum_{alpha=RootOf(2*Z^3-2*Z^2-2*Z+1)} (1/37)*(5 - 9*alpha^2 + 12*alpha)* alpha^(-1 - n). a(n) = 2*a(n-2) + Sum_{i=0..n-1} a(i). - Yuchun Ji, Dec 29 2018 MAPLE spec := [S, {S=Sequence(Prod(Z, Union(Z, Z, Sequence(Z))))}, unlabeled]: seq(combstruct[count](spec, size=n), n=0..20); MATHEMATICA LinearRecurrence[{2, 2, -2}, {1, 1, 4}, 30] (* G. C. Greubel, May 12 2019 *) PROG (PARI) my(x='x+O('x^30)); Vec((1-x)/(1-2*x-2*x^2+2*x^3)) \\ G. C. Greubel, May 12 2019 (MAGMA) R:=PowerSeriesRing(Integers(), 30); Coefficients(R!( (1-x)/(1 -2*x-2*x^2+2*x^3) )); // G. C. Greubel, May 12 2019 (Sage) ((1-x)/(1-2*x-2*x^2+2*x^3)).series(x, 30).coefficients(x, sparse=False) # G. C. Greubel, May 12 2019 (GAP) a:=[1, 1, 4];; for n in [4..30] do a[n]:=2*a[n-1]+2*a[n-2]-2*a[n-3]; od; a; # G. C. Greubel, May 12 2019 CROSSREFS Cf. A077937, A052987. Sequence in context: A175655 A000639 A190795 * A058855 A297339 A290138 Adjacent sequences:  A052525 A052526 A052527 * A052529 A052530 A052531 KEYWORD nonn,easy AUTHOR encyclopedia(AT)pommard.inria.fr, Jan 25 2000 EXTENSIONS More terms from James A. Sellers, Jun 06 2000 STATUS approved

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Last modified December 12 12:30 EST 2019. Contains 329958 sequences. (Running on oeis4.)