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 A105476 Number of compositions of n when each even part can be of two kinds. 27
 1, 1, 3, 6, 15, 33, 78, 177, 411, 942, 2175, 5001, 11526, 26529, 61107, 140694, 324015, 746097, 1718142, 3956433, 9110859, 20980158, 48312735, 111253209, 256191414, 589951041, 1358525283, 3128378406, 7203954255, 16589089473, 38200952238, 87968220657 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Row sums of A105475. Starting (1, 3, 6, 15,...) = sum of (n-1)-th row terms of triangle A140168. - Gary W. Adamson, May 10 2008 a(n) is also the number of compositions of n using 1's and 2's such that each run of like numbers can be grouped arbitrarily. For example, a(4) = 15 because 4 = (1)+(1)+(1)+(1) = (1+1)+(1)+(1) = (1)+(1+1)+(1) = (1)+(1)+(1+1) = (1+1)+(1+1) = (1+1+1)+(1) = (1)+(1+1+1) = (1+1+1+1) = (2)+(1)+(1) = (1)+(2)+(1) = (1)+(1)+(2) = (2)+(1+1) = (1+1)+(2) = (2)+(2) = (2+2). - Martin J. Erickson (erickson(AT)truman.edu), Dec 09 2008 An elephant sequence, see A175655. For the central square four A[5] vectors, with decimal values 69, 261, 321 and 324, lead to this sequence (without the first leading 1). For the corner squares these vectors lead to the companion sequence A006138. - Johannes W. Meijer, Aug 15 2010 Inverse INVERT transform of the left shifted sequence gives A000034. Eigensequence of the triangle   1,   2, 1,   1, 2, 1,   2, 1, 2, 1,   1, 2, 1, 2, 1,   2, 1, 2, 1, 2, 1,   1, 2, 1, 2, 1, 2, 1,   2, 1, 2, 1, 2, 1, 2, 1 ... - Paul Barry, Feb 10 2011 Pisano period lengths: 1, 3, 1, 6, 24, 3, 24, 6, 1, 24,120, 6,156, 24, 24, 12, 16, 3, 90, 24,... - R. J. Mathar, Aug 10 2012 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 Index entries for linear recurrences with constant coefficients, signature (1,3). FORMULA G.f.: (1-x^2) / (1-x-3*x^2). a(n) = a(n-1) + 3*a(n-2) for n>=3. a(n) = 3*A006138(n-2), n>=2. a(n) = ((2+sqrt(13))*(1+sqrt(13))^n-(2-sqrt(13))*(1-sqrt(13))^n)/(3*2^n*sqrt(13)) for n>0. - Bruno Berselli, May 24 2011 G.f.: 1/(1 - Sum_{k>=1} x^k*(1+x^k) ). - Joerg Arndt, Mar 09 2014 G.f.: 1/(1 - (x/(1-x)) - x^2/(1-x^2)) = 1/(1 - (x+2*x^2+x^3+2*x^4+x^5+2*x^6+...) ); in general 1/(1 - Sum_{j>=1} m(j)*x^j ) is the g.f. for compositions with m(k) sorts of part k. - Joerg Arndt, Feb 16 2015 a(n) = 3^((n-1)/2)*( 2*sqrt(3)*Fibonacci(n, 1/sqrt(3)) + Fibonacci(n, 1/sqrt(3)) ). - G. C. Greubel, Jan 15 2020 E.g.f.: 1/3 + (2/39)*exp(x/2)*(13*cosh((sqrt(13)*x)/2) + 2*sqrt(13)*sinh((sqrt(13)*x)/2)). - Stefano Spezia, Jan 15 2020 EXAMPLE a(3)=6 because we have (3),(1,2),(1,2'),(2,1),(2',1) and (1,1,1). MAPLE G:=(1-z^2)/(1-z-3*z^2): Gser:=series(G, z=0, 35): 1, seq(coeff(Gser, z^n), n=1..33); MATHEMATICA CoefficientList[Series[(1-x^2)/(1-x-3x^2), {x, 0, 35}], x] (* or *) Join[{1}, LinearRecurrence[{1, 3}, {1, 3}, 50]] (* Vladimir Joseph Stephan Orlovsky, Jul 17 2011; typo fixed by Vincenzo Librandi, Jul 21 2013 *) Table[Round[Sqrt[3]^(n-3)*(2*Sqrt[3]*Fibonacci[n+1, 1/Sqrt[3]] +Fibonacci[n, 1/Sqrt[3]])], {n, 0, 40}] (* G. C. Greubel, Jan 15 2020 *) PROG (PARI) Vec((1-x^2)/(1-x-3*x^2)+O(x^40)) \\ Charles R Greathouse IV, Jun 13 2013 (MAGMA) I:=[1, 1, 3]; [n le 3 select I[n] else Self(n-1)+3*Self(n-2): n in [1..35]]; // Vincenzo Librandi, Jul 21 2013 (MAGMA) R:=PowerSeriesRing(Integers(), 33); Coefficients(R!( 1/(1-(x/(1-x))-x^2/(1-x^2)))); // Marius A. Burtea, Jan 15 2020 (Sage) def A105476_list(prec):     P. = PowerSeriesRing(ZZ, prec)     return P( (1-x^2)/(1-x-3*x^2) ).list() A105476_list(40) # G. C. Greubel, Jan 15 2020 (GAP) a:=[1, 3];; for n in [3..40] do a[n]:=a[n-1]+3*a[n-2]; od; Concatenation([1], a); # G. C. Greubel, Jan 15 2020 CROSSREFS Cf. A006130, A105475, A105963, A274977. Sequence in context: A289006 A336632 A152167 * A000599 A063832 A006647 Adjacent sequences:  A105473 A105474 A105475 * A105477 A105478 A105479 KEYWORD nonn,easy AUTHOR Emeric Deutsch, Apr 09 2005 STATUS approved

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Last modified April 11 15:49 EDT 2021. Contains 342886 sequences. (Running on oeis4.)