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 A005252 a(n) = Sum_{k=0..floor(n/4)} binomial(n-2k,2k). (Formerly M1048) 15
 1, 1, 1, 1, 2, 4, 7, 11, 17, 27, 44, 72, 117, 189, 305, 493, 798, 1292, 2091, 3383, 5473, 8855, 14328, 23184, 37513, 60697, 98209, 158905, 257114, 416020, 673135, 1089155, 1762289, 2851443, 4613732, 7465176, 12078909, 19544085, 31622993 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS The Twopins/t sequence (see Guy). Number of closed walks of length n at a vertex of the graph with adjacency matrix [1,1,0,0;0,0,0,1;1,0,0,0;0,0,1,1]. - Paul Barry, Mar 15 2004 a(n+3) = number of n-bit sequences that avoid both 010 and 0110. Example: for n=3, there are 8 3-bit sequences and only 010 fails to qualify, so a(6)=7. - David Callan, Mar 25 2004 a(n) = A173021(2^(n-1) - 1) for n>0. [Reinhard Zumkeller, Feb 07 2010] a(n) is the number of length n binary words that have an even number of 0's and every 0 is immediately followed by a 1. a(6) = 7 because we have: 010111, 011011, 011101, 101011, 101101, 110101, 111111. - Geoffrey Critzer, Jan 08 2014 a(n) = number of vertices of the Fibonacci cube Gamma(n-1) having an even number of ones. The Fibonacci cube Gamma(n) can be defined as the graph whose vertices are the binary strings of length n without two consecutive 1's  and in which two vertices are adjacent when their Hamming distance is exactly 1. Example: a(4) = 2; indeed, the Fibonacci cube Gamma(3) has the five vertices 000, 010, 001, 100, 101, two of which have an even number of ones. See the E. Munarini et al. reference, p. 323. - Emeric Deutsch, Jun 28 2015 a(n) is the number of even permutations p of 1,2,..,n such that |p(i)-i|<=1 for i=1,2,..,n. - Dmitry Efimov, Jan 08 2016 This sequence (prefixed with 0) is an autosequence of the first kind, whose second kind companion is (2 followed by abs(A111734)). - Jean-François Alcover, Oct 30 2017 REFERENCES John H. Conway and R. K. Guy, The Book of Numbers, Copernicus Press, p. 205 of first edition. R. K. Guy, Anyone for Twopins?, in D. A. Klarner, editor, The Mathematical Gardner. Prindle, Weber and Schmidt, Boston, 1981, pp. 2-15. MacKay, Information Theory, Inference and Learning Algorithms, CUP, 2003, p251. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). E. L. Tan, On the cycle graph of a graph and inverse cycle graphs, Ph.D. Dissertation, Univ. of Philippines, Diliman, Quezon City, 1987. E. L. Tan, On Fibonacci numbers and cycle graphs, Matimyas Matemaka (Published by the Mathematical Society of the Philippines), 13 (No. 2, 1990), 1-4. LINKS T. D. Noe, Table of n, a(n) for n=0..500 R. Austin and R. K. Guy, Binary sequences without isolated ones, Fib. Quart., 16 (1978), 84-86. R. K. Guy, Anyone for Twopins?, in D. A. Klarner, editor, The Mathematical Gardner. Prindle, Weber and Schmidt, Boston, 1981, pp. 2-15. [Annotated scanned copy, with permission] V. C. Harris, C. C. Styles, A generalization of Fibonacci numbers, Fib. Quart. 2 (1964) 277-289, sequence u(n,2,2). V. E. Hoggatt, Jr., 7-page typed letter to N. J. A. Sloane with suggestions for new sequences, circa 1977. V. E. Hoggatt, Jr. and M. Bicknell, Diagonal sums of generalized Pascal triangles, Fib. Quart., 7 (1969), 341-358, 393. INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 424 Milan Janjic, Binomial Coefficients and Enumeration of Restricted Words, Journal of Integer Sequences, 2016, Vol 19, #16.7.3 S. Klavzar, Structure of Fibonacci cubes: a survey, J. Comb. Optim., 25, 2013, 505-522. E. Munarini, N. Z. Salvi, Structural and enumerative properties of the Fibonacci cubes, Discrete Math., 255, 2002, 317-324. Simon Plouffe, Approximations de séries génératrices et quelques conjectures, Dissertation, Université du Québec à Montréal, 1992. Simon Plouffe, 1031 Generating Functions and Conjectures, Université du Québec à Montréal, 1992. E. L. Tan, Letter to N. J. A. Sloane, Feb 1992 OEIS Wiki, Autosequence. Index entries for linear recurrences with constant coefficients, signature (2,-1,0,1). FORMULA Second differences give sequence shifted twice. - E. L. Tan, Univ. Phillipines. G.f.: (1-x)/((1-x-x^2)(1-x+x^2)). Simon Plouffe in his 1992 dissertation. a(n) = Fib(n+1)/2+A010892(n)/2; a(n)=(((1+sqrt(5))/2)^(n+1)/sqrt(5)-((1-sqrt(5))/2)^(n+1)/sqrt(5)+cos(Pi*n/3)+sin(Pi*n/3)/sqrt(3))/2. - Paul Barry, Mar 15 2004 a(n) = 2*a(n-1)-a(n-2)+a(n-4); a(0) = a(1) = a(2) = a(3) = 1 . - Philippe Deléham, May 01 2006 lim(n->oo,a(n)/a(n+1))=(sqrt(5)-1)/2. - Sergei N. Gladkovskii, Jan 05 2014 G.f.: (1+ Q(0)*x^4/2)/(1-x), where Q(k) = 1 + 1/(1 - x*( 4*k+2 -x +x^3)/( x*( 4*k+4 -x +x^3) +1/Q(k+1) )); (continued fraction). - Sergei N. Gladkovskii, Jan 07 2014 a(n) = Fibonacci(n+1) + (-1)^(n+1)*A106511(n+2). - Katharine Ahrens, May 05 2019 MAPLE ZL:=[S, {a = Atom, b = Atom, S = Prod(X, Sequence(Prod(X, X))), X = Sequence(b, card >= 2)}, unlabelled]: seq(combstruct[count](ZL, size=n), n=2..40); # Zerinvary Lajos, Mar 26 2008 MATHEMATICA Table[Sum[Binomial[n-2k, 2k], {k, 0, Floor[n/4]}], {n, 0, 50}] (* or *) LinearRecurrence[{2, -1, 0, 1}, {1, 1, 1, 1}, 50] (* Harvey P. Dale, Dec 09 2011 *) Table[HypergeometricPFQ[{1/4-n/4, 1/2-n/4, 3/4-n/4, -n/4}, {1/2, 1/2-n/2, -n/2}, 16], {n, 0, 38}] (* Jean-François Alcover, Oct 04 2012 *) PROG (Haskell) a005252 n = sum \$ map (\x -> a007318 (n - x) x) [0, 2 .. 2 * div n 4] -- Reinhard Zumkeller, Jul 05 2013 (PARI) Vec((1-x)/((1-x-x^2)*(1-x+x^2)) + O(x^100)) \\ Altug Alkan, Jan 08 2015 (MAGMA) I:=[1, 1, 1, 1]; [n le 4 select I[n] else 2*Self(n-1)-Self(n-2)+Self(n-4): n in [1..50]]; // Vincenzo Librandi, Jan 09 2016 CROSSREFS First differences of A024490. Cf. A007318, A024490. Sequence in context: A035295 A289010 A006999 * A023430 A023429 A023428 Adjacent sequences:  A005249 A005250 A005251 * A005253 A005254 A005255 KEYWORD nonn,easy,nice AUTHOR EXTENSIONS More terms from (and formula corrected by) James A. Sellers, Feb 06 2000 Definition revised at the suggestion of Alessandro Orlandi by N. J. A. Sloane, Aug 16 2009 STATUS approved

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Last modified October 18 23:39 EDT 2019. Contains 328211 sequences. (Running on oeis4.)