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A006054 a(n) = 2*a(n-1) + a(n-2) - a(n-3).
(Formerly M1396)
53
0, 0, 1, 2, 5, 11, 25, 56, 126, 283, 636, 1429, 3211, 7215, 16212, 36428, 81853, 183922, 413269, 928607, 2086561, 4688460, 10534874, 23671647, 53189708, 119516189, 268550439, 603427359, 1355888968, 3046654856, 6845771321, 15382308530, 34563733525 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

Let u(k), v(k), w(k) be defined by u(1)=1, v(1)=0, w(1)=0 and u(k+1)=u(k)+v(k)+w(k), v(k+1)=u(k)+v(k), w(k+1)=u(k); then {u(n)} = 1,1,3,6,14,31,... (A006356 with an extra initial 1), {v(n)} = 0,1,2,5,11,25,... (this sequence with its initial 0 deleted) and {w(n)} = {u(n)} prefixed by an extra 0 = A077998 with an extra initial 0. - Benoit Cloitre, Apr 05 2002. Also u(k)^2+v(k)^2+w(k)^2 = u(2k). - Gary W. Adamson, Dec 23 2003

Form the graph with matrix A=[1, 1, 1; 1, 0, 0; 1, 0, 1]. Then A006054 counts walks of length n between the vertex of degree 1 and the vertex of degree 3. - Paul Barry, Oct 02 2004

Form the digraph with matrix [1,1,0;1,0,1;1,1,1]. A006054(n) counts walks of length n between the vertices with loops. - Paul Barry, Oct 15 2004

a(n), n>1 = round(k*A006356(n-1)), where k = 0.3568958678... = 1/(1+2*cos pi/7). - Gary W. Adamson, Jun 06 2008

Nonzero terms = INVERT transform of (1, 1, 2, 2, 3, 3,...). Example: 56 = (1, 1, 2, 5, 11, 25) dot (3, 3, 2, 2, 1, 1) = (3 + 3 + 4 + 10 + 11 + 25). - Gary W. Adamson, Apr 20 2009

-a(n+1) appears in the formula for the nonpositive powers of rho:= 2*cos(Pi/7), the ratio of the smaller diagonal in the heptagon to the side length s=2*sin(Pi/7), when expressed in the basis <1,rho,sigma>, with sigma:=rho^2-1, the ratio of the larger heptagon diagonal to the side length, as follows. rho^(-n) = C(n)*1+C(n-1)*rho-a(n+1)*sigma, n>=0, with C(n)=A077998(n), C(-1):=0. See the Steinbach reference, and a comment under A052547.

If, with the above notations, the power basis of the field Q(rho) is taken one has for nonpositive powers of rho, rho^(-n) = a(n+2)*1 + A077998(n-1)*rho - a(n+1)*rho^2. For nonnegative powers see A006053. See also the Steinbach reference. - Wolfdieter Lang, May 06 2011

a(n) appears also in the nonnegative powers of sigma,(defined in the above comment, where also the basis is given). See a comment in A106803.

The sequence b(n):=(-1)^(n+1)*a(n) forms the negative part (i.e. with nonpositive indices) of the sequence (-1)^n*A006053(n+1). In this way we obtain what we shall call the Ramanujan-type sequence number 2a for the argument 2Pi/7 (see the comment to Witula's formula in A006053). We have b(n)=-2*b(n-1)+b(n-2)+b(n-3) and b(n)*49^(1/3) = (c(1)/c(4))^(1/3)*(c(1))^(-n) + (c(2)/c(1))^(1/3)*(c(2))^(-n) + (c(4)/c(2))^(1/3)*(c(4))^(-n) = (c(2)/c(1))^(1/3)*(c(1))^(-n+1) + (c(4)/c(2))^(1/3)*(c(2))^(-n+1) + (c(1)/c(4))^(1/3)*(c(4))^(-n+1), where c(j) := 2*cos(2Pi*j/7) (for the proof see comments to A215112). - Roman Witula, Aug 06 2012

(1, 1, 2, 5, 11, 25, 56,...) * (1, 0, 1, 0, 1,...) = the variant of A006356: (1, 1, 3, 6, 14, 31,...). - Gary W. Adamson, May 15 2013

The limit of a(n+1)/a(n) for n -> infinity is, for all generic sequences with this recurrence of signature (2,1,-1), sigma = rho^2-1, approximately 2.246979603, the length ratio (largest diagonal)/side in the regular heptagon (7-gon). For rho = 2*cos(Pi/7) and sigma see a comment above, and the P. Steinbach reference. Proof: a(n+1)/a(n) = 2 + 1/(a(n)/a(n-1)) - 1/((a(n)/a(n-1))*(a(n-1)/a(n-2))), leading in the limit to sigma^3 -2*sigma^2 - sigma + 1, which is solved by sigma = rho^2-1, due to C(7, rho) = 0 , with the minimal polynomial C(7, x) = x^3 - x^2 - 2*x + 1 of rho (see A187360). - Wolfdieter Lang, Nov 07 2013

REFERENCES

Jay Kappraff, Beyond Measure, A Guided Tour Through Nature, Myth and Number, World Scientific, 2002.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..150

C. P. de Andrade, J. P. de Oliveira Santos, E. V. P. da Silva and K. C. P. Silva, Polynomial Generalizations and Combinatorial Interpretations for Sequences Including the Fibonacci and Pell Numbers, Open Journal of Discrete Mathematics, 2013, 3, 25-32 doi:10.4236/ojdm.2013.31006. - From N. J. A. Sloane, Feb 20 2013

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 434

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.

R. Sachdeva and A. K. Agarwal, Combinatorics of certain restricted n-color composition functions, Discrete Mathematics, 340, (2017), 361-372.

P. Steinbach, Golden fields: a case for the heptagon, Math. Mag. 70 (1997), no. 1, 22-31.

Alexey Ustinov, Supercontinuants, arXiv:1503.04497 [math.NT], 2015.

Roman Witula, Ramanujan Type Trigonometric Formulas: The General Form for the Argument 2*Pi/7, Journal of Integer Sequences, Vol. 12 (2009), Article 09.8.5.

R. Witula, D. Slota and A. Warzynski, Quasi-Fibonacci Numbers of the Seventh Order, J. Integer Seq., 9 (2006), Article 06.4.3.

Index entries for linear recurrences with constant coefficients, signature (2,1,-1).

FORMULA

G.f.: x^2/(1-2*x-x^2+x^3).

Sum_{k=0..n+2} a(k) = A077850(n). - Philippe Deléham, Sep 07 2006

Let M = the 3 X 3 matrix [1,1,0; 1,2,1; 0,1,2], then M^n*[1,0,0] = [A080937(n-1), A094790(n), A006054(n-1)]. E.g., M^3*[1,0,0] = [5,9,5] = [A080937(2), A094790(3), A006054(2)]. - Gary W. Adamson, Feb 15 2006

a(n+1) = A187070(2n+1) = A187068(2n+3). - L. Edson Jeffery, Mar 10 2011

a(n+3) = sum(k=1..n, sum(j=0..k, binomial(j,n-3*k+2*j)*(-1)^(j-k)*binomial(k,j)*2^(-n+3*k-j))), a(0)=0, a(1)=0, a(2)=1. - Vladimir Kruchinin, May 05 2011

7*a(n) = (c(2)-c(4))*(1+c(1))^n + (c(4)-c(1))*(1+c(2))^n + (c(1)-c(2))*(1+c(4))^n, where c(j):=2*Cos(2Pi*j/7) - for the proof see Witula et al. papers. - Roman Witula, Aug 07 2012

MAPLE

A006054:=z**2/(1-2*z-z**2+z**3); # Simon Plouffe in his 1992 dissertation

MATHEMATICA

LinearRecurrence[{2, 1, -1}, {0, 0, 1}, 60] (* Vladimir Joseph Stephan Orlovsky, Feb 10 2012 *)

PROG

(Maxima)

a(n):=if n<2 then 0 else if n=2 then 1 else b(n-2);

b(n):=sum(sum(binomial(j, n-3*k+2*j)*(-1)^(j-k)*binomial(k, j)*2^(-n+3*k-j), j, 0, k), k, 1, n); \\ Vladimir Kruchinin, May 05 2011

(PARI) x='x+O('x^66);

Vec(x^2/(1-2*x-x^2+x^3)) \\ Joerg Arndt, May 05 2011

(Haskell)

a006054 n = a006053_list !! n

a006054_list = 0 : 0 : 1 : zipWith (+) (map (2 *) $ drop 2 a006054_list)

   (zipWith (-) (tail a006054_list) a006054_list)

-- Reinhard Zumkeller, Oct 14 2011

CROSSREFS

Cf. A006356, A007583, A005578, A080937, A094790.

Row sums of A144159 and A180264.

Cf. A006053, A214683, A215112, A214699, A214779.

Sequence in context: A017919 A017920 A228765 * A106805 A094981 A239812

Adjacent sequences:  A006051 A006052 A006053 * A006055 A006056 A006057

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified July 20 22:51 EDT 2017. Contains 289629 sequences.