0,3

For n>0, a(n) is the degree (n-1) "numbral" power of 5 (see A048888 for the definition of numbral arithmetic). Example: a(3)=21, since the numbral square of 5 is 5(*)5 = 101(*)101(base 2) = 101 OR 10100 = 10101(base 2) = 21, where the OR is taken bitwise. - John W. Layman, Dec 18 2001

a(n) is composite for all n > 2 and has factors x, (3*x+2*(-1)^n) where x belongs to A001045. In binary the terms>0 are 1, 101, 10101, 1010101, etc. - John McNamara, Jan 16 2002

Number of n X 2 binary arrays with path of adjacent 1's from upper left corner to right column. - R. H. Hardin, Mar 16 2002

Collatz-function iteration started at with a(n) will surely end at 1 in 2*n steps. - Labos Elemer, Sep 30 2002

Second binomial transform of A001045. - Paul Barry, Mar 28 2003

All members of sequence are also generalized octagonal numbers (A001082). - Matthew Vandermast, Apr 10 2003

Also sum of squares of divisors of 2^n: a(n) = A001157(A000079(n)). - Paul Barry, Apr 11 2003

Binomial transform of A000244 (with leading zero). - Paul Barry, Apr 11 2003

Number of walks of length 2n between two vertices at distance 2 in the cycle graph C_6. For n=2 we have for example 5 walks of length 4 from vertex A to C: ABABC, ABCBC, ABCDC, AFABC and AFEDC. - Herbert Kociemba, May 31 2004

Also number of walks of length 2n+1 between two vertices at distance 3 in the cycle graph C_12. - Herbert Kociemba, Jul 05 2004

a(n+1) is the number of steps that are made when generating all n-step random walks that begin in a given point P on a two-dimensional square lattice. To make one step means to mark one vertex on the lattice (compare A080674). - Pawel P. Mazur (Pawel.Mazur(AT)pwr.wroc.pl), Mar 13 2005

a(n+1) = sum of square divisors of 4^n. - Paul Barry, Oct 13 2005

a(n+1) is the decimal number generated by the binary bits in the n-th generation of the Rule 250 elementary cellular automaton. - Eric W. Weisstein, Apr 08 2006

a(k) = [M^k]_2,1, where M is the 3 by 3 matrix defined as follows: M = [1,1,1;1,3,1;1,1,1]. - Simone Severini, Jun 11 2006

a(n-1) / a(n) = percentage of wasted storage if a single image is stored as a pyramid with a each subsequent higher resolution layer containing four times as many pixels as the previous layer. n is the number of layers. - Victor Brodsky (victorbrodsky(AT)gmail.com), Jun 15 2006

n is in the sequence if and only if C(4n+1,n) (A052203) is odd. - Paul Barry, Mar 26 2007

This sequence also gives the number of distinct 3-colorings of the odd cycle C(2*n-1). - Keith Briggs (keith.briggs(AT)bt.com), Jun 19 2007

All numbers of the form n*4^n+(4^n-1)/3 have the property that they are sums of two squares and also their indices are the sum of two squares. This follows from the identity n*4^n + (4^n-1)/3 = 4(4(..4(4n+1)+1)+1)+1..)+1. - Artur Jasinski, Nov 12 2007

Successive numbers contain only the digit 1 in base 4 positional system: 1, 11, 111, 1111, etc. - Artur Jasinski, Sep 30 2008

Let A be the Hessenberg matrix of order n, defined by: A[1,j]=1, A[i,i]:=5, (i>1), A[i,i-1]=-1, and A[i,j]=0 otherwise. Then, for n>=1, a(n)=charpoly(A,1). - Milan Janjic, Jan 27 2010

This is the sequence A(0,1;3,4;2) = A(0,1;4,0;1) of the family of sequences [a,b:c,d:k] considered by G. Detlefs, and treated as A(a,b;c,d;k) in the W. Lang link given below. - Wolfdieter Lang, Oct 18 2010

6*a(n)+1 = every second Mersenne number >=M3 hence all Mersenne primes greater than M2 must be a 6*a(n)+1 of this sequence. - Roderick MacPhee, Nov 01 2010

From the MathWorld PrimeKnot entry: "Let N(n) be the number of distinct prime knots with n crossings, counting chiral versions of the same knot separately. Then (((2^(n-2)-1)/3 =< N(n) =< e^n, according to Ernst and Summers." Note that (2^n-1)/3 is an integer only when n is even, hence this relates to (4^n - 1)/3. - Jonathan Vos Post, Nov 21 2010

Smallest number having alternating bit sum n. Cf. A065359.

For n=1,2,..., the last digit of a(n) is 1,5,1,5,... . - Washington Bomfim, Jan 21 2011

Rule 50 elementary cellular automaton generates this sequence. This sequence also appears in the second column of array in A173588. - Paul Muljadi, Jan 27 2011

Sequence found by reading the line from 0, in the direction 0, 5,... and the line from 1, in the direction 1, 21,..., in the square spiral whose edges are the Jacobsthal numbers A001045 and whose vertices are the numbers A000975. These parallel lines are two semi-diagonals in the spiral. - Omar E. Pol, Sep 10 2011

a(n), n>=1, is also the inverse of 3, denoted by 3^(-1), Modd(2^(2*n-1)). For Modd n see a comment on A203571. E.g., a(2)=5, 3*5=15==1 (Modd 8), because floor(15/8)=1 is odd and -15==1 (mod 8). For n=1 note that 3*1=3==1 (Modd 2) because floor(3/2)=1 and -3==1 (mod 2). The inverse of 3 taken Modd 2^(2*n) coincides with 3^(-1) (mod 2^(2*n)) given in A007583(n), n>=1. - Wolfdieter Lang, Mar 12 2012

If an AVL tree has a leaf at depth n, then the tree can contain no more than a(n+1) nodes total. - Mike Rosulek, Nov 20 2012

Also, this is the Lucas sequence V(5,4). - Bruno Berselli, Jan 10 2013

Also, for n > 0, a(n) is odd number whose Collatz trajectory contains no odd number other than n and 1. - Jayanta Basu, Mar 24 2013

Sum_{n>=1} 1/a(n) converges to (3*(log[4/3]-QPolyGamma[0,1,1/4]))/log[4] = 1.26329305810027133188797266393139146884324008972340089723642183177... - K. G. Stier, Jun 23 2014

Consider n spheres in R^n: the i-th one (i=1, ..., n) has radius r(i) = 2^(1-i) and the coordinates of its center are (0, 0, ..., 0, r(i), 0, ..., 0) where r(i) is in position i. The coordinates of the intersection point in the positive orthant of these spheres are (2/a(n), 4/a(n), 8/a(n), 16/a(n), ...). For example in R^2, circles centered at (1, 0) and (0, 1/2), and with radii 1 and 1/2, meet at (2/5, 4/5). - Jean M. Morales, May 19 2015

A. Fletcher, J. C. P. Miller, L. Rosenhead and L. J. Comrie, An Index of Mathematical Tables. Vols. 1 and 2, 2nd ed., Blackwell, Oxford and Addison-Wesley, Reading, MA, 1962, Vol. 1, p. 112.

J. Riordan, Combinatorial Identities, Wiley, 1968, p. 217.

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

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

T. D. Noe, Table of n, a(n) for n=0..200

H. Bottomley, Illustration of initial terms

A. Frosini and S. Rinaldi, On the Sequence A079500 and Its Combinatorial Interpretations, J. Integer Seq., Vol. 9 (2006), Article 06.3.1.

INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 373

Sung-Hyuk Cha, On Complete and Size Balanced k-ary Tree Integer Sequences, International Journal of Applied Mathematics and Informatics, Issue 2, Volume 6, 2012, pp. 67-75. - From N. J. A. Sloane, Dec 24 2012

C. Ernst and D. W. Sumners, The Growth of the Number of Prime Knots, Math. Proc. Cambridge Philos. Soc. 102, 303-315, 1987

E. Estrada and J. A. de la Pena, Integer sequences from walks in graphs, Notes on Number Theory and Discrete Mathematics, Vol. 19, 2013, No. 3, 78-84

R. Flórez, R. A. Higuita, A. Mukherjee, Alternating Sums in the Hosoya Polynomial Triangle, Article 14.9.5 Journal of Integer Sequences, Vol. 17 (2014).

Wolfdieter Lang, Notes on certain inhomogeneous three term recurrences.

J. V. Leyendekkers and A.G. Shannon, Modular Rings and the Integer 3, Notes on Number Theory & Discrete Mathematics, 17 (2011), 47-51.

Mircea Merca, A Note on Cosine Power Sums J. Integer Sequences, Vol. 15 (2012), Article 12.5.3.

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.

A. G. Shannon, Some recurrence relations for binary sequence matrices, NNTDM 17 (2011), 4, 913. - From N. J. A. Sloane, Jun 13 2012

T. N. Thiele, Interpolationsrechnung, Teubner, Leipzig, 1909, p. 35.

Eric Weisstein's World of Mathematics, Repunit, Rule 250, Prime Knot.

Wikipedia, Elementary cellular automaton

Wikipedia, Lucas sequence: Specific names.

Index entries for linear recurrences with constant coefficients, signature (5,-4).

a(n+1) = Sum_{m=0..n} A060921(n, m). G.f.: x/((1-x)*(1-4*x)). - Wolfdieter Lang, Apr 24 2001

a(n) = Sum_{k=0..n-1} 4^k; a(n) = A001045(2*n). - Paul Barry, Mar 17 2003

E.g.f.: (exp(4*x)-exp(x))/3. - Paul Barry, Mar 28 2003

a(n) = (A007583(n)-1)/2. - N. J. A. Sloane, May 16 2003

a(n) = A000975(2*n)/2. - N. J. A. Sloane, Sep 13 2003

a(n) = A084160(n)/2. - N. J. A. Sloane, Sep 13 2003

a(n+1) = 4*a(n) + 1, with a(0) = 0. - Philippe Deléham, Feb 25 2004

a(n) = Sum_{i=0..n-1} C(2*n-1-i, i)*2^i. - Mario Catalani (mario.catalani(AT)unito.it), Jul 23 2004

a(n+1) = Sum_{k=0..n} binomial(n+1, k+1)*3^k. - Paul Barry, Aug 20 2004

a(n) = center term in M^n * [1 0 0], where M = the 3 X 3 matrix [1 1 1 / 1 3 1 / 1 1 1]. M^n * [1 0 0] = [A007583(n-1) a(n) A007583(n-1)]. E.g., a(4) = 85 since M^4 * [1 0 0] = [43 85 43] = [A007583(3) a(4) A007583(3)]. - Gary W. Adamson, Dec 18 2004

a(n) = Sum_{k=0..n}{j=0..n} C(n, j)*C(j, k)*A001045(j-k). - Paul Barry, Feb 15 2005

a(n) = Sum_{k=0..n} C(n, k)*A001045(n-k)*2^k = Sum_{k=0..n} C(n, k)*A001045(k)*2^(n-k). - Paul Barry, Apr 22 2005

a(n) = A125118(n,3) for n>2. - Reinhard Zumkeller, Nov 21 2006

a(n) = Sum_{k=0..n} 2^(n-k)*A128908(n,k), n>=1. - Philippe Deléham, Oct 19 2008

a(n) = Sum_{k=0..n} A106566(n,k)*A100335(k). - Philippe Deléham, Oct 30 2008

If we define f(m,j,x) = Sum_{k=j..m} binomial(m,k)*stirling2(k,j)*x^(m-k) then a(n-1)=f(2*n,4,-2), n>=2. - Milan Janjic, Apr 26 2009

a(n) = A014551(n) * A001045(n). - R. J. Mathar, Jul 08 2009

a(n) = 4*a(n-1) + a(n-2) - 4*a(n-3) = 5*a(n-1) - 4*a(n-2), a(0)=0, a(1)=1, a(2)=5. - Wolfdieter Lang, Oct 18 2010

a(0) = 0, a(n+1) = a(n)+2^(2*n). - Washington Bomfim, Jan 21 2011

A036555(a(n)) = 2*n. - Reinhard Zumkeller, Jan 28 2011

a(n) = Sum_{k=1..floor((n+2)/3)} C(2*n+1,n+2-3*k). - Mircea Merca, Jun 25 2011

a(n) = Sum_{i=1..n} binomial(2n+1,2i)/3. - Wesley Ivan Hurt, Mar 14 2015

[seq((4^n-1)/3, n=0..40)];

A002450:=1/(4*z-1)/(z-1); # Simon Plouffe in his 1992 dissertation, dropping the initial zero

Table[(4^n-1)/3, {n, 0, 100}] (* Vladimir Joseph Stephan Orlovsky, Sep 29 2008 *)

LinearRecurrence[{5, -4}, {0, 1}, 30] (* Harvey P. Dale, Jun 23 2013 *)

(MAGMA) [ (4^n-1)/3: n in [0..25] ]; // Klaus Brockhaus, Oct 28 2008

(PARI) a(n) = (4^n-1)/3;

(Haskell)

a002450 = (`div` 3) . a024036

a002450_list = iterate ((+ 1) . (* 4)) 0

-- Reinhard Zumkeller, Oct 03 2012

(Maxima) makelist((4^n-1)/3, n, 0, 30); /* Martin Ettl, Nov 05 2012 */

(MAGMA) [n le 2 select n-1 else 5*Self(n-1)-4*Self(n-2): n in [1..70]]; // Vincenzo Librandi, Jun 13 2015

Partial sums of powers of 4, A000302.

When converted to binary, produces A094028.

Cf. A002446, A024036, A020988, A080674, A047849, A007583, A080355, A112627, A113860, A129735, A018215.

Cf. A160967, A139391.

Subsequence of A003714.

Primitive factors: A129735.

Sequence in context: A255451 A028948 A084241 * A187063 A026855 A097113

Adjacent sequences:  A002447 A002448 A002449 * A002451 A002452 A002453

nonn,easy,nice

N. J. A. Sloane

More terms from Artur Jasinski, Jan 27 2006 and Nov 12 2007

approved