0,2

Molien series for ternary self-dual codes over GF(3) of length 12n containing 11...1.

(1+x)*(1+x^2) / ((1-x)*(1-x^2)*(1-x^3)) is the Poincaré series [or Poincare series] (or Molien series) for H^*(O_3(q); F_2).

a(n) is the position of the n-th triangular number in the running sum of the (pseudo-Orloj) sequence 1,2,1,2,1,2,1...., cf. A028355. - Wouter Meeussen, Mar 10 2002

a(n) = [a(n-1) + (number of even terms so far in the sequence)]. Example: 14 is [10 + 4 even terms so far in the sequence (they are 0,2,4,10)]. See A096777 for the same construction with odd integers. - Eric Angelini, Aug 05 2007

The number of partitions of 2*n into at most 3 parts. - Colin Barker, Mar 31 2015

Also a(n) equals the number of linearly-independent terms at 2n-th order in the power series expansion of a trigonal Rotational Energy Surface. An optimal basis for the expansion follows either decomposition: g1(x) = (1+x)(1+x^2)g2(x) or g1(x) = (1+x^2)x^(-1)g3(x), where g1(x), g2(x), g3(x) are the generating functions for sequences A007980, A001399, A001840. - Bradley Klee, Aug 06 2015

Also a(n) equals the number of linearly-independent terms at 4n-th order in the power series expansion of the symmetrized weight enumerator of a self-dual code of length n over Z4 that contains a vector (+/-)1^n and has all norms divisible by 8. An optimal basis for the expansion follows the decomposition: g1(x) = (1+x)(1+x^2)g2(x) where g1(x), g2(x) are the generating functions for sequences A007980, A001399. (Cf. Calderbank and Sloane, Corollary 5.) - Bradley Klee, Aug 06 2015

Also, a(n) is equal to the number of partitions of 2n+3 of length 3. Letting n=4, there are a(4)=10 partitions of 2n+3=11 of length 3: (9,1,1), (8,2,1), (7,3,1), (7,2,2), (6,4,1), (6,3,2), (5,5,1), (5,4,2), (5,3,3), (4,4,3). - John M. Campbell, Jan 30 2016

a(n) is the number of partitions of n into parts 1 (of two kinds), part 2 (occurring at most once), and parts 3. - Joerg Arndt, Oct 12 2020

Conjecture: a(n) is the maximum number of pieces a triangle can be cut into by n cevians. - Anton Zakharov, Apr 04 2017

Zakharov's conjecture is true (see Fried link). - Sela Fried, Sep 25 2025

Also, a(n) is the number of graphs which are double-triangle descendants of K_5 with n+6 triangles and 3 more vertices than triangles. See Laradji/Mishna/Yeats reference, proposition 3.6 for details. - Karen A. Yeats, Feb 21 2020

A. Adem and R. J. Milgram, Cohomology of Finite Groups, Springer-Verlag, 2nd. ed., 2004; p. 233.

Vincenzo Librandi, Table of n, a(n) for n = 0..1000

Raghavendra N. Bhat, Cristian Cobeli, and Alexandru Zaharescu, A lozenge triangulation of the plane with integers, arXiv:2403.10500 [math.NT], 2024.

A. R. Calderbank and N. J. A. Sloane, Double circulant codes over Z_4, J. Algeb. Combin., 6 (1997) 119-131 (Abstract, pdf, ps).

Sela Fried, On cevians in a triangle, 2025.

Sela Fried, Proofs of Ten Conjectures From the OEIS, Journal of Integer Sequences, Vol. 29 (2026), Article 26.1.8. See pp. 16-18.

Mohamed Laradji, Marni Mishna, and Karen Yeats, Some results on double triangle descendants of K_5, arXiv:1904.06923 [math.CO], 2019.

C. L. Mallows and N. J. A. Sloane, Weight enumerators of self-orthogonal codes over GF(3), SIAM J. Alg. Discrete Methods, 2 (1981), 452-460.

Paul Tabatabai and Dieter P. Gruber, Knights and Liars on Graphs, J. Int. Seq., Vol. 24 (2021), Article 21.5.8.

Anton Zakharov, cevians.

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

Index entries for Molien series.

Index entries for two-way infinite sequences.

G.f.: (1 + x^2) / ((1 - x)^2 * (1 - x^3)). - Michael Somos, Jun 07 2003

a(n) = a(n-1) + a(n-3) -a(n-4) + 2 = a(-3-n) for all n in Z. - Michael Somos, Jun 07 2003

a(n) = ceiling((n+1)*(n+2)/3). - Paul Boddington, Jan 26 2004

a(n) = A192736(n+1) / (n+1). - Reinhard Zumkeller, Jul 08 2011

From Bruno Berselli, Oct 22 2010: (Start)

a(n) = ((n+1)*(n+2)+(2*cos(2*Pi*n/3)+1)/3)/3 = Sum_{i=1..n+1} A004396(i).

a(n) = 2*a(n-1) - a(n-2) + a(n-3) - 2*a(n-4) + a(n-5) for n>4.

a(n) = A002378(n+1)/3 if 3 divides A002378(n+1), a(n) = (A002378(n)+1)/3 otherwise. (End)

a(n) = A001840(n+1) + A001840(n-1). - R. J. Mathar, Aug 23 2015

From Michael Somos, Aug 23 2015: (Start)

Euler transform of length 4 sequence [2, 1, 1, -1].

a(n) = A001399(2*n) = A008796(2*n) = A008796(2*n + 3) = A069905(2*n + 3) = A211540(2*n + 5).

a(2*n) = A238705(n+1).

a(3*n - 1) = A049451(n).

a(3*n) = A003215(n).

a(3*n + 1) = A049450(n+1).

2*a(3*n - 1) = A005449(n).

2*a(3*n + 1) = A000326(n+1).

a(n+1) - a(n) = A004396(n+2). (End)

a(n) = floor((n^2+3*n+3)/3). - Giacomo Guglieri, May 01 2019

a(n) = A000212(n) + n+1. - Yuchun Ji, Oct 12 2020

Sum_{n>=0} 1/a(n) = (tanh(Pi/(2*sqrt(3)))-1)*Pi/sqrt(3) + 3. - Amiram Eldar, May 20 2023

E.g.f.: (exp(x)*(7 + 12*x + 3*x^2) + 2*exp(-x/2)*cos(sqrt(3)*x/2))/9. - Stefano Spezia, Apr 08 2026

G.f. = 1 + 2*x + 4*x^2 + 7*x^3 + 10*x^4 + 14*x^5 + 19*x^6 + 24*x^7 + ...

with (combinat):seq(count(Partition((2*n+1)), size=3), n=1..56); # Zerinvary Lajos, Mar 28 2008

Table[Ceiling[n (n+1)/3], {n, 56}]

CoefficientList[Series[(1+x^2)/((1-x)^2*(1-x^3)), {x, 0, 60}], x] (* Vincenzo Librandi, Feb 25 2012 *)

a[ n_] := Quotient[ n^2, 3] + n + 1; (* Michael Somos, Aug 23 2015 *)

LinearRecurrence[{2, -1, 1, -2, 1}, {1, 2, 4, 7, 10}, 60] (* Harvey P. Dale, Aug 24 2016 *)

(PARI) {a(n) = if( n<-1, a(-3-n), polcoeff( (1 + x^2) / ( (1 - x)^2 * (1 - x^3)) + x*O(x^n), n))}; /* Michael Somos, Jun 07 2003 */

(PARI) {a(n) = n^2\3 + n+1}; /* Michael Somos, Aug 23 2015 */

(PARI) a(n) = #partitions(2*n, , [1, 3]); \\ Michel Marcus, Feb 12 2016

(PARI) a(n) = #partitions(2*n+3, , [3, 3]); \\ Michel Marcus, Feb 12 2016

Cf. A000326, A001399, A001840, A002378, A003215, A004396, A005449, A007980.

Cf. A008796, A028355, A049450, A069905, A096777, A192736, A211540, A238705.

Sequence in context: A055607 A024512 A047808 * A022339 A025711 A117634

Adjacent sequences: A007977 A007978 A007979 * A007981 A007982 A007983

nonn,easy,changed

approved