

A008486


Expansion of (1 + x + x^2)/(1  x)^2.


61



1, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, 63, 66, 69, 72, 75, 78, 81, 84, 87, 90, 93, 96, 99, 102, 105, 108, 111, 114, 117, 120, 123, 126, 129, 132, 135, 138, 141, 144, 147, 150, 153, 156, 159, 162, 165, 168, 171, 174, 177, 180, 183, 186
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OFFSET

0,2


COMMENTS

Also the Engel expansion of exp^(1/3); cf. A006784 for the Engel expansion definition.  Benoit Cloitre, Mar 03 2002
Coordination sequence for planar net 6^3 (the graphite net, or the graphene crystal)  that is, the number of atoms at graph distance n from any fixed atom.  N. J. A. Sloane, Jan 06 2013
Coordination sequence for 2dimensional cyclotomic lattice Z[zeta_3].
Conjecture: This is also the maximum number of edges possible in a planar simple graph with n+2 vertices.  Dmitry Kamenetsky, Jun 29 2008
The conjecture is correct. Proof: For n=0 the theorem holds, the maximum planar graph has n+2=2 vertices and 1 edge. Now suppose that we have a connected planar graph with at least 3 vertices. If it contains a face that is not a triangle, we can add an edge that divides this face into two without breaking its planarity. Hence all maximum planar graphs are triangulations. Euler's formula for planar graphs states that in any planar simple graph with V vertices, E edges and F faces we have V+FE=2. If all faces are triangles, then F=2E/3, which gives us E=3V6. Hence for n>0 each maximum planar simple graph with n+2 vertices has 3n edges.  Michal Forisek, Apr 23 2009
a(n) = sum of natural numbers m such that n  1 <= m <= n + 1. Generalization: If a(n,k) = sum of natural numbers m such that n  k <= m <= n + k (k >= 1) then a(n,k) = (k + n)*(k + n + 1)/2 = A000217(k+n) for 0 <= n <= k, a(n,k) = a(n1,k) +2k + 1 = ((k + n  1)*(k + n)/2) + 2k + 1 = A000217(k+n1) +2k +1 for n >= k + 1 (see e.g. A008486).  Jaroslav Krizek, Nov 18 2009
a(n) = partial sums of A158799(n). Partial sums of a(n) = A005448(n).  Jaroslav Krizek, Dec 06 2009
Integers n dividing a(n) = a(n1)  a(n2) with initial conditions a(0)=0, a(1)=1 (see A128834 with offset 0).  Thomas M. Bridge, Nov 03 2013
a(n) is conjectured to be the number of polygons added after n iterations of the polygon expansions (type A, B, C, D & E) shown in the Ngaokrajang link. The patterns are supposed to become the planar Archimedean net 3.3.3.3.3.3, 3.6.3.6, 3.12.12, 3.3.3.3.6 and 4.6.12 respectively when n  > infinity.  Kival Ngaokrajang, Dec 28 2014
Number of reduced words of length n in Coxeter group on 3 generators S_i with relations (S_i)^2 = (S_i S_j)^3 = I.  Ray Chandler, Nov 21 2016
Conjecture: let m = n + 2, p is the polyhedron formed by the convex hull of m points, q is the number of quadrilateral faces of p (see the Wikipedia link below), and f(m) = a(n)  q. Then f(m) would be the solution of the Thompson problem for all m in 3space.  Sergey Pavlov, Feb 03 2017
Also, sequence defined by a(0)=1, a(1)=3, c(0)=2, c(1)=4; and thereafter a(n) = c(n1) + c(n2), and c consists of the numbers missing from a.  Ivan Neretin, Mar 28 2017


LINKS

Vincenzo Librandi, Table of n, a(n) for n = 0..10000
David Applegate, The movie version
M. Beck and S. Hosten, Cyclotomic polytopes and growth series of cyclotomic lattices, arXiv:math/0508136 [math.CO], 20052006.
J.G. Eon, Algebraic determination of generating functions for coordination sequences in crystal structures, Acta Cryst. A58 (2002), 4753.
A. S. Fraenkel, New games related to old and new sequences, INTEGERS, Electronic J. of Combinatorial Number Theory, Vol. 4, Paper G6, 2004. (See Table 5.)
Branko Grünbaum and Geoffrey C. Shephard, Tilings by regular polygons, Mathematics Magazine, 50 (1977), 227247.
Kival Ngaokrajang, Illustration of polygon expansions (planar net)
N. J. A. Sloane, The uniform planar nets and their Anumbers [Annotated scanned figure from Gruenbaum and Shephard (1977)]
University of Manchester, Graphene
Wikipedia, Thomson problem
Index entries for linear recurrences with constant coefficients, signature (2, 1).


FORMULA

a(0) = 1; a(n) = 3*n, n >= 1.
Euler transform of length 3 sequence [3, 0, 1].  Michael Somos, Aug 04 2009
a(n) = a(n1) + 3 for n >= 2.  Jaroslav Krizek, Nov 18 2009
a(n) = 0^n + 3*n.  Vincenzo Librandi, Aug 21 2011
a(n) = a(n) unless n = 0.  Michael Somos, May 05 2015


EXAMPLE

G.f. = 1 + 3*x + 6*x^2 + 9*x^3 + 12*x^4 + 15*x^5 + 18*x^6 + 21*x^7 + 24*x^8 + ...
From Omar E. Pol, Aug 20 2011: (Start)
Illustration of initial terms as triangles:
. o
. o o o
. o o o o o
. o o o o o o o
. o o o o o o o o o
. o o o o o o o o o o o o o o o o o o o o o
.
. 1 3 6 9 12 15
(End)


MATHEMATICA

CoefficientList[Series[(1 + x + x^2) / (1  x)^2, {x, 0, 80}], x] (* Vincenzo Librandi, Nov 23 2014 *)
a[ n_] := If[ n == 0, 1, 3 n]; (* Michael Somos, Apr 17 2015 *)


PROG

(PARI) {a(n) = if( n==0, 1, 3 * n)}; /* Michael Somos, May 05 2015 */
(MAGMA) [0^n+3*n: n in [0..90] ]; // Vincenzo Librandi, Aug 21 2011
(Haskell)
a008486 0 = 1; a008486 n = 3 * n
a008486_list = 1 : [3, 6 ..]  Reinhard Zumkeller, Apr 17 2015


CROSSREFS

Partial sums give A005448.
List of coordination sequences for uniform planar nets: A008458 (the planar net 3.3.3.3.3.3), A008486 (6^3), A008574(4.4.4.4 and 3.4.6.4), A008576 (4.8.8), A008579(3.6.3.6), A008706 (3.3.3.4.4), A072154 (4.6.12), A219529(3.3.4.3.4), A250120 (3.3.3.3.6), A250122 (3.12.12).
Cf. A008585.
Cf. A022424.
Sequence in context: A209258 A008585 A031193 * A135943 A194416 A036686
Adjacent sequences: A008483 A008484 A008485 * A008487 A008488 A008489


KEYWORD

nonn,easy


AUTHOR

N. J. A. Sloane


STATUS

approved



