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A253186 Number of connected unlabeled multigraphs with 3 vertices and n edges. 4
0, 0, 1, 2, 3, 4, 6, 7, 9, 11, 13, 15, 18, 20, 23, 26, 29, 32, 36, 39, 43, 47, 51, 55, 60, 64, 69, 74, 79, 84, 90, 95, 101, 107, 113, 119, 126, 132, 139, 146, 153, 160, 168, 175, 183, 191, 199, 207, 216, 224, 233, 242, 251, 260, 270, 279, 289, 299, 309, 319, 330 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

COMMENTS

a(n) is also the number of ways to partition n into 2 or 3 parts.

a(n) is also the dimension of linear space of three-dimensional 2n-homogeneous polynomial vector fields, which have an octahedral symmetry (for a given representation), which are solenoidal, and which are vector fields on spheres. - Giedrius Alkauskas, Sep 30 2017

Apparently a(n) = A244239(n-6) for n > 4. - Georg Fischer, Oct 09 2018

LINKS

Danny Rorabaugh, Table of n, a(n) for n = 0..10000

Giedrius Alkauskas, Projective and polynomial superflows. I, arxiv.org/1601.06570 [math.AG] (2017), Section 5.3.

R. J. Mathar, Statistics on Small Graphs, arXiv:1709.09000  [math.CO] (2017) Eq. (23).

Gordon Royle, Small Multigraphs

Index entries for linear recurrences with constant coefficients, signature (1,1,0,-1,-1,1).

FORMULA

a(n) = A004526(n) + A069905(n).

a(n) = floor(n/2) + floor((n^2 + 6)/12).

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

EXAMPLE

On vertex set {a, b, c}, every connected multigraph with n = 5 edges is isomorphic to a multigraph with one of the following a(5) = 4 edge multisets: {ab, ab, ab, ab, ac}, {ab, ab, ab, ac, ac}, {ab, ab, ab, ac, bc}, and {ab, ab, ac, ac, bc}.

MATHEMATICA

CoefficientList[Series[- x^2 (x^3 - x - 1) / ((1 - x) (1 - x^2) (1 - x^3)), {x, 0, 70}], x] (* Vincenzo Librandi, Mar 24 2015 *)

LinearRecurrence[{1, 1, 0, -1, -1, 1}, {0, 0, 1, 2, 3, 4}, 61] (* Robert G. Wilson v, Oct 11 2017 *)

a[n_]:=Floor[n/2] + Floor[(n^2 + 6)/12]; Array[a, 70, 0] (* Stefano Spezia, Oct 09 2018 *)

PROG

(Sage) [floor(n/2) + floor((n^2 + 6)/12) for n in range(70)]

(MAGMA) [Floor(n/2) + Floor((n^2 + 6)/12): n in [0..70]]; // Vincenzo Librandi, Mar 24 2015

CROSSREFS

Cf. A001399, A003082, A014395-A014398, A050535, A076864. Column 3 of A191646.

Sequence in context: A064313 A011865 A085680 * A321195 A249020 A258470

Adjacent sequences:  A253183 A253184 A253185 * A253187 A253188 A253189

KEYWORD

nonn,easy

AUTHOR

Danny Rorabaugh, Mar 23 2015

STATUS

approved

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Last modified April 26 09:52 EDT 2019. Contains 322472 sequences. (Running on oeis4.)