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A002028 Number of connected graphs on n labeled nodes, each node being colored with one of 3 colors, such that no edge joins nodes of the same color.
(Formerly M2603 N1030)
0
1, 3, 6, 42, 618, 15990, 668526, 43558242, 4373213298, 677307561630, 162826875512646, 61183069270120842, 36134310487980825258, 33673533885068169649830, 49646105434209446798290206, 116002075479856331220877149042, 430053223599741677879550609246498, 2531493110297317758855120762121050990 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

REFERENCES

R. C. Read, personal communication.

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).

LINKS

Table of n, a(n) for n=0..17.

Read, R. C.; Wright, E. M., Colored graphs: A correction and extension, Canad. J. Math. 22 1970 594-596.

FORMULA

E.g.f.: log(A(x))+1 where A(x) is the E.g.f. for A191371. - Geoffrey Critzer, Jun 02 2011

a(n) = m_n(3) using the functions defined in A002032. - Sean A. Irvine, May 29 2013

MATHEMATICA

f[{k_, r_, m_}]:= Binomial[m+r+k, k] Binomial[m+r, r] 2^(k r +k m + r m);

  a = Sum[Total[Map[f, Compositions[n, 3]]] x^n/n!, {n, 0, 20}];

  Range[0, 20]! CoefficientList[Series[Log[a]+1, {x, 0, 20}], x] (* Geoffrey Critzer, Jun 02 2011 *)

CROSSREFS

Cf. A002031, A002032.

Sequence in context: A079095 A125889 A300599 * A202858 A116315 A001886

Adjacent sequences:  A002025 A002026 A002027 * A002029 A002030 A002031

KEYWORD

nonn

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified October 20 17:24 EDT 2018. Contains 316392 sequences. (Running on oeis4.)