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A000686 Number of 4-colored labeled graphs on n nodes, divided by 4.
(Formerly M4449 N1884)
4
1, 7, 85, 1777, 63601, 3882817, 403308865, 71139019777, 21276992674561, 10778161937857537, 9238819435213784065, 13390649605615389843457, 32796747486424209782108161, 135669064080920007649863745537, 947468281528010179181982467702785, 11166618111585805201637975219611631617 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Sequence represents 1/4 of the number of 4-colored labeled graphs on n nodes. Indeed, on p. 413 of the Read paper, column 4 is 4, 28, 340, 7108, ... - Emeric Deutsch, May 06 2004

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=1..16.

S. R. Finch, Bipartite, k-colorable and k-colored graphs (4*A000686)

R. C. Read, The number of k-colored graphs on labelled nodes, Canad. J. Math., 12 (1960), 410—414.

R. C. Read, Letter to N. J. A. Sloane, Oct. 29, 1976

FORMULA

a(n) = (1/4)Sum_{k=0..n} binomial(n, k)*2^(k(n-k))*b(k), where b(0)=1 and b(k)=3*A000685(k) for k > 0. - Emeric Deutsch, May 06 2004

From Peter Bala, Apr 12 2013: (Start)

a(n) = (1/4)*A223887(n).

a(n) = (1/4)*Sum_{k = 0..n} binomial(n,k)*2^(k*(n-k))*b(k)*b(n-k), where b(n) := Sum_{k = 0..n} binomial(n,k)*2^(k*(n-k)).

Let E(x) = Sum_{n >= 0} x^n/(n!*2^C(n,2)). Then a generating function for this sequence is 1/4*(E(x)^4 - 1) = Sum_{n >= 0} a(n)*x^n/(n!*2^C(n,2)) = x + 7*x^2/(2!*2) + 85*x^3/(3!*2^3) + .... (End)

MATHEMATICA

b[n_] := Sum[ 2^((i-j)*j + i*(n-i))*Binomial[n, i]*Binomial[i, j], {i, 0, n}, {j, 0, i}]; a[n_] := 1/4*Sum[ Binomial[n, k]*2^(k*(n-k))*b[k], {k, 0, n}]; Table[a[n], {n, 1, 14}] (* Jean-François Alcover, Dec 07 2011, after Emeric Deutsch *)

PROG

(PARI)

N=66;  x='x+O('x^N);

E=sum(n=0, N, x^n/(n!*2^binomial(n, 2)) );

tgf=E^4-1;  v=Vec(tgf);

v=vector(#v, n, v[n] * n! * 2^(n*(n-1)/2) ) / 4

/* Joerg Arndt, Apr 10 2013 */

CROSSREFS

Cf. A000683, A000685, A223887.

Sequence in context: A060237 A000424 A207214 * A102923 A220246 A196257

Adjacent sequences:  A000683 A000684 A000685 * A000687 A000688 A000689

KEYWORD

nonn,easy,nice

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from Pab Ter (pabrlos(AT)yahoo.com) and Emeric Deutsch, May 05 2004

STATUS

approved

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Last modified February 20 00:28 EST 2019. Contains 320329 sequences. (Running on oeis4.)