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 A223887 Number of 4-colored labeled graphs on n vertices. 9
 1, 4, 28, 340, 7108, 254404, 15531268, 1613235460, 284556079108, 85107970698244, 43112647751430148, 36955277740855136260, 53562598422461559373828, 131186989945696839128432644, 542676256323680030599454982148 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS A simple graph G is a k-colorable graph if it is possible to assign one of k' <= k colors to each vertex of G so that no two adjacent vertices receive the same color. Such an assignment of colors is called a coloring function for the graph. A k-colored graph is a k-colorable graph together with its coloring function. This sequence gives the number of labeled 4-colored graphs on n vertices. An example is given below. See A047863 for labeled 2-colored graphs on n vertices and A191371 for labeled 3-colored graphs on n vertices. See A076316 for labeled 4-colorable graphs on n vertices and A224068 for the count of labeled graphs colored using exactly 4 colors. REFERENCES F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, 1973. LINKS Andrew Howroyd, Table of n, a(n) for n = 0..50 S. R. Finch, Bipartite, k-colorable and k-colored graphs S. R. Finch, Bipartite, k-colorable and k-colored graphs, June 5, 2003. [Cached copy, with permission of the author] R. C. Read, The number of k-colored graphs on labelled nodes, Canad. J. Math., 12 (1960), 410—414. R. P. Stanley, Acyclic orientation of graphs Discrete Math. 5 (1973), 171-178. North Holland Publishing Company. Eric Weisstein's World of Mathematics, k-Colorable Graph FORMULA a(n) = 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 E(x)^4 = sum {n >= 0} a(n)*x^n/(n!*2^C(n,2)) = 1 + 4*x + 28*x^2/(2!*2) + 340*x^3/(3!*2^3) + .... In general, for k = 1, 2, ..., E(x)^k is a generating function for labeled k-colored graphs (see Stanley). EXAMPLE a(2) = 28: There are two labeled 4-colorable graphs on 2 nodes, namely A)  1    2  B)  1    2     o    o      o----o Using 4 colors there are 16 ways to color the graph of type A and 4*3 = 12 ways to color the graph of type B so that adjacent vertices do not share the same color. Thus there are in total 28 labeled 4-colored graphs on 2 vertices. PROG (PARI) N=66;  x='x+O('x^N); E=sum(n=0, N, x^n/(n!*2^binomial(n, 2)) ); tgf=E^4;  v=concat(Vec(tgf)); v=vector(#v, n, v[n] * (n-1)! * 2^((n-1)*(n-2)/2) ) /* Joerg Arndt, Apr 10 2013 */ CROSSREFS Column k=4 of A322280. Equals 4 * A000686, A047863 (labeled 2-colored graphs), A076316, A191371 (labeled 3-colored graphs), A224068. Sequence in context: A113371 A280570 A080636 * A140425 A193198 A165193 Adjacent sequences:  A223884 A223885 A223886 * A223888 A223889 A223890 KEYWORD nonn,easy AUTHOR Peter Bala, Apr 10 2013 STATUS approved

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Last modified January 25 03:49 EST 2020. Contains 331241 sequences. (Running on oeis4.)