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 A058875 Triangle T(n,k) = C_n(k)/2^(k*(k-1)/2) where C_n(k) = number of k-colored labeled graphs with n nodes (n >= 1, 1 <= k <= n). 6
 1, 1, 1, 1, 6, 1, 1, 40, 24, 1, 1, 360, 640, 80, 1, 1, 4576, 24000, 7040, 240, 1, 1, 82656, 1367296, 878080, 62720, 672, 1, 1, 2122240, 122056704, 169967616, 23224320, 487424, 1792, 1, 1, 77366400, 17282252800, 53247344640, 13440516096 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS From Peter Bala, Apr 12 2013: (Start) A coloring of a simple graph G is a choice of color for each graph vertex such that no two vertices sharing the same edge have the same color. Let E(x) = Sum_{n >= 0} x^n/(n!*2^C(n,2)) = 1 + x + x^2/(2*2!) + x^3/(2^3*3!) + .... Read has shown that (E(x) - 1)^k is a generating function for labeled graphs on n nodes that can be colored using exactly k colors. Cases include A213441 (k = 2), A213442 (k = 3) and A224068 (k = 4). If colorings of a graph using k colors are counted as the same if they differ only by a permutation of the colors then a generating function is 1/k!*(E(x) - 1)^k , which is a generating function for the k-th column of A058843. Removing a further factor of 2^C(k,2) gives 1/(k!*2^C(k,2))*(E(x) - 1)^k as a generating function for the k-th column of this triangle. (End) REFERENCES F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 18, Table 1.5.1. LINKS Andrew Howroyd, Table of n, a(n) for n = 1..1275 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. Eric Weisstein's World of Mathematics, k-Colorable Graph FORMULA C_n(k) = Sum_{i=1..n-1} binomial(n, i)*2^(i*(n-i))*C_i(k-1)/k. From Peter Bala, Apr 12 2013: (Start) Recurrence equation: T(n,k) = 1/2^(k-1)*Sum_{i = 1..n-1} binomial(n-1,i)*2^(i*(n-i))*T(i,k-1). Let E(x) = Sum_{n >= 0} x^n/(n!*2^C(n,2)) = 1 + x + x^2/(2!*2) + x^3/(3!*2^3) + .... Then a generating function for this triangle is E(x*(E(z) - 1)) = 1 + x*z + (x + x^2 )*z^2/(2!*2) + (x + 6*x^2 + x^3)*z^3/(3!*2^3) + (x + 40*x^2 + 24*x^3 + x^4)*z^4/(4!*2^6) + .... Cf. A008277 with e.g.f. exp(x*(exp(z) - 1)). The row polynomials R(n,x) satisfy the recurrence equation R(n,x) = x*sum {k = 0..n-1} binomial(n-1,k)*2^(k*(n-k))*R(k,x/2) with R(0,x) = 1. The row polynomials appear to have only real zeros. Column 2 = 1/(2!*2)*A213441; column 3 = 1/(3!*2^3)*A213442; column 4 = 1/(4!*2^6)*A224068. (End) T(n,k) = A058843(n,k)/2^binomial(k,2). - Andrew Howroyd, Nov 30 2018 EXAMPLE Triangle begins:   1;   1,     1;   1,     6,       1;   1,    40,      24,      1;   1,   360,     640,     80,     1;   1,  4576,   24000,   7040,   240,   1;   1, 82656, 1367296, 878080, 62720, 672, 1;   ... MATHEMATICA maxn=8; t[_, 1]=1; t[n_, k_]:=t[n, k]=Sum[Binomial[n, j]*2^(j*(n-j))*t[j, k-1]/k, {j, 1, n-1}]; Flatten[Table[t[n, k]/2^Binomial[k, 2], {n, 1, maxn}, {k, 1, n}]]  (* Geoffrey Critzer, Oct 06 2012, after code from Jean-François Alcover in A058843 *) PROG (PARI) b(n)={n!*2^binomial(n, 2)} T(n, k)={b(n)*polcoef((sum(j=1, n, x^j/b(j)) + O(x*x^n))^k, n)/b(k)} \\ Andrew Howroyd, Nov 30 2018 CROSSREFS Apart from scaling, same as A058843. Columns give A058872 and A000683, A058873 and A006201, A058874 and A006202, also A006218. Cf. A213441, A213442, A224068. Sequence in context: A203338 A158116 A172343 * A156764 A156765 A015117 Adjacent sequences:  A058872 A058873 A058874 * A058876 A058877 A058878 KEYWORD nonn,easy,tabl AUTHOR N. J. A. Sloane, Jan 07 2001 STATUS approved

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Last modified December 14 17:41 EST 2019. Contains 329979 sequences. (Running on oeis4.)