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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). 1
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

Table of n, a(n) for n=1..41.

S. R. Finch, Bipartite, k-colorable and k-colored graphs

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)

EXAMPLE

1; 1,1; 1,6,1; 1,40,24,1; ...

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-Fran├žois Alcover in A058843 *)

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 November 15 11:35 EST 2018. Contains 317238 sequences. (Running on oeis4.)