%I #77 Nov 03 2024 09:32:39
%S 1,2,6,26,162,1442,18306,330626,8488962,309465602,16011372546,
%T 1174870185986,122233833963522,18023122242478082,3765668654914699266,
%U 1114515608405262434306,467221312005126294077442,277362415313453291571118082
%N Number of labeled graphs with 2-colored nodes where black nodes are only connected to white nodes and vice versa.
%C Row sums of A111636. - _Peter Bala_, Sep 30 2012
%C Column 2 of Table 2 in Read. - _Peter Bala_, Apr 11 2013
%C It appears that 5 does not divide a(n), that a(n) is even for n>0, that 3 divides a(2n) for n>0, that 7 divides a(6n+5), and that 13 divides a(12n+3). - _Ralf Stephan_, May 18 2013
%D H. S. Wilf, Generatingfunctionology, Academic Press, NY, 1990, p. 79, Eq. 3.11.2.
%H T. D. Noe, <a href="/A047863/b047863.txt">Table of n, a(n) for n = 0..50</a>
%H P. J. Cameron, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL3/groups.html">Sequences realized by oligomorphic permutation groups</a>, J. Integ. Seqs. Vol. 3 (2000), #00.1.5.
%H S. R. Finch, <a href="http://www.people.fas.harvard.edu/~sfinch/">Bipartite, k-colorable and k-colored graphs</a>
%H S. R. Finch, <a href="/A191371/a191371.pdf">Bipartite, k-colorable and k-colored graphs</a>, June 5, 2003. [Cached copy, with permission of the author]
%H A. Gainer-Dewar and I. M. Gessel, <a href="http://arxiv.org/abs/1304.0139">Enumeration of bipartite graphs and bipartite blocks</a>, arXiv:1304.0139 [math.CO], 2013.
%H D. A. Klarner, <a href="/A000798/a000798_11.pdf">The number of graded partially ordered sets</a>, J. Combin. Theory, 6 (1969), 12-19. [Annotated scanned copy]
%H Y. Puri and T. Ward, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL4/WARD/short.html">Arithmetic and growth of periodic orbits</a>, J. Integer Seqs., Vol. 4 (2001), #01.2.1.
%H R. C. Read, <a href="http://cms.math.ca/10.4153/CJM-1960-035-0">The number of k-colored graphs on labelled nodes</a>, Canad. J. Math., 12 (1960), 410-414.
%H R. P. Stanley, <a href="http://www-math.mit.edu/~rstan/pubs/pubfiles/18.pdf">Acyclic orientation of graphs</a> Discrete Math. 5 (1973), 171-178. North Holland Publishing Company.
%H Martin Svatoš, Peter Jung, Jan Tóth, Yuyi Wang, and Ondřej Kuželka, <a href="https://arxiv.org/abs/2302.04606">On Discovering Interesting Combinatorial Integer Sequences</a>, arXiv:2302.04606 [cs.LO], 2023, p. 17.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/k-ColorableGraph.html">k-Colorable Graph</a>
%H H. S. Wilf, <a href="http://www.math.upenn.edu/~wilf/DownldGF.html">Generatingfunctionology</a>, 2nd edn., Academic Press, NY, 1994, p. 88, Eq. 3.11.2.
%F a(n) = Sum_{k=0..n} binomial(n, k)*2^(k*(n-k)).
%F a(n) = 4 * A000683(n) + 2. - _Vladeta Jovovic_, Feb 02 2000
%F E.g.f.: Sum_{n>=0} exp(2^n*x)*x^n/n!. - _Paul D. Hanna_, Nov 27 2007
%F O.g.f.: Sum_{n>=0} x^n/(1 - 2^n*x)^(n+1). - _Paul D. Hanna_, Mar 08 2008
%F From _Peter Bala_, Apr 11 2013: (Start)
%F 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 is E(x)^2 = 1 + 2*x + 6*x^2/(2!*2) + 26*x^3/(3!*2^3) + .... In general, E(x)^k, k = 1, 2, ..., is a generating function for labeled k-colored graphs (see Stanley). For other examples see A191371 (k = 3) and A223887 (k = 4).
%F If A(x) = 1 + 2*x + 6*x^2/2! + 26*x^3/3! + ... denotes the e.g.f. for this sequence then sqrt(A(x)) = 1 + x + 2*x^2/2! + 7*x^3/3! + ... is the e.g.f. for A047864, which counts labeled 2-colorable graphs. (End)
%F a(n) ~ c * 2^(n^2/4+n+1/2)/sqrt(Pi*n), where c = Sum_{k = -infinity..infinity} 2^(-k^2) = EllipticTheta[3, 0, 1/2] = 2.128936827211877... if n is even and c = Sum_{k = -infinity..infinity} 2^(-(k+1/2)^2) = EllipticTheta[2, 0, 1/2] = 2.12893125051302... if n is odd. - _Vaclav Kotesovec_, Jun 24 2013
%e For n=2, {1,2 black, not connected}, {1,2 white, not connected}, {1 black, 2 white, not connected}, {1 black, 2 white, connected}, {1 white, 2 black, not connected}, {1 white, 2 black, connected}.
%e G.f. = 1 + 2*x + 6*x^2 + 26*x^3 + 162*x^4 + 1442*x^5 + 18306*x^6 + ...
%t Table[Sum[Binomial[n,k]2^(k(n-k)),{k,0,n}],{n,0,20}] (* _Harvey P. Dale_, May 09 2012 *)
%t nmax = 20; CoefficientList[Series[Sum[E^(2^k*x)*x^k/k!, {k, 0, nmax}], {x, 0, nmax}], x] * Range[0, nmax]! (* _Vaclav Kotesovec_, Jun 05 2019 *)
%o (PARI) {a(n)=n!*polcoeff(sum(k=0,n,exp(2^k*x +x*O(x^n))*x^k/k!),n)} \\ _Paul D. Hanna_, Nov 27 2007
%o (PARI) {a(n)=polcoeff(sum(k=0, n, x^k/(1-2^k*x +x*O(x^n))^(k+1)), n)} \\ _Paul D. Hanna_, Mar 08 2008
%o (PARI) N=66; x='x+O('x^N); egf = sum(n=0, N, exp(2^n*x)*x^n/n!);
%o Vec(serlaplace(egf)) \\ _Joerg Arndt_, May 04 2013
%o (Python)
%o from sympy import binomial
%o def a(n): return sum([binomial(n, k)*2**(k*(n - k)) for k in range(n + 1)]) # _Indranil Ghosh_, Jun 03 2017
%o (Magma)
%o A047863:= func< n | (&+[Binomial(n,k)*2^(k*(n-k)): k in [0..n]]) >;
%o [A047863(n): n in [0..40]]; // _G. C. Greubel_, Nov 03 2024
%o (SageMath)
%o def A047863(n): return sum(binomial(n,k)*2^(k*(n-k)) for k in range(n+1))
%o [A047863(n) for n in range(41)] # _G. C. Greubel_, Nov 03 2024
%Y Column k=2 of A322280.
%Y Cf. A135079 (variant).
%Y Cf. A000683, A001831, A002031, A033995, A047864, A052332, A111636, A191371, A223887.
%K nonn,nice,changed
%O 0,2
%A _N. J. A. Sloane_
%E Better description from _Christian G. Bower_, Dec 15 1999