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 A179534 Number of labeled split graphs on n vertices. 2
 1, 2, 8, 58, 632, 9654, 202484, 5843954, 233064944, 12916716526, 998745087980, 108135391731689, 16434082400952295, 3513344943520006169, 1058030578581541938616, 449389062270642127911734 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A split graph is a graph whose vertices can be partitioned into a clique and an independent set. - Justin M. Troyka, Oct 28 2018 REFERENCES V. Bina, Enumeration of Labeled Split Graphs and Counts of Important Superclasses. In Adacher L., Flamini, M., Leo, G., Nicosia, G., Pacifici, A., Picialli, V. (Eds.). CTW 2011, Villa Mondragone, Frascati, pp. 72-75 (2011). LINKS E. A. Bender, L. B. Richmond, and N. C. Wormald, Almost all chordal graphs split, J. Austral. Math. Soc. (Series A), 38 (1985), 214-221. V. Bina, Multidimensional probability distributions: Structure and learning, PHD Thesis. Fac. of Management, University of Economics in Prague (2011). J. M. Troyka, Split graphs: combinatorial species and asymptotics, arXiv:1803.07248 [math.CO], 2018. FORMULA a(n) = 1 + Sum_{k=2..n} binomial(n,k) ( (2^k-1)^(n-k) - Sum_{j=1..n-k) j*k*binomial(n-k,j)*(2^(k-1)-1)^(n-k-j) /(j+1) ). From Justin M. Troyka, Oct 28 2018: (Start) a(n) = [ Sum_{k=0..n} binomial(n,k) 2^(k(n-k)) ] - [ n Sum_{k=0..n-1} binomial(n-1,k)*2^(k(n-k-1)) ] (see the Troyka link, Cor. 3.4). a(n) = A047863(n) - n*A047863(n-1) (see the Troyka link, Cor. 3.4). a(n) ~ A047863(n) (see Bender, Richmond, and Wormald, Cor. 1). (End) MAPLE A179534 := proc(n) local a, k; a := 1 ; for k from 2 to n do a := a+binomial(n, k)*( (2^k-1)^(n-k) -add(j*k*binomial(n-k, j)*(2^(k-1)-1)^(n-k-j)/(j+1), j=1..n-k) ) end do: a ; end proc: # R. J. Mathar, Jun 21 2011 MATHEMATICA a[n_] := 1 + Sum[Binomial[n, k]*((2^k-1)^(n-k) - Sum[j*k*Binomial[n-k, j]*(2^(k-1)-1)^(n-k-j)/(j+1), {j, 1, n-k}]), {k, 2, n}]; Array[a, 20] (* Stefano Spezia, Oct 29 2018 *) PROG (R) print(1); print(1); for (n in 2:20) { N.spl <- 1; for (k in 2:n) { tmp <- 0; for (j in 1:(n-k)) { tmp <- tmp + j/(j+1)*k*choose(n-k, j)*(2^(k-1)-1)^(n-k-j); }; N.spl <- N.spl + choose(n, k)*((2^k-1)^(n-k) - tmp); }; print(n); print(N.spl); } CROSSREFS Cf. A048194, A058862, A006125. Sequence in context: A319590 A005804 A162067 * A256034 A086907 A132186 Adjacent sequences:  A179531 A179532 A179533 * A179535 A179536 A179537 KEYWORD nonn AUTHOR Vladislav Bina, Jul 18 2010 STATUS approved

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Last modified February 15 16:25 EST 2019. Contains 320136 sequences. (Running on oeis4.)