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A049312 Number of graphs with a distinguished bipartite block, by number of vertices. 10
1, 2, 4, 8, 17, 38, 94, 258, 815, 3038, 13804, 78760, 580456, 5647602, 73645352, 1297920850, 31031370360, 1007551636038, 44432872400460, 2661065508648436, 216457998880015366, 23920728651724212120, 3593384834863975164882, 734240676501745813835934 (list; graph; refs; listen; history; text; internal format)



Calculate number of connected bipartite graphs + number of connected bipartite graphs with no duality automorphism, apply EULER transform.

Inverse Euler transform is A318870.


R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1976.


Alois P. Heinz, Table of n, a(n) for n = 0..40

P. J. Cameron, Sequences realized by oligomorphic permutation groups, J. Integ. Seqs. Vol. 3 (2000), #00.1.5.

Karen L. Collins, Ann N. Trenk, Finding Balance: Split Graphs and Related Classes, arXiv:1706.03092 [math.CO], June 2017.

M. Guay-Paquet, A. H. Morales and E. Rowland, Structure and enumeration of (3+ 1)-free posets, arXiv preprint arXiv:1212.5356 [math.CO], 2012-2013. - From N. J. A. Sloane, Feb 01 2013

J. M. Troyka, Split graphs: combinatorial species and asymptotics, arXiv:1803.07248 [math.CO], 2018.


a(n) ~ 1/n! A047863(n) = 1/n! Sum_{k=0..n} binomial(n,k) * 2^(k(n-k)) (see the Troyka link, Thm. 3.7). - Justin M. Troyka, Oct 29 2018


a(2)=4: null graph with 0, 1 or 2 vertices in the distinguished block and complete graph with 1 vertex in distinguished block.


b:= proc(n, i) option remember; `if`(n=0, {0}, `if`(i<1, {},

      {seq(map(p-> p+j*x^i, b(n-i*j, i-1) )[], j=0..n/i)}))


g:= proc(n, k) option remember; add(add(2^add(add(igcd(i, j)*

      coeff(s, x, i)* coeff(t, x, j), j=1..degree(t)),

      i=1..degree(s))/mul(i^coeff(s, x, i)*coeff(s, x, i)!,

      i=1..degree(s))/mul(i^coeff(t, x, i)*coeff(t, x, i)!,

      i=1..degree(t)), t=b(n+k$2)), s=b(n$2))


A:= (n, k)-> g(min(n, k), abs(n-k)):

a:= d-> add(A(n, d-n), n=0..d):

seq(a(n), n=0..20);  # Alois P. Heinz, Aug 01 2014


b[n_, i_] := b[n, i] = If[n == 0, {0}, If[i<1, {}, Flatten @ Table[ Map[ Function[ {p}, p+j*x^i], b[n-i*j, i-1]], {j, 0, n/i}]]];

g[n_, k_] := g[n, k] = Sum[ Sum[ 2^Sum[Sum[GCD[i, j]*Coefficient[s, x, i]*Coefficient[t, x, j], {j, 1, Exponent[t, x]}], {i, 1, Exponent[s, x]}]/Product[i^Coefficient[s, x, i]*Coefficient[s, x, i]!, {i, 1, Exponent[s, x]}]/Product[i^Coefficient[t, x, i]*Coefficient[t, x, i]!, {i, 1, Exponent[t, x]}], {t, b[n+k, n+k]}], {s, b[n, n]}];

A[n_, k_] := g[Min[n, k], Abs[n-k]];

a[d_] := Sum[A[n, d-n], {n, 0, d}];

Table[a[n], {n, 0, 20}] (* Jean-Fran├žois Alcover, Feb 25 2015, after Alois P. Heinz *)


Row sums of A028657.

Cf. A048194, A318870.

Sequence in context: A090901 A101516 A118928 * A132043 A055545 A241671

Adjacent sequences:  A049309 A049310 A049311 * A049313 A049314 A049315




Peter J. Cameron


More terms from Vladeta Jovovic, Jun 17 2000



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Last modified January 15 19:04 EST 2019. Contains 319170 sequences. (Running on oeis4.)