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 A139621 Triangle read by rows: T(n,k) is the number of connected directed multigraphs with loops and no vertex of degree 0, with n arcs and k vertices. 5
 1, 1, 1, 1, 4, 3, 1, 8, 15, 8, 1, 16, 57, 66, 27, 1, 25, 163, 353, 295, 91, 1, 40, 419, 1504, 2203, 1407, 350, 1, 56, 932, 5302, 12382, 13372, 6790, 1376, 1, 80, 1940, 16549, 58237, 96456, 80736, 33628, 5743, 1, 105, 3743, 46566, 237904, 573963, 717114, 482730, 168645, 24635 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS Length of the n-th row: n+1. LINKS Andrew Howroyd, Table of n, a(n) for n = 0..1325 R. J. Mathar, Statistics on Small Graphs, arXiv:1709.09000 (2017) Table 71. FORMULA T(n,1) = 1. T(n,2) = A136564(n,2) - floor(n/2). EXAMPLE Triangle begins:      1      1     1      1     4     3      1     8    15     8      1    16    57    66    27      1    25   163   353   295    91      1    40   419  1504  2203  1407   350      1    56   932  5302 12382 13372  6790  1376 T(2 arcs, 2 vertices) = 4: one graph 1->1, 2->1; one graph with 1->1, 1->2; one graph with 2->1, 2->1, one graph with 1->2, 2->1. T(2 arcs, 3 vertices) = 3: one graph 2->1, 3->1; one graph 2->1, 3->2; one graph 2->1, 2->3. PROG (PARI) InvEulerMT(u)={my(n=#u, p=log(1+x*Ser(u)), vars=variables(p)); Vec(sum(i=1, n, moebius(i)*substvec(p + O(x*x^(n\i)), vars, apply(v->v^i, vars))/i) )} permcount(v) = {my(m=1, s=0, k=0, t); for(i=1, #v, t=v[i]; k=if(i>1&&t==v[i-1], k+1, 1); m*=t*k; s+=t); s!/m} edges(v, t) = {prod(i=2, #v, prod(j=1, i-1, my(g=gcd(v[i], v[j])); t(v[i]*v[j]/g)^(2*g))) * prod(i=1, #v, t(v[i])^v[i])} G(n, x)={my(s=0); forpart(p=n, s+=permcount(p)/edges(p, i->1-x^i)); s/n!} T(n)={Mat([Col(p+O(y^n), -n) | p<-InvEulerMT(vector(n, k, G(k, y + O(y^n))))])} {my(A=T(10)); for(n=1, #A, print(A[n, 1..n]))} \\ Andrew Howroyd, Oct 22 2019 CROSSREFS Cf. A129620, A136564, A139622, A137975 (row sums), A000238 (diagonal). Sequence in context: A128007 A098458 A165914 * A305621 A196841 A165732 Adjacent sequences:  A139618 A139619 A139620 * A139622 A139623 A139624 KEYWORD nonn,tabl AUTHOR Benoit Jubin, May 01 2008 EXTENSIONS Prepended a(0)=1 to have a regular triangle, Joerg Arndt, Apr 14 2013 More terms from R. J. Mathar, Jul 31 2017 Terms a(34) and beyond from Andrew Howroyd, Oct 22 2019 STATUS approved

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Last modified May 6 19:52 EDT 2021. Contains 343586 sequences. (Running on oeis4.)