OFFSET
0,3
COMMENTS
A digraph is semi-strong if all its weakly connected components are strongly connected. - Andrew Howroyd, Jan 14 2022
A digraph is semi-strong iff the following implication holds for all x,y in [n]: If there is a directed path from x to y then x and y are in the same strongly connected component. - Geoffrey Critzer, Oct 07 2023
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..50
V. A. Liskovets, Some easily derivable sequences, J. Integer Sequences, 3 (2000), #00.2.2.
FORMULA
E.g.f.: 1/(1-B(x)) where B(x) is e.g.f. for A054947. - Vladeta Jovovic, Mar 11 2003
E.g.f. A(x) satisfies: Sum_{n>=0} 2^(n^2-n)*x^n/n! / A(2^n*x) = 1. - Paul D. Hanna, Oct 27 2012
E.g.f.: exp(B(x)) where B(x) is the e.g.f. of A003030. - Andrew Howroyd, Jan 14 2022
MATHEMATICA
a[n_] := a[n] = Module[{A}, A = 1+Sum[a[k]*x^k/k!, {k, 1, n-1}]; n!*SeriesCoefficient[Sum[2^(k^2-k)*x^k/k!/(A /. x -> 2^k*x) , {k, 0, n}], {x, 0, n}]]; a[0]=1; Table[a[n], {n, 0, 13}] (* Jean-François Alcover, Dec 15 2014, after Paul D. Hanna *)
PROG
(PARI) a(n)=local(A=1+sum(k=1, n-1, a(k)*x^k/k!)+x*O(x^n)); n!*polcoeff(sum(k=0, n, 2^(k^2-k)*x^k/k!/subst(A, x, 2^k*x)), n)
for(n=0, 10, print1(a(n), ", ")) \\ Paul D. Hanna, Oct 27 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, May 24 2000
EXTENSIONS
More terms from Vladeta Jovovic, Mar 11 2003
Changed offset to 0 and added a(0)=1 by Paul D. Hanna, Oct 27 2012
STATUS
approved