This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A339155 #8 Nov 28 2020 01:21:46
%S A339155 1,0,0,1,1,3,5,13,29,70,165,409,1001,2505,6278,15904,40447,103567,
%T A339155 266229,687668,1782573,4637731,12103112,31679212,83135973,218713492,
%U A339155 576683119,1523740365,4033915677,10698680606,28422818782,75629586540,201539697208,537818080714
%N A339155 Number of essentially parallel oriented series-parallel networks with n elements and without unit elements in parallel.
%C A339155 A series configuration is an ordered concatenation of two or more parallel configurations and a parallel configuration is the unit element or a multiset of two or more series configurations. a(n) is the number of parallel configurations with n unit elements.
%F A339155 G.f.: B(x)/(1 + B(x)) where B(x) is the g.f. of A339156.
%e A339155 In the following examples, elements in series are juxtaposed and elements in parallel are separated by '|'. The unit element is denoted by 'o'.
%e A339155 a(1) = 1: (o).
%e A339155 a(4) = 1: (oo|oo).
%e A339155 a(5) = 1: (oo|ooo).
%e A339155 a(6) = 3: (oo|oooo), (ooo|ooo), (oo|oo|oo).
%e A339155 a(7) = 4: (oo|ooooo), (oo|o(oo|oo)), (oo|(oo|oo)o), (ooo|oooo), (oo|oo|ooo).
%o A339155 (PARI) EulerT(v)={Vec(exp(x*Ser(dirmul(v,vector(#v,n,1/n))))-1, -#v)}
%o A339155 seq(n)={my(p=x+O(x^2)); for(n=2, n, p=x+x*Ser(EulerT(Vec(p^2/(1+p), -n)))); Vec(1 - 1/(1+p))}
%Y A339155 Cf. A003430, A339152, A339154, A339156.
%K A339155 nonn
%O A339155 1,6
%A A339155 _Andrew Howroyd_, Nov 26 2020