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 A339152 #6 Nov 26 2020 21:59:12
%S A339152 1,0,0,1,1,3,4,9,16,33,63,131,261,545,1123,2359,4948,10502,22307,
%T A339152 47731,102367,220600,476626,1033450,2246252,4895935,10694744,23414838,
%U A339152 51364180,112891831,248548836,548123924,1210612692,2677682900,5930586249,13151963729,29201456634
%N A339152 Number of essentially parallel nonequivalent series-parallel networks with n elements and without unit elements in parallel.
%C A339152 Equivalence is up to rearrangement of the order of elements in both series and parallel configurations.
%C A339152 A series configuration is a multiset of two or more parallel configurations and a parallel configuration is a multiset of two or more series configurations. The unit element is considered to be a parallel configuration.
%F A339152 Inverse Euler transform of A339153.
%e A339152 In the following examples, elements in series are juxtaposed and elements in parallel are separated by '|'. The unit element is denoted by 'o'.
%e A339152 a(1) = 1: (o).
%e A339152 a(4) = 1: (oo|oo).
%e A339152 a(5) = 1: (oo|ooo).
%e A339152 a(6) = 3: (oo|oooo), (ooo|ooo), (oo|oo|oo).
%e A339152 a(7) = 4: (oo|ooooo), (oo|o(oo|oo)), (ooo|oooo), (oo|oo|ooo).
%o A339152 (PARI) EulerT(v)={Vec(exp(x*Ser(dirmul(v,vector(#v,n,1/n))))-1, -#v)}
%o A339152 seq(n)={my(S=vector(n), P=vector(n)); P[1]=1; for(n=2, #S, my(t=EulerT(S[1..n])[n]); S[n]=EulerT(P[1..n])[n]; P[n]=t); P}
%Y A339152 Cf. A000669, A339151, A339153.
%K A339152 nonn
%O A339152 1,6
%A A339152 _Andrew Howroyd_, Nov 26 2020