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 A339151 #8 Nov 26 2020 21:58:48
%S A339151 0,1,1,1,2,3,6,11,21,41,81,164,333,684,1417,2965,6229,13182,28019,
%T A339151 59862,128376,276408,597041,1293763,2811181,6124479,13374201,29270703,
%U A339151 64191331,141041901,310444472,684445543,1511345542,3342101662,7400605768,16408670095
%N A339151 Number of essentially series nonequivalent series-parallel networks with n elements and without unit elements in parallel.
%C A339151 Equivalence is up to rearrangement of the order of elements in both series and parallel configurations.
%C A339151 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.
%e A339151 In the following examples, elements in series are juxtaposed and elements in parallel are separated by '|'. The unit element is denoted by 'o'.
%e A339151 a(2) = 1: (oo).
%e A339151 a(3) = 1: (ooo).
%e A339151 a(4) = 1: (oooo).
%e A339151 a(5) = 2: (ooooo), (o(oo|oo)).
%e A339151 a(6) = 3: (oooooo), (oo(oo|oo)), (o(oo|ooo)).
%e A339151 a(7) = 6: (ooooooo), (ooo(oo|oo)), (oo(oo|ooo)), (o(oo|oooo)), (o(ooo|ooo)), (o(oo|oo|oo)).
%o A339151 (PARI) EulerT(v)={Vec(exp(x*Ser(dirmul(v,vector(#v,n,1/n))))-1, -#v)}
%o A339151 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); S}
%Y A339151 Cf. A000669, A339152, A339153.
%K A339151 nonn
%O A339151 1,5
%A A339151 _Andrew Howroyd_, Nov 26 2020