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 A221991 #8 May 13 2013 01:22:34
%S A221991 2,4,6,8,10,20,22,36,42,66,90,110,142,184,232,284,342,400,458,532,604,
%T A221991 678,756,838,928,1026,1126,1230,1336,1446,1558,1686,1816,1954,2092,
%U A221991 2242,2392,2550,2712,2880,3052,3232,3412,3604,3796,3994,4192,4404,4626,4854,5082
%N A221991 a(n) is the number of terms in the expansion of (x-y)(x^2-y^2)*(x^3-y^3)*(x^5-y^5)*...*(x^p_i-y^p_i), where p_i is the i-th prime.
%C A221991 In the definition one can take y=1. Thus the sequence becomes the number of terms in the polynomial of the product{k=0..n} (1-x^p_i), where p_i is the i-th prime and p_0 = 1.
%C A221991 Offset is 1 to keep it parallel to other like sequences.
%t A221991 f[n_] := Length@ ExpandAll[(1 - x) Product[(1 - x^Prime[k]), {k, n}]]; Array[f, 51, 0]
%Y A221991 Cf. A000124, A086781, A086795, A086796, A086817, A225549.
%K A221991 nonn
%O A221991 1,1
%A A221991 _Robert G. Wilson v_, May 12 2013