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 A121884 #14 Feb 15 2018 12:57:02
%S A121884 1,1,2,3,1,2,2,3,2,3,2,3,4,1,2,3,2,4,2,4,5,1,4,2,1,4,3,2,3,4,2,4,5,6,
%T A121884 3,2,2,5,6,8,9,10,2,2,3,2,3,4,6,7,4,1,2,4,3,2,4,5,2,4,6,1,2,3,4,6,7,
%U A121884 10,2,3,4,6,7,8,10,3,2,4,6,8,2,3,2,5,2,4,3,1,4,6,8,2,5,6,8,9,10,12,2,1,2,4,6
%N A121884 Excess of n-th semiprime over previous prime.
%C A121884 a(n) = 1 iff n is in A077068. a(n) = 2 iff n is in A062721.
%H A121884 Robert G. Wilson v, <a href="/A121884/b121884.txt">Table of n, a(n) for n=1..1000</a>
%F A121884 a(n) = Min{A001358(n)-p for p < A001358(n) and p in A000040(k)}.
%t A121884 semiPrimeQ[n_] := Plus @@ Last /@ FactorInteger[n] == 2; lst1 = Select[ Range@ 325, semiPrimeQ@# &]; lst = Select[ Range@ 500, semiPrimeQ@# &]; lst - (NextPrime[ #, -1] & /@ lst) (* _Robert G. Wilson v_, Mar 16 2009 *)
%t A121884 #-NextPrime[#,-1]&/@Select[Range[400],PrimeOmega[#]==2&] (* _Harvey P. Dale_, Feb 15 2018 *)
%Y A121884 Cf. A000040, A001358, A062721, A077068.
%K A121884 easy,nonn
%O A121884 1,3
%A A121884 _Jonathan Vos Post_, Aug 31 2006
%E A121884 a(31)-a(103) from _Robert G. Wilson v_, Mar 16 2009