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 A175509 #8 Feb 02 2015 05:30:02 %S A175509 -2,-2,-3,2,-2,4,-4,2,1,5,-4,1,7,9,9,1,-3,9,8,7,7,9,11,5,-2,4,-3,-1,6, %T A175509 11,12,16,17,19,2,-5,-11,-10,-1,7,10,16,9,12,21,13,15,16,15,22,11,10, %U A175509 11,13,9,17,7,9,11,12,17,6,-4,3,7,7,15,27,21,25,28,35,37,43,43,41,31,31,20 %N A175509 a(n) = ((2*n)-th semiprime) minus 2*(n-th semiprime). %F A175509 a(n) = A099981(n) - A075818(n). %e A175509 a(1) = -2 because ((2*1)-th semiprime) - 2*(1st semiprime) = 6 - 2*4 = -2. %Y A175509 Cf. A066066. %K A175509 sign %O A175509 1,1 %A A175509 _Juri-Stepan Gerasimov_, Jun 02 2010 %E A175509 Entries checked by _R. J. Mathar_, Aug 12 2010