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 A172180 #4 Mar 30 2012 18:52:38 %S A172180 1,1,1,1,1,1,1,1,4,5,9,9,9,12,15,19,17,21,22,23,27,28,33,39,39,39,42, %T A172180 41,43,53,55,60,59,67,66,71,75,76,81,85,86,93,93,93,93,101,111,112, %U A172180 113,115,120,120,129,133,138,141,141,144,147,147,153,165,168,168,171,183,185 %N A172180 The absolute value of n-th prime of the form 3*n-+1 minus n-th nonprime of the form 3*n-+1. %F A172180 a(n)=abs(A045344(n)-A171993(n)). %p A172180 a(1)=abs(2-1)=1, a(2)=abs(5-4)=1, a(3)=abs(7-8)=1, a(4)=abs(11-10)=1, a(5)=abs(13-14)=1, a(6)=abs(17-16)=1, a(7)=abs(19-20)=1, a(8)=abs(23-22)=1, a(9)=abs(29-25)=4. %Y A172180 Cf. A045344, A171993. %K A172180 nonn %O A172180 1,9 %A A172180 _Juri-Stepan Gerasimov_, Jan 28 2010 %E A172180 Corrected (28 inserted, a third 39 inserted etc.) by _R. J. Mathar_, May 19 2010