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 A133067 #9 Nov 21 2013 13:07:20 %S A133067 2,13,247,4063,16775167,4294934527,68719345663,1152921504069976063, %T A133067 1329227995784915872327346307976921087, %U A133067 95780971304118053647396689042151819065498660774084607 %N A133067 Perfect numbers divided by 2, minus 1. %F A133067 a(n)=(A000396(n)/2)-1. %e A133067 a(3)=247 because the 3rd perfect number is 496 and 496/2 is 248. a(3)=248-1. %t A133067 p = Select[2^Range[400] - 1, PrimeQ]; p*(p+1)/4 - 1 (* _Vladimir Joseph Stephan Orlovsky_, Feb 02 2012 *) %Y A133067 Cf. A000396, A133028. %K A133067 nonn %O A133067 1,1 %A A133067 _Omar E. Pol_, Nov 07 2007