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 A065917 #3 May 07 2013 10:10:24 %S A065917 1,3,9,15,45,75,105,315,525,735,945,1155,3465,5775,8085,10395,12705, %T A065917 15015,45045,75075,105105,135135,165165,195195,225225,255255,765765, %U A065917 1276275,1786785,2297295,2807805,3318315,3828825,4339335,4849845 %N A065917 Boundaries of primorial intervals [1,3]; [3,9],[9,15]; [15,45], etc. %F A065917 a(n+1)= a(n)+q(j) for primorial q(j) < a(n+1) <= q(j+1)/2. %e A065917 a(4)= 15= a(3)+q(2)= 9+ 6, a(7)= 105= a(6)+q(3)= 75+ 30, %e A065917 a(5)= 45= a(4)+q(3)=15+30, a(8)= 315= a(7)+q(4)=105+210. %Y A065917 Primorial q(j)= A002110(j), interval count A005521(j+1)-1. %K A065917 nonn,easy %O A065917 1,2 %A A065917 _Frank Ellermann_, Dec 08 2001