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 A085917 #8 Jul 01 2020 23:08:56 %S A085917 5,4,2,1,3,5,4,2,1,2,1,3,6,3,3,3,3,6,3,3,3,2,1,3,6,3,6,3,6,3,3,11,16, %T A085917 8,4,2,1,8,4,2,1,15,13,15,16,8,4,2,1,6,3,17,15,12,6,3,4,2,1,3,6,3,3,5, %U A085917 3,2,1,5,6,3,48,24,12,6,3,12,6,3,10,5,4,2,1,3,3,31,21,17,15,11,13,24,12,6,3 %N A085917 Least k such that k*2^n - 1 is a semiprime. %C A085917 The first few values of n such that 509203*2^n - 1 is a semiprime, where k = 509203 (the conjectured smallest Riesel number), are: 3,4,16,34,61,82,124,142,163,171,... Conjecture: there are infinitely many semiprimes of this form. %H A085917 Sean A. Irvine, <a href="/A085917/b085917.txt">Table of n, a(n) for n = 1..500</a> %e A085917 a(33)=16 because k*2^33 - 1 is not a semiprime for k=1,2,...15, but 16*2^33 - 1 = 223 * 616318177 is. %Y A085917 Cf. A001358, A085427, A076337, A085245. %K A085917 nonn %O A085917 1,1 %A A085917 _Jason Earls_, Aug 16 2003