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 A085245 #14 Jul 02 2020 02:41:11 %S A085245 4,2,1,2,1,1,1,6,3,2,1,1,1,6,3,2,1,2,1,1,3,2,1,3,8,4,2,1,3,2,1,1,3,7, %T A085245 5,5,8,4,2,1,4,2,1,3,3,7,6,3,15,9,29,28,14,7,6,3,3,8,4,2,1,4,2,1,14,7, %U A085245 12,6,3,3,9,5,12,6,3,8,4,2,1,3,29,18,9,18,9,10,5,13,8,4,2,1,15,12,6,3,9,6 %N A085245 Least k such that k*2^n + 1 is a semiprime. %C A085245 The first few values of n such that 78557*2^n + 1 is a semiprime, where k = 78557 (the conjectured smallest Sierpinski number), are: 2, 3, 7, 15, 17, 18, 24, 60, 71, 89, 92, 107, 140, 143, 163,... Conjecture: there are infinitely many semiprimes of this form. %H A085245 Sean A. Irvine, <a href="/A085245/b085245.txt">Table of n, a(n) for n = 1..500</a> %e A085245 a(51)=29 because k*2^51 + 1 is not a semiprime for k=1,2,...28, but 29*2^51 + 1 = 63839 * 1022920073887 is. %o A085245 (PARI) a(n) = my(k=1); while (bigomega(k*2^n + 1) != 2, k++); k; \\ _Michel Marcus_, Jul 02 2020 %Y A085245 Cf. A001358, A035050, A076336. %K A085245 nonn %O A085245 1,1 %A A085245 _Jason Earls_, Aug 11 2003