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 A023519 #12 Jun 08 2019 10:40:35 %S A023519 1,5,7,17,19,71,5,7,109,3,449,3,379,881,5,3,3,7,3,29,2591,3,11,3,13, %T A023519 31,7,5,7,3079,5,4159,3,9521,5,7,3,3,5,3,3,97,5,7,5,17,3,3,5,7,13339, %U A023519 3,31,5,3,3,3,7,3,11,39761,11,5,23869,7,5,23,3,59,151,19,3,43,3,3,11,3,19,39799,13 %N A023519 Least odd prime divisor of prime(n)*prime(n-1) - 1, or 1 if prime(n)*prime(n-1) - 1 is a power of 2. %C A023519 Assumes the not generally accepted convention prime(0) = 1. - _Michel Marcus_, Jun 06 2019 %F A023519 a(n) = A078701(A023515(n)). - _Michel Marcus_, Jun 06 2019 %o A023519 (PARI) p(n) = if (n==0, 1, prime(n)); %o A023519 f(n) = my(v = select(x->((x%2)==1), factor(n)[, 1])); if (#v, vecmin(v), 1); %o A023519 a(n) = f(p(n)*p(n-1) - 1); \\ _Michel Marcus_, Jun 06 2019 %Y A023519 Cf. A023515, A078701. %K A023519 nonn %O A023519 1,2 %A A023519 _Clark Kimberling_ %E A023519 Terms corrected by _Sean A. Irvine_, Jun 05 2019