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 A023505 #12 Jun 06 2019 02:41:54 %S A023505 1,1,1,3,5,3,1,3,11,7,3,3,5,3,23,13,29,3,3,5,3,3,41,11,3,5,3,53,3,7,3, %T A023505 5,17,3,37,3,3,3,83,43,89,3,5,3,7,3,3,3,113,3,29,7,3,5,1,131,67,3,3,5, %U A023505 3,73,3,5,3,79,3,3,173,3,11,179,3,3,3,191,97,3,5,3,11,3,5,3,3 %N A023505 Least odd prime divisor of prime(n) - 1, or 1 if prime(n) - 1 is a power of 2. %C A023505 Note that a(n)=1 for n= 1,2,3,7,55,6543, ... . - _Michel Marcus_, Oct 01 2013 %F A023505 a(n) = A078701(A006093(n)). - _R. J. Mathar_, Feb 06 2019 %o A023505 (PARI) a(n) = my(p = prime(n) - 1, v = p/(2^valuation(p, 2))) ; if (v == 1, 1, factor(v)[1, 1]); \\ _Michel Marcus_, Oct 01 2013 %Y A023505 Cf. A006093, A078701. %K A023505 nonn %O A023505 1,4 %A A023505 _Clark Kimberling_