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 A281331 #40 Mar 12 2017 12:55:30
%S A281331 1,1,1,1,1,691,1,3617,43867,283,131,103,657931,9349,1721,37,
%T A281331 151628697551,26315271553053477373,154210205991661,137616929,
%U A281331 1520097643918070802691,59,383799511,653,417202699,577,39409,113161,67,2003,157,1226592271,839,37,688531,3112655297839
%N A281331 Smallest prime factor of |A001067(n)|, or 1 if |A001067(n)| = 1.
%H A281331 Wikipedia, <a href="https://en.wikipedia.org/wiki/Kummer's_congruence">Kummer's congruences</a>
%F A281331 a(n) = A020639(|A001067(n)|).
%F A281331 If n = A112548(m)/2, a(n) = |A001067(n)|.
%F A281331 a(18*m-2) = 37 for m > 0.
%e A281331 |A001067(10)| = 174611 = 283*617. So a(10) = 283.
%e A281331 |A001067(16)| = 7709321041217 = 37*683*305065927. So a(16) = 37.
%t A281331 a[n_] := FactorInteger[Abs[Numerator[BernoulliB[2*n] / (2*n)]]][[1, 1]]; Table[a[n], {n, 1, 36}] (* _Indranil Ghosh_, Mar 12 2017 *)
%o A281331 (PARI) a(n) = my(num = abs(numerator(bernfrac(2*n)/(2*n)))); if (num==1, 1, factor(num)[1,1]); \\ _Michel Marcus_, Jan 21 2017
%Y A281331 Cf. A000928, A001067, A020639, A033563, A112548, A281332.
%K A281331 nonn
%O A281331 1,6
%A A281331 _Seiichi Manyama_, Jan 20 2017
%E A281331 More terms from _Michel Marcus_, Jan 21 2017