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 A125255 #18 Dec 17 2024 00:50:30
%S A125255 3,7,11,3,19,23,3,31,5,3,43,47,3,5,59,3,67,71,3,79,83,3,7,5,3,103,107,
%T A125255 3,5,7,3,127,131,3,139,11,3,151,5,3,163,167,3,5,179,3,11,191,3,199,7,
%U A125255 3,211,5,3,223,227,3,5,239,3,13,251,3,7,263,3,271,5,3,283,7,3,5,13,3,307
%N A125255 Smallest prime divisor of 4n-1.
%C A125255 All divisors of 4n-1 are odd.
%H A125255 Nick Hobson, <a href="/A125255/b125255.txt">Table of n, a(n) for n = 1..1000</a>
%F A125255 a(3k+1) = 3, k = 0,1,2,... a(5k+4) = 5 if k is not a multiple of 3; k = 1,2,4,5,7,8,... - _Alexander Adamchuk_, Nov 28 2006
%F A125255 a(n) = A020639(4n-1). - _R. J. Mathar_, Jan 23 2007
%e A125255 The prime divisors of 4*9 - 1 = 35 are 5 and 7, so a(9) = 5.
%t A125255 Table[FactorInteger[4n-1][[1,1]],{n,77}] (* _James C. McMahon_, Dec 16 2024 *)
%o A125255 (PARI) vector(77, n, factor(4*n-1)[1,1])
%Y A125255 Cf. A004767, A107745, A125254.
%K A125255 easy,nonn
%O A125255 1,1
%A A125255 _Nick Hobson_, Nov 26 2006