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 A125254 #12 Feb 11 2024 14:21:25
%S A125254 3,7,11,3,19,23,3,31,7,3,43,47,3,11,59,3,67,71,3,79,83,3,7,19,3,103,
%T A125254 107,3,23,7,3,127,131,3,139,11,3,151,31,3,163,167,3,7,179,3,11,191,3,
%U A125254 199,7,3,211,43,3,223,227,3,47,239,3,19,251,3,7,263,3,271,11,3,283,7,3,59,23
%N A125254 Smallest prime divisor of 4n-1 that is of the form 4k-1.
%C A125254 4n-1 always has a prime divisor congruent to 3 modulo 4.
%D A125254 T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, page 147.
%H A125254 Nick Hobson, <a href="/A125254/b125254.txt">Table of n, a(n) for n = 1..1000</a>
%e A125254 The divisors of 4*9 - 1 = 35 are 5 and 7, so a(9) = 7.
%t A125254 Table[SelectFirst[Transpose[FactorInteger[4n-1]][[1]],Mod[#,4]==3&],{n,80}] (* The program uses the SelectFirst function from Mathematica version 10 *) (* _Harvey P. Dale_, Mar 05 2015 *)
%o A125254 (PARI) vector(76, n, f=factor(4*n-1); r=0; until(f[r,1]%4==3, r++); f[r,1])
%Y A125254 Cf. A057205, A111863, A125255.
%K A125254 easy,nonn
%O A125254 1,1
%A A125254 _Nick Hobson_, Nov 26 2006