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 A242981 #14 Feb 16 2025 08:33:22
%S A242981 13992265,1504651681,14782305601,292869912385,2387706608354185,
%T A242981 4484354372982001,70895950685971489,807481759780458361,
%U A242981 1659264916949696161,118023300545190612481,140695625117781970705,11710597360056333492601,19818019625768288167321
%N A242981 Carmichael numbers of the form (6*k + 1)*(12*k + 1)*(18*k + 1), where only one of the three numbers 6*k + 1, 12*k + 1, 18*k + 1 is prime.
%H A242981 Giovanni Resta, <a href="/A242981/b242981.txt">Table of n, a(n) for n = 1..26</a> (terms < 4*10^33)
%H A242981 Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_739.htm">Puzzle 739</a>
%H A242981 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/CarmichaelNumber.html">Carmichael Number</a>
%H A242981 <a href="/index/Ca#Carmichael">Index entries for sequences related to Carmichael numbers</a>
%o A242981 (Magma) lst:=[]; for k in [1..2482095] do t:={n: n in [1..3] | IsPrime(6*k*n+1)}; if #Set(t) eq 1 then c:=&*[6*k*n+1: n in [1..3]]; if IsOne(c mod CarmichaelLambda(c)) then lst:=Append(lst, c); end if; end if; end for; lst;
%Y A242981 Cf. A002997, A033502, A242980.
%K A242981 nonn
%O A242981 1,1
%A A242981 _Arkadiusz Wesolowski_, May 28 2014