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A242980 Carmichael numbers of the form (6*k + 1)*(12*k + 1)*(18*k + 1), where only two of the three numbers 6*k + 1, 12*k + 1, 18*k + 1 are prime.

%I A242980 #13 Feb 16 2025 08:33:22
%S A242980 172081,1773289,4463641,47006785,295643089,798770161,1150270849,
%T A242980 1420379065,1976295241,18390744505,122160500281,134642101321,
%U A242980 215741809801,228944441089,263610459505,321140603665,374464040689,444722065201,676328168881,1009514855521
%N A242980 Carmichael numbers of the form (6*k + 1)*(12*k + 1)*(18*k + 1), where only two of the three numbers 6*k + 1, 12*k + 1, 18*k + 1 are prime.
%H A242980 Giovanni Resta, <a href="/A242980/b242980.txt">Table of n, a(n) for n = 1..2665</a> (terms < 4*10^30)
%H A242980 Carlos Rivera, <a href="http://www.primepuzzles.net/puzzles/puzz_739.htm">Puzzle 739</a>
%H A242980 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/CarmichaelNumber.html">Carmichael Number</a>
%H A242980 <a href="/index/Ca#Carmichael">Index entries for sequences related to Carmichael numbers</a>
%o A242980 (Magma) lst:=[]; for k in [1..920] do t:={n: n in [1..3] | IsPrime(6*k*n+1)}; if #Set(t) eq 2 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 A242980 Cf. A002997, A033502, A242981.
%K A242980 nonn
%O A242980 1,1
%A A242980 _Arkadiusz Wesolowski_, May 28 2014