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%I #10 Sep 08 2021 21:14:29
%S 4,16,40,56,160,7280
%N Numbers k such that (3^lambda(k) - 1)/k is prime, where lambda(k) is the Carmichael lambda function (A002322).
%C The corresponding primes are 2, 5, 2, 13, 41, 73.
%e 4 is in the sequence since lambda(4) = 2 and (3^2 - 1)/4 = 2 is prime.
%t aQ[n_] := PrimeQ[(3^CarmichaelLambda[n]-1)/n]; a={}; Do[If[aQ[k], AppendTo[a,k]], {k,1,10000}]; a
%o (PARI) isok(n) = (denominator(p=(3^lcm(znstar(n)[2])-1)/n)==1) && isprime(p); \\ _Michel Marcus_, Dec 29 2017
%Y Cf. A002322.
%K nonn,more
%O 1,1
%A _Amiram Eldar_, Dec 29 2017
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