A227180 - OEIS

%I #16 Dec 22 2016 00:35:37

%S 4,6,8,9,10,12,14,16,18,20,22,24,26,27,30,32,34,36,38,40,42,44,46,48,

%T 50,54,56,58,60,62,64,68,72,74,78,80,81,82,84,86,88,90,92,94,96,98,

%U 100,102,104,106,108,110,114,116,118,120,122,126,128,132,134,136,138,140,142,144,146,150,152,156,158,160,162,164,166,168,170

%N Composite numbers n such that b^(n-1) == 1 (mod n) implies b == -1 or +1 (mod n).

%C The sequence is the union of A111305 with {3^k | k > 1}.

%C The composite numbers not in this sequence are the Fermat pseudoprimes A181780.

%t FQ[k_]:= Block[{},GCD[EulerPhi[k],k-1]==1||IntegerQ[Log[3,k]]];Select[Range[4,170],FQ]

%o (PARI) is(n)=for(b=2, n-2, if(Mod(b, n)^(n-1)==1, return(0))); !isprime(n) \\ _Charles R Greathouse IV_, Dec 22 2016

%Y Cf. A111305, A181780, A209211.

%K nonn

%O 1,1

%A _Emmanuel Vantieghem_, Jul 03 2013