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%I #29 Sep 08 2022 08:45:58
%S 1,2,3,4,5,6,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,
%T 83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167,173,
%U 179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263
%N Numbers n such that phi(n) divides 2*(n-1).
%C This sequence contains all prime numbers. Only two composite numbers are known in this sequence: 4, 6.
%C No more composites below 10^10. - _Charles R Greathouse IV_, Aug 24 2011
%C Larger composite terms, if any, must be Carmichael numbers (A002997). - _Ivan Neretin_, Aug 30 2015
%C None of the 246683 Carmichael numbers < 10^16 are in the sequence. - _Robert Israel_, Sep 06 2015
%p with(numtheory): A193461:=n->`if`((2*(n-1) mod phi(n)) = 0, n, NULL): seq(A193461(n), n=1..300); # _Wesley Ivan Hurt_, Sep 05 2015
%t Union@Table[If[IntegerQ[2*(n-1)/EulerPhi[n]],n],{n,300}]
%o (PARI) is(n)=(2*n-2)%eulerphi(n)==0 \\ _Charles R Greathouse IV_, Jun 14 2013
%o (Magma) [n: n in [1..300] | ((2*n-2) mod EulerPhi(n) eq 0)]; // _Vincenzo Librandi_, Sep 01 2015
%Y Cf. A002997.
%K nonn
%O 1,2
%A _José María Grau Ribas_, Jul 26 2011
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