OFFSET
1,2
COMMENTS
This sequence contains all prime numbers. Only two composite numbers are known in this sequence: 4, 6.
No more composites below 10^10. - Charles R Greathouse IV, Aug 24 2011
Larger composite terms, if any, must be Carmichael numbers (A002997). - Ivan Neretin, Aug 30 2015
None of the 246683 Carmichael numbers < 10^16 are in the sequence. - Robert Israel, Sep 06 2015
MAPLE
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
MATHEMATICA
Union@Table[If[IntegerQ[2*(n-1)/EulerPhi[n]], n], {n, 300}]
PROG
(PARI) is(n)=(2*n-2)%eulerphi(n)==0 \\ Charles R Greathouse IV, Jun 14 2013
(Magma) [n: n in [1..300] | ((2*n-2) mod EulerPhi(n) eq 0)]; // Vincenzo Librandi, Sep 01 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
José María Grau Ribas, Jul 26 2011
STATUS
approved