OFFSET
1,2
COMMENTS
A039698 with the primes removed. For every prime p, 2p is in the sequence. - Ray Chandler, May 26 2008
LINKS
Iain Fox, Table of n, a(n) for n = 1..10000 (first 1000 terms from Harry J. Smith)
EXAMPLE
Solutions to 1+phi(x)=13 are {13, 21, 26, 28, 36, 42} of which the 5 composites are in the sequence.
MAPLE
select(n -> not isprime(n) and isprime(1+numtheory:-phi(n)), [$1..1000]); # Robert Israel, Dec 29 2017
MATHEMATICA
Select[Complement[Range@ #, Prime@ Range@ PrimePi@ #] &@ 150, PrimeQ[EulerPhi@ # + 1] &] (* Michael De Vlieger, Jul 01 2016 *)
PROG
(PARI) isok(k) = { !isprime(k) && isprime(eulerphi(k) + 1) } \\ Harry J. Smith, Nov 10 2009
(Magma) [n: n in [1..200] |not IsPrime(n) and IsPrime(EulerPhi(n)+1)]; // Vincenzo Librandi, Jul 02 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Dec 03 2001
STATUS
approved