OFFSET
1,1
COMMENTS
a(21) > 10^9.
All listed composite terms are multiples of the two known primes in this sequence, 11 and 1006003, the only known base 3 Wieferich primes.
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..78 (terms < 10^15)
MATHEMATICA
Select[Range[1000], Mod[3^EulerPhi[#], #^2] == 1 &] (* Alonso del Arte, Jun 02 2014 *)
PROG
(PARI) for(n=2, 10^9, if(Mod(3, n^2)^(eulerphi(n))==1, print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
Felix Fröhlich, May 27 2014
EXTENSIONS
Terms a(21) and beyond from Giovanni Resta, Jan 27 2020
STATUS
approved