OFFSET
1,1
COMMENTS
Prime factors of numbers of the form 3^3^i - 1: p divides 3^3^i - 1 if and only if the multiplicative order of 3 modulo p is a power of 3 not exceeding 3^i.
EXAMPLE
13 is a term since the multiplicative order of 3 modulo 13 is 3 = 3^1, which means that 13 is a factor of 3^3^1 - 1.
109 is a term since the multiplicative order of 3 modulo 109 is 27 = 3^3, which means that 109 is a factor of 3^3^3 - 1.
PROG
(PARI) isA367648(n) = isprime(n) && (n!=3) && isprimepower(3*znorder(Mod(3, n)))
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
Jianing Song, Nov 25 2023
EXTENSIONS
a(18)-a(19) from Michel Marcus, Nov 27 2023
a(20)-a(25) from Max Alekseyev, Jul 22 2024
STATUS
approved