OFFSET
1,2
COMMENTS
Note that 3^3^i - 1 divides 3^3^(i+1) - 1, so this sequence is also numbers k such that k divides 3^3^i - 1 for all sufficiently large i.
Also numbers k such that there exists i >= 1 such that k divides 3^^i - 1, where 3^^i = 3^3^...^3 (i times) = A014220(i-1).
Also numbers k such that ord(3,k) is a power of 3, where ord(a,k) is the multiplicative order of a modulo k: 3^3^i == 1 (mod k) if and only if ord(3,k) divides 3^i, so such i exists if and only if ord(3,k) is a power of 3.
If a term k is not squarefree, then it is divisible by p^2, where p is a Wieferich prime to base 3 (A014127) such that ord(3,p) is a power of 3. No such p is known.
EXAMPLE
Suppose that q is an odd prime power such that ord(3,q) = 3^e. e = 1 gives q = 13; e = 2 gives q = 757; e = 3 gives q = 109, 433, 8209; e = 4 gives q = 3889, 1190701, 12557612956332313.
PROG
(PARI) isA357265(k) = (k%3!=0) && isprimepower(3*znorder(Mod(3, k)))
CROSSREFS
KEYWORD
nonn
AUTHOR
Jianing Song, Nov 11 2023
STATUS
approved