1,1

Numbers k such that 3^k == 3 (mod k) and k is divisible by the square of a Mirimanoff prime (or base-3 Wieferich prime), A014127.

A non-coprime pseudoprime in base b is a number k such that b^k == b (mod k) and that gcd(b, k) > 1, and the non-coprime pseudoprime in base 3 (726, 1053426, 6498426, ...) that are not squarefree are listed in A306450 while the others terms in this sequence (121, 3751, 4961, ...) are listed in A244065. So this sequence is the union of A244065 and A306450.

Intersection of A122780 and A013929.

Amiram Eldar, Table of n, a(n) for n = 1..1000

121 is a term because 3^120 == (3^5)^24 == 1 (mod 121) and 121 = 11^2.

Although 3^725 = 243 rather than 1 mod 726, we see that nevertheless 3^726 = 3 mod 726, and since 726 = 2 * 3 * 11^2, 726 is in the sequence. - Alonso del Arte, Mar 16 2019

Select[Range[5000], PowerMod[3, #, #] == 3 && MoebiusMu[#] == 0 &] (* Alonso del Arte, Mar 16 2019 *)

(PARI) forcomposite(n=1, 10^7, if(Mod(3, n)^n==3 && !issquarefree(n), print1(n, ", ")))

Cf. A013929, A014127, A122780, A158358, A244065, A306450.

Sequence in context: A356630 A385606 A385607 * A238250 A361658 A293588

Adjacent sequences: A306449 A306450 A306451 * A306453 A306454 A306455

Jianing Song, Feb 17 2019

approved