A360761 Primes p that divide both 3^k-2 and 5^k-1 for some k.

31, 601, 2593, 20478961, 204700049, 668731841

Offset

1,1

Comments

If prime p divides 3^k-2 and 5^k-1, then p divides 3^j-2 and 5^j-1 for all j such that j == k (mod p-1).

Primes p such that the equation 3^(x*A070677(p)) == 2 (mod p) has a solution.

Values of k: 24, 108, 64, 376020, 67141466, 487515840, ... - Chai Wah Wu, Feb 24 2023

Links

Table of n, a(n) for n=1..6.

Example

a(3) = 2593 is a term because 2593 is prime, 3^64 == 2 (mod 2593) and 5^64 == 1 (mod 2593).

Maple

R:= NULL: count:= 0: p:= 5: with(numtheory):
while count < 4 do
  p:= nextprime(p);
if  mlog(2, 3 &^ order(5, p) mod p, p) <> FAIL then R:= R, p; count:= count+1 fi
od:
R;

Prog

(Python)
from itertools import islice
from sympy import discrete_log, nextprime, n_order
def A360761_gen(): # generator of terms
    p = 5
    while True:
        try:
            discrete_log(p:=nextprime(p), 2, pow(3, n_order(5, p), p))
        except:
            continue
        yield p
A360761_list = list(islice(A360761_gen(), 4)) # Chai Wah Wu, Feb 23 2023

Crossrefs

Cf. A070677.

Sequence in context: A240420 A022755 A003533 * A005462 A028201 A028184

Adjacent sequences: A360758 A360759 A360760 * A360762 A360763 A360764

Keyword

nonn,more

Author

Robert Israel, Feb 19 2023

Extensions

a(5)-a(6) from Chai Wah Wu, Feb 23 2023

Status

approved