A270817 Integers k such that (2^k - 1) + (3^k - 1) + (5^k - 1) is prime.

1, 3, 4, 9, 11, 69, 117, 449, 675, 1119, 1959, 2687, 2859, 8001, 8175, 24269

Offset

1,2

Comments

Inspired by A268067.

Corresponding primes are 7, 157, 719, 1973317, 49007317, ...

Links

Table of n, a(n) for n=1..16.

Example

4 is a term because (2^4 - 1) + (3^4 - 1) + (5^4 - 1) = 719 is a prime number.

Mathematica

Select[Range@ 3000, PrimeQ[(2^# - 1) + (3^# - 1) + (5^# - 1)] &] (* Michael De Vlieger, Mar 23 2016 *)

Prog

(PARI) lista(nn) = for(n=1, nn, if(ispseudoprime(-3 + 2^n + 3^n + 5^n), print1(n, ", ")));
(Python)
from sympy import isprime
def afind(limit, startk=1):
    pow2, pow3, pow5 = 2**startk, 3**startk, 5**startk
    for k in range(startk, limit+1):
        if isprime(pow2 + pow3 + pow5 - 3): print(k, end=", ")
        pow2 *= 2; pow3 *= 3; pow5 *= 5
afind(1200) # Michael S. Branicky, Sep 08 2021

Crossrefs

Cf. A268064, A268067.

Sequence in context: A366753 A062798 A327060 * A182828 A294229 A035256

Adjacent sequences: A270814 A270815 A270816 * A270818 A270819 A270820

Keyword

nonn,more

Author

Altug Alkan, Mar 23 2016

Extensions

a(14)-a(15) from Michael S. Branicky, Sep 08 2021

a(16) from Michael S. Branicky, Apr 13 2023

Status

approved