A020469 Primes that contain digits 6 and 7 only.

7, 67, 677, 67777, 76667, 76777, 666667, 677767, 767677, 777677, 6676667, 6676777, 6677677, 6677767, 6677777, 6766667, 6766777, 6776677, 7666667, 7667677, 7667767, 7766767, 7766777, 7777667, 66666667, 66677777, 66776777, 67667777, 67766767, 67776677, 67776767

Offset: 1

Author

David W. Wilson

Keywords

• nonn
• base

Links

• Michael S. Branicky, Table of n, a(n) for n = 1..10000 (terms 1..1000 from Vincenzo Librandi)

Crossrefs

Cf. A020449-A020472.

Programs

Mathematica

Flatten[Table[Select[FromDigits/@Tuples[{6,7},n],PrimeQ],{n,8}]] (* Vincenzo Librandi, Jul 27 2012 *)

Python

from sympy import isprime
from itertools import count, islice, product
def agen(): # generator of terms
    yield 7
    for d in count(2):
        for first in product("67", repeat=d-1):
            t = int("".join(first) + "7")
            if isprime(t): yield t
print(list(islice(agen(), 31))) # Michael S. Branicky, Nov 15 2022