Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #15 Nov 15 2022 10:46:54
%S 7,67,677,67777,76667,76777,666667,677767,767677,777677,6676667,
%T 6676777,6677677,6677767,6677777,6766667,6766777,6776677,7666667,
%U 7667677,7667767,7766767,7766777,7777667,66666667,66677777,66776777,67667777,67766767,67776677,67776767
%N Primes that contain digits 6 and 7 only.
%H Michael S. Branicky, <a href="/A020469/b020469.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Vincenzo Librandi)
%t Flatten[Table[Select[FromDigits/@Tuples[{6,7},n],PrimeQ],{n,8}]] (* _Vincenzo Librandi_, Jul 27 2012 *)
%o (Python)
%o from sympy import isprime
%o from itertools import count, islice, product
%o def agen(): # generator of terms
%o yield 7
%o for d in count(2):
%o for first in product("67", repeat=d-1):
%o t = int("".join(first) + "7")
%o if isprime(t): yield t
%o print(list(islice(agen(), 31))) # _Michael S. Branicky_, Nov 15 2022
%Y Cf. A020449-A020472.
%K nonn,base
%O 1,1
%A _David W. Wilson_