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Smallest n-digit prime containing only the digits 4 and 7, or 0 if no such prime exists.
2

%I #15 Oct 10 2019 03:57:20

%S 7,47,0,4447,44777,0,4444747,44447747,0,4444444447,44444444747,0,

%T 4444444447777,44444447447447,0,4444444447447777,44444444444444477,0,

%U 4444444444444444777,44444444444444444447,0,4444444444444444444747,44444444444444444444477,0

%N Smallest n-digit prime containing only the digits 4 and 7, or 0 if no such prime exists.

%H Alois P. Heinz, <a href="/A036944/b036944.txt">Table of n, a(n) for n = 1..500</a>

%t Flatten[Table[Select[FromDigits/@Tuples[{4,7},n],PrimeQ,1],{n,22}]/.{}->{0}] (* _Harvey P. Dale_, Jun 28 2012 *)

%Y Cf. A036229, A020465, A036318.

%K nonn,base

%O 1,1

%A _Patrick De Geest_, Jan 04 1999