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A087514 Primes consisting only of digits 1 and 7 occurring with equal frequency. 8

%I #11 Jul 28 2019 00:30:27

%S 17,71,11171777,11177717,17111777,17171177,17171771,17177117,17711717,

%T 17717171,71117177,71171717,71717117,77111717,77711171,1111717777,

%U 1117117777,1117177177,1117771177,1117771717,1177117177,1177177711

%N Primes consisting only of digits 1 and 7 occurring with equal frequency.

%C There are 18 digit pairs which can produce such primes. (1,0),(7,0),(1,3),(1,4),(1,6),(1,7),(1,9),(2,3),(2,9),(3,4),(3,5),(3,7),(3,8),(4,7),(4,9),(5,9),(6,7),(7,9).

%H Harvey P. Dale, <a href="/A087514/b087514.txt">Table of n, a(n) for n = 0..1000</a>

%t Sort[Select[FromDigits/@Flatten[Table[Permutations[PadRight[{},2n,{1,7}]],{n,5}],1],PrimeQ]] (* _Harvey P. Dale_, Jul 27 2019 *)

%Y Cf. A087510, A087515, A087516.

%K base,nonn

%O 0,1

%A _Amarnath Murthy_ and _Paul D. Hanna_, Sep 11 2003

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