A087533 Primes consisting only of digits 4 and 7 occurring with equal frequency.

47, 47447747, 77474447, 4447747747, 4744477747, 4744747477, 4744747747, 4747444777, 4747747447, 4774744477, 4774747447, 7444477477, 7447474477, 7744744747, 7744747447, 7747444477, 7747447447, 7747744447, 44444477777477, 44447447774777, 44447474777477

Offset: 1

Author

Paul D. Hanna and Amarnath Murthy, Sep 12 2003

Keywords

• base
• nonn

Comments

There are 18 digit pairs which can produce such primes. (1,0),(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),(8,9).

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A087510, A087511, A087532.

Programs

Mathematica

Table[Select[FromDigits/@Permutations[PadRight[{},2n,{4,7}]],PrimeQ],{n,7}]//Flatten//Sort (* Harvey P. Dale, May 05 2016 *)

PARI

\\ Needs B() from A087510.
concat(vector(6,k,B(k,4,7,isprime))) \\ Andrew Howroyd, Sep 20 2024

Extensions

Edited by Charles R Greathouse IV, Oct 28 2009

Corrected by Harvey P. Dale, May 05 2016

Offset changed by Andrew Howroyd, Sep 20 2024