A112785 Members of twin prime pairs using only digits 7 and 9.

79997, 79999, 777977, 777979, 7977797, 7977799, 77997779, 77997977, 77997979, 99777779, 799999799, 977797979, 977999777, 977999779, 979797779, 999799979, 7777997999, 7797799979, 7979997797, 7979997799, 7999777997, 7999777999, 7999797977, 7999797979

Offset: 1

Author

Amarnath Murthy, Jan 02 2006

Keywords

• base
• nonn

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Subsequence of A020471.

Programs

Mathematica

Select[Flatten[Table[FromDigits/@Tuples[{7,9},n],{n,2,11}]],PrimeQ[#] && Or@@PrimeQ[#+{2,-2}]&] (* Harvey P. Dale, Nov 06 2013 *)

PARI

istwin(n)=ispseudoprime(n)&&ispseudoprime(if(n%3==1,n-2,n+2))
do(D)=my(v=List(),t); for(d=5,D,for(i=0,2^d-1, t=sum(j=0,d-1, if(bittest(i,j),9,7)*10^j); if(istwin(t), listput(v,t)))); Vec(v)
\\ Charles R Greathouse IV, Nov 06 2013

Extensions

Edited and extended by D. S. McNeil, Nov 27 2010

Corrected and extended by Harvey P. Dale, Nov 06 2013