OFFSET
1,1
COMMENTS
Because prime numbers must end in odd digits other than 5, the only beginning/end digits combinations that satisfy the definition are (2,3), (6,7), and (8,9). [From Harvey P. Dale, Jan 29 2012]
FORMULA
a(x) = 10^(x+1)*n+floor(10^x*7/9)*10+n+1. Output if a(x) is prime.
MATHEMATICA
Select[Flatten[Table[{FromDigits[Join[PadRight[{2}, n, 7], {3}]], FromDigits[ Join[PadRight[{6}, n, 7], {7}]], FromDigits[ Join[PadRight[ {8}, n, 7], {9}]]}, {n, 2, 70}]], PrimeQ] (* Harvey P. Dale, Jan 29 2012 *)
PROG
(PARI) n10np1(n) = { local(x, y, k); for(x=1, n, for(k=1, 8, y=10^(x+1)*k+floor(10^x*7/9)*10+k+1; if(isprime(y), print1(y", ")) ) ) }
CROSSREFS
KEYWORD
easy,nonn,base
AUTHOR
Cino Hilliard, Jul 11 2005
STATUS
approved