A096209 - OEIS

%I #21 Jan 12 2020 12:49:01

%S 109,10999999999,1099999999999,1099999999999999999,

%T 109999999999999999999999,1099999999999999999999999999999,

%U 10999999999999999999999999999999999999

%N Primes of the form 10 followed by 9's only.

%C Or, primes of the form 10^k + 10^(k-1) - 1.

%C Number of 9's in a(n) is given by A111391(n). - _Daniel Starodubtsev_, Jan 06 2020

%t t = {}; Do[m = n; p = 10^(n + 1) + (10^n - 1); If[PrimeQ[p], AppendTo[t, p]], {n, 165}]; t (* _Robert G. Wilson v_ *)

%o (PARI) f(n) = for(x=1,n,y=10^x+10^(x-1)-1;if(ispseudoprime(y),print1(y", ")))

%Y Cf. A113628, A113629, A113630, A111391.

%K base,nonn

%O 1,1

%A _Cino Hilliard_, Jul 28 2004

%E Entry revised by _N. J. A. Sloane_, Apr 01 2006