OFFSET
1,1
COMMENTS
Or, primes of the form 10^k + 10^(k-1) - 1.
Number of 9's in a(n) is given by A111391(n). - Daniel Starodubtsev, Jan 06 2020
MATHEMATICA
t = {}; Do[m = n; p = 10^(n + 1) + (10^n - 1); If[PrimeQ[p], AppendTo[t, p]], {n, 165}]; t (* Robert G. Wilson v *)
PROG
(PARI) f(n) = for(x=1, n, y=10^x+10^(x-1)-1; if(ispseudoprime(y), print1(y", ")))
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Cino Hilliard, Jul 28 2004
EXTENSIONS
Entry revised by N. J. A. Sloane, Apr 01 2006
STATUS
approved