OFFSET
1,1
COMMENTS
Also numbers k such that (8*10^(k+1) - 71)/9 is prime.
There are no other terms <= 2500. - Rick L. Shepherd, Mar 02 2004
a(18) > 10^5. - Robert Price, Nov 01 2014
LINKS
FORMULA
a(n) = A099421(n+1) - 1. - Robert Price, Nov 01 2014
MATHEMATICA
Do[ If[ PrimeQ[ 80*(10^n - 1)/9 + 1 ], Print[n]], {n, 15000}]
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 09 2000
EXTENSIONS
a(9) (giving a probable prime) from Rick L. Shepherd, Mar 02 2004
a(10)-a(15) from N. J. A. Sloane, Feb 20 2005
a(16)-a(17) derived from A099421 by Robert Price, Nov 01 2014
STATUS
approved