OFFSET
1,1
COMMENTS
Numbers n such that 10^n-1 concatenated with 10^(n-1)+1 is prime.
a(8) > 10^4.
EXAMPLE
The largest and smallest odd 3-digit numbers are 999 and 101, respectively. Since 999101 is prime, 3 is a member of the sequence.
The largest and smallest odd 4-digit numbers are 9999 and 1001, respectively. Since 99991001 is not prime, 4 is not a member of this sequence.
PROG
(PARI) f(n) = 10^(n-1)*(10^(n+1)-9)+1
for(n=1, 10^4, if(ispseudoprime(f(n)), print1(n, ", ")))
CROSSREFS
KEYWORD
nonn,base,more,hard
AUTHOR
Derek Orr, Nov 22 2014
STATUS
approved