Milton Brown wrote

> 9901 (9909901,...) and 9091 (90901,...)

> which also quite often are prime. I think David

> indicated where these have been studied before.

The latter is

ID Number: A001562 (Formerly M3767 and N1537)

Sequence: 5,7,19,31,53,67,293,641,2137,3011

Name: (10^p + 1)/11 is prime.

References J. Brillhart et al., Factorizations of b^n +- 1.

in the EIS. But the former does not figure in the

Cunningham project tables for cyclotomic

factors of 10^n +/- 1.

Perhaps you meant

f(n)=(10^(6*n)+1)/(10^(2*n)+1)

f(1)= 9901 is prime

f(2)= 99990001 is prime

f(3)= 999999000001 is prime

f(4)= 9999999900000001 is prime

Puzzle: Find the next prime of this form :-)

David