Using the definition of H Lifchitz:
"A PRP is a probable prime number, a number that nobody knows how to

prove or disprove its primality."

see also http://ourworld.compuserve.com/homepages/hlifchitz/

There are an infinite number of primes that can not be proven to be
prime.

If we consider the number 10^(d+0^m) + n as prime, where
(d+0^m) means a digit followed by m 0's, then the average distance
between primes on either side of this is ln(d+0^m) * ln 10.
So, n is approximately ln(d+0^m) * ln 10 / 2.

Like my number 10^20000 + 56149.

There are an infinite number of these numbers,
just keep making m larger.

Milton L. Brown
miltbrown@...