How many primes, written in usual base 10, have digits that are alternating 1s and 0s, beginning and ending with one?

For example (not necessarily prime):
1, 101, 10101, ...

(In reply to No Subject by e.g.)

Nice. This line

"If, after dividing by the denominator, there still remain two factors in the numerator, the number won't be a prime."

I found a bit confusing at first, but I see now.

Posted by Brian Wainscott on 2004-02-18 19:03:50