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 Oh god! I read the question wrong. by Mitch Mullings)

"All others are divisible by 3"

Ah, no.

10101 = 3*3367
1010101 = 10101*100+1 = 3*336700+1

so clearly 1010101 is not a multiple of 3. Check out eg's post for a complete solution

Posted by Brian Wainscott on 2004-02-23 15:25:28