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Binary Primes (Posted on 2004-02-18) Difficulty: 4 of 5
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, ...

See The Solution Submitted by Aaron    
Rating: 3.5000 (2 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Second thought | Comment 3 of 12 |
10101 or any number with 3n 1's is divisible by 3

so the # of 1's can't be even (except for 101) or a multiple of 3
  Posted by Larry on 2004-02-18 14:25:58
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