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No primes (Posted on 2011-01-08) Difficulty: 2 of 5
Prove that no number in the sequence 949, 9449, 94449... is prime.

See The Solution Submitted by Math Man    
Rating: 4.3333 (3 votes)

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re: No primes here (spoiler) .... a typo | Comment 2 of 4 |
(In reply to No primes here (spoiler) by Steve Herman)

Good reasoning and a correct answer.

 

Just  change 1 into 5 to fix a small typo:

......'Trying 7 and 37 as factors of 9444449, we find that 9444449 = 7*1349207.  If the number of 4's (mod 6) = 1, then it is therefore a multiple of 7.  (We could prove this by calculating divisors mod 7 of 10^n, which necessarily repeat with period 6.)"
 
...for the benefit of other readers.

  Posted by Ady TZIDON on 2011-01-08 17:19:31
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