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38s and Bunches of 1s (Posted on 2017-06-30) Difficulty: 3 of 5
Prove every number in the sequence 38, 381, 3811, 38111, 381111, ... is composite.

See The Solution Submitted by Brian Smith    
Rating: 4.5000 (2 votes)

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Another way of seeing part of it | Comment 5 of 7 |
for any term x, the next term is 10x+1, 
the term after that is 10(10x+1)+1 = 100x+11,
the term after that is 10(10(10x+1)+1)+1 = 1000x+111

111 = 3*37 so if any term is divisible by 3, the third term after that is too.  Same goes for 37.

I'm not sure how this explains the 2333333 

  Posted by Jer on 2017-06-30 14:03:47
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