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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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Some Thoughts re: Another way of seeing part of it | Comment 6 of 7 |
(In reply to Another way of seeing part of it by Jer)

I perhaps oversimplified the corresponding expression in my last post.

1/9(7*10^n - 1)(7*10^n + 7^2*10^(2n) + 1) =

1/3(7*10^n - 1)*1/3(7*10^n + 7^2*10^(2n) + 1) =

23 | 233 | 2333 | 23333 | 233333 * 1657 | 163567 | 16335667 | 1633356667 | 163333566667

etc.


  Posted by broll on 2017-06-30 23:14:23
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