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Terminating in seven sevens (Posted on 2009-06-26) Difficulty: 2 of 5
Determine the minimum value of a positive base ten integer N, such that each of the last seven digits of N3 is 7 but the eighth digit from the right in N3 is not 7.

See The Solution Submitted by K Sengupta    
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re: computer solution Comment 2 of 2 |
(In reply to computer solution by Charlie)

I had tried to build this number up from one 7.  The problem was on my calculator I ran out of digits at 5 and didn't see a pattern to continue.

3^3 ends in 7

of a3, 53^3 ends in 77

of a53, 753 ends in 777


This is a pretty efficient way to search.  I'm sure a quick program could extend this to many digits.

  Posted by Jer on 2009-06-26 20:32:47

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