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Long chain (Posted on 2015-04-10) Difficulty: 3 of 5
Create a chain of consecutive digits starting with 1 and following 9 by 0: 123456789012345...

How many digits are needed to obtain the 1st prime number?

See The Solution Submitted by Ady TZIDON    
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re(2): computer exploration | Comment 3 of 7 |
(In reply to re: computer exploration by Ady TZIDON)

I had already determined that the possible candidate(s) would end in 1 or 7:

The digital root of a prime number, but for 3, is never a multiple of 3. A prime number, but for 2, never ends in an even number.

The digital root of the chained number is a multiple of 3 except
when the number ends in 1, 4, or 7, in which case the digital root is 1.  Thus, the chained number, if prime, ends either in a 1 or 7.

  Posted by Dej Mar on 2015-04-11 00:23:31

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