Carmichael numbers of the form (6n+1)(12n+1)(18n+1)

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Homework Statement



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Homework Equations





The Attempt at a Solution



I'm fine with the second part (n = 6). But the first part is eluding me, I've been told it's quite simple.

I feel like it's something to do with the fact that for each prime factor of C_n, p1, p2, p3, we have

p-1|C_n-1

But don't really know how to use this.
 
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Does it help to consider acn-1 mod (6n+1) etc?
 
Pick a so that gcd(a,c_n)=1. So gcd(a,p1)=1. So a^(p1-1)=1 mod p1. See a good place to use your divisibility fact?
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
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