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I Recently Learnt about Euler's Totient Theorem where $$ a^{\phi(n)}\equiv 1 \pmod n $$ Now I Am Stuck When Im Supposed To Prove The Following Summation $$ n = \sum_{d|n}^{}\phi(d) $$ So I Needed Some Hints To Prove The Following Summation. Majorly I'm Not Able To Understand what does $d|n$ means in the summation.

lulu
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mrtechtroid
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    The notation $d,|,n$ means that the sum is taken over the (positive) divisors of $n$. Note too that your first congruence only holds if $\gcd(a,n)=1$. That sort of detail is important, don't ignore it. – lulu Sep 14 '21 at 11:36
  • here is another duplicate. – lulu Sep 14 '21 at 11:39

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