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Proof about phi function

  1. Mar 19, 2013 #1
    1. The problem statement, all variables and given/known data
    [itex] \sum_{k<n} k= \frac{n \phi (n)}{2} [/itex]
    [itex] \phi [/itex] is Euler's phi function or Euler's totient function
    3. The attempt at a solution
    So the sum should be
    [itex] 1+2+3+.....+(n-2)+(n-1) [/itex]
    which will equal [itex] \frac{n^2-n}{2} [/itex]
    using the formula for the sum of integers by gauss.
    I can get this equal to the right side if n is prime
    then [itex] \phi (n) = n-1 [/itex] but for example if n=12 and k=11 this doesn't seem to work.
    It seem that n need to be prime for this to work, unless im missing something.
    Well I guess if k is less than n, that means its any integer less than n and if their
    gcd(n,k)=1 then n must be prime.
  2. jcsd
  3. Mar 19, 2013 #2
    The summation is taken over all k < n such that k and n are relatively prime (in other words, such that gcd(k,n) = 1). For example, if n = 12, the sum is

    [tex]1 + 5 + 7 + 11 = 24[/tex]
    (because 1, 5, 7 and 11 are all the integers less than 12 that are relatively prime to 12). Also

    [tex]\frac{12\phi(12)}{2} = \frac{12 \times 4}{2} = 24[/tex]
    Write out similar equations for several other values of n and try to find a pattern.
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