Find the sum from n = 1 to infinity of phi(n)/n^3. You can use the following facts. pi^2 / 6 = sum from n = 1 to infinity of 1/n^2, and the sum from n = 1 to infinity of 1/n^3 = a. (Note: phi(n) is the number of integers below n and above one that satisfy gcd(n,k) = 1)
Hint: Start with a given, and work it out till you get the desired expression.
Level: 6
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