this may be a silly question, but it's bothering me for a while.
I am going through the book "What is Mathematics?" by Richard Courant.
In the supplement to the first chapter, he speaks of the Euler function and the general way of finding the total number of relative primes of any number n.
For this, he gives a formula, and then says "The proof is quite elementary, but will be omitted here."
I tried to prove it without success, then looked up online to see various articles on multiplicative functions which don't seem elementary at all.
So do you think this is a joke in the spirit of Fermat? (his famous "elegant proof but too big for the margin?")
Or is it that multiplicative function is considered 'elementary'?