Calculate Euler's totient function
Mathematica, 27 22 bytes

Range@#~GCD~#~Count~1&amp;


An unnamed function that takes and returns an integer.

Not much to explain here, except that @ is prefix notation for function calls and ~...~ is (left-associative) infix notation, so the above is the same as:

Count[GCD[Range[#], #], 1] &amp;