#75
Euler's Totient Function
 

Difficulty:N/A
Topics:medium


Two numbers are coprime if their greatest common divisor equals 1. Euler's totient function f(x) is defined as the number of positive integers less than x which are coprime to x. The special case f(1) equals 1. Write a function which calculates Euler's totient function.
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(= (__ 1) 1)
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(= (__ 10) (count '(1 3 7 9)) 4)
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(= (__ 40) 16)
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(= (__ 99) 60)


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