Simplify the following text for a Grade 8 native English-speaking person.
Original: The function used here is the totient function , usually called the Euler totient or Euler 's totient , after the Swiss mathematician Leonhard Euler , who studied it .
Simplified: The function so defined is the totient function . The totient is usually called the Euler totient or Euler 's totient , after the Swiss mathematician Leonhard Euler , who studied it .