Understanding Phi Function and Multiplicativity

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SUMMARY

The discussion focuses on the calculation of the Euler's Totient Function, specifically phi(20^100), using its multiplicative properties. The breakdown shows that phi(20^100) can be expressed as phi(2^200 * 5^100), leading to the final result of 2^201 * 5^99. Key formulas discussed include phi(p^a) = (p-1)(p^(a-1)) for prime p and the multiplicative property phi(a*b) = phi(a)*phi(b) when gcd(a,b)=1. Understanding these properties is essential for accurate computation of the totient function.

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  • Understanding of Euler's Totient Function
  • Familiarity with prime factorization
  • Knowledge of multiplicative functions in number theory
  • Basic algebraic manipulation skills
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  • Study the properties of Euler's Totient Function in detail
  • Learn about prime factorization techniques
  • Explore the concept of multiplicative functions in number theory
  • Practice calculating phi for various composite numbers
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Mathematicians, number theorists, and students studying advanced mathematics, particularly those interested in number theory and its applications.

hanelliot
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phi(20^100)
= phi(4^100 * 5^100)
= phi(2^200 * 5^100)
= (2^200 - 2^199)(5^100 - 5^99)
= 2^199(2-1) * 5^99(5-1)
= 2^199 * 5^99 * 4
= 2^201 * 5^99.

I don't understand line 4-7. Can anyone explain?
 
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phi(p^a) = (p-1)(p^(a-1)) for p prime
 
also since phi is multiplicative phi(a*b) = phi(a)*phi(b) when gcd(a,b)=1
 

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