# THE EULER TOTIENT FUNCTION Φ The Euler function, or totient function φ is a very important number theoretic function having a deep relationship khổng lồ prime numbers & the so-called order of integers.

The Euler function φ: NN is a mapping associating to each positive integer n the number φ(n) of integers m n relatively prime lớn n. (In other words: φ(n) is the number of positive integers m n with gcd(m, n) = 1.) E.g., φ(6) = 2 (since only 1 und 5 are relatively prime lớn 6), or φ(15) = 8 (for 1, 2, 4, 7, 8, 11, 13, & 14 are relatively prime lớn 15). The following table shows the function values for the first natural numbers.

n φ(n)
 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 1 2 2 4 2 6 4 6 4 10 4 12 6 8

bởi you discover some formation rule? If so, you should go on researching, because you could solve number theoretic problems being mở cửa so far... If not, you might be interested in some of the most important properties of the Euler function. The first one is that there exists a formula to lớn compute φ(n): If the prime factorization of n is given by n = p1a1 ... Pkak, we have

 φ(n) = n (1 - 1/p1) ... (1 - 1/pk).

Example: 12 = 22 · 3, i.e.

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φ(12) = 12 · (1 - 1/2) · (1 - 1/3) = 12 · 2 / (2 · 3) = 4. A further property of the totient function is the sum formula: The sum over the Euler function values φ(d) of all divisors d of an integer number n exactly gives n. E.g. For n=12:

φ(1) + φ(2) + φ(3) + φ(4) + φ(6) + φ(12) = 1 + 1 + 2 + 2 + 2 + 4 = 12.

Moreover, the order ordn(m) of an integer m modulo n always divides φ(n). Closely related to lớn the Euler function is the Carmichael function λ.

### References

Friedhelm Padberg: Elementare Zahlentheorie. Spektrum Akademischer Verlag, Heidelberg Berlin 1996 Hans Riesel: Prime Numbers và Computer Methods for Factorization. Birkhäuser Boston Basel Berlin 1994
 © de Vries 2002  