# How many programs for the Euler phi?

How many ways can you think of , for writing a program that gives Euler's Totient function ?

Here are 2 that use 2 different definitions of the phi function.

$\mathbf{1.}$ This program uses the basic definition of $\phi(n)$ , which is

### $\phi(n)$ is the number of natural numbers less than $n$, which are coprime to $n$

 1 2 3 4 5 6 7 8 >>> from fractions import gcd >>> def phi(n): """Euler's Totient Function... phi(n)""" li=[] for i in range(n): if gcd(n,i)==1: li.append(i) return len(li) 

Now as we took $\text{gcd}(n,i)=1$ ,

we actually took them coprime, counting all of them and getting them in the list li.

We print len(li) which is length of the list, the number of natural numbers less than $n$, which are coprime to $n$.

$\mathbf{2.}$ This program uses a formula by using primes,

### For a natural number $n$ , we can define $\phi(n)$ as $\phi(n) = n\times \prod _{p\mid n} \Bigl(1-\frac{1}{p}\Bigr)$

or in other words, if $n = p_1^{a_1}p_2^{a_2} ... p_n^{a_n}$, then $\phi(n) = n \Bigl(1-\frac{1}{p_1}\Bigr)\Bigl(1-\frac{1}{p_2}\Bigr)....\Bigl(1-\frac{1}{p_n}\Bigr)$

  1 2 3 4 5 6 7 8 9 10 >>> def checkprime(n): return not(n<2 or any(n%i==0 for i in range(2,int(n**0.5)+1))) >>> def phi(n): """Euler Totient Function... phi(n)""" k=n for i in range(n): if checkprime(i) is True and n%i==0: k= k- k/i return k 

Hence for every prime number $p$ dividing $n$, the program transforms $n\rightarrow n - \frac{n}{p}$ , at the end resulting into the value $\phi(n)$

Anyone knowing any other method is welcome to post it :)

6 years, 3 months ago

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Wow! Still no comments ? Pls wait till the 4th , I'll post a Java solution then :)

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