What is Euler's Totient Function?
Number theory is one of the most important topics in the field of Math and can be used to solve a variety of problems. Many times one might have come across problems that relate to the prime factorization of a number, to the divisors of a number, to the multiples of a number and so on.
Euler's Totient function is a function that is related to getting the number of numbers that are coprime to a certain number that are less than or equal to it. In short , for a certain number we need to find the count of all numbers where and .
A naive method to do so would be to Brute-Force the answer by checking the gcd of and every number less than or equal to and then incrementing the count whenever a of is obtained. However, this can be done in a much faster way using Euler's Totient Function.
According to Euler's product formula, the value of the Totient function is below the product over all prime factors of a number. This formula simply states that the value of the Totient function is the product after multiplying the number by the product of for each prime factor of .
So,
Algorithm steps:
- Generate a list of primes.
- While dealing with a certain , check and store all the primes that perfectly divide .
- Now, it is just needed to use these primes and the above formula to get the result.
Implementation:
set<> primes;
static void mark(int num,int max,int[] arr)
{
int i=2,elem;
while((elem=(num*i))<=max)
{
arr[elem-1]=1;
i++;
}
}
GeneratePrimes()
{
int arr[max_prime];
for(int i=1;i<arr.length;i++)
{
if(arr[i]==0)
{
list.add(i+1);
mark(i+1,arr.length-1,arr);
}
}
}
main()
{
GeneratePrimes();
int N=nextInt();
int ans=N;
for(int k:set)
{
if(N%k==0)
{
ans*=(1-1/k);
}
}
print(ans);
}
There are a few subtle observations that one can make about Euler's Totient Function.
- The sum of all values of Totient Function of all divisors of is equal to .
- The value of Totient function for a certain prime will always be as the number will always have a of with all numbers less than or equal to it except itself.
- For 2 number A and B, if then = .
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