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Primitive root of a prime number n modulo n
• Difficulty Level : Medium
• Last Updated : 26 Nov, 2019

Given a prime number n, the task is to find its primitive root under modulo n. Primitive root of a prime number n is an integer r between[1, n-1] such that the values of r^x(mod n) where x is in range[0, n-2] are different. Return -1 if n is a non-prime number.

Examples:

```Input : 7
Output : Smallest primitive root = 3
Explanation: n = 7
3^0(mod 7) = 1
3^1(mod 7) = 3
3^2(mod 7) = 2
3^3(mod 7) = 6
3^4(mod 7) = 4
3^5(mod 7) = 5

Input : 761
Output : Smallest primitive root = 6
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

A simple solution is to try all numbers from 2 to n-1. For every number r, compute values of r^x(mod n) where x is in range[0, n-2]. If all these values are different, then return r, else continue for next value of r. If all values of r are tried, return -1.

An efficient solution is based on below fact.
If the multiplicative order of a number r modulo n is equal to Euler Totient Function Φ(n) (Note that Euler Totient Function for a prime n is n-1), then it is a primitive root [Source : Wiki]

```1- Euler Totient Function phi = n-1 [Assuming n is prime]
1- Find all prime factors of phi.
2- Calculate all powers to be calculated further
using (phi/prime-factors) one by one.
3- Check for all numbered for all powers from i=2
to n-1 i.e. (i^ powers) modulo n.
4- If it is 1 then 'i' is not a primitive root of n.
5- If it is never 1 then return i;.
```

Although there can be multiple primitive root for a prime number but we are only concerned for smallest one.If you want to find all roots then continue the process till p-1 instead of breaking up on finding first primitive root.

## C++

 `// C++ program to find primitive root of a``// given number n``#include``using` `namespace` `std;`` ` `// Returns true if n is prime``bool` `isPrime(``int` `n)``{``    ``// Corner cases``    ``if` `(n <= 1)  ``return` `false``;``    ``if` `(n <= 3)  ``return` `true``;`` ` `    ``// This is checked so that we can skip``    ``// middle five numbers in below loop``    ``if` `(n%2 == 0 || n%3 == 0) ``return` `false``;`` ` `    ``for` `(``int` `i=5; i*i<=n; i=i+6)``        ``if` `(n%i == 0 || n%(i+2) == 0)``            ``return` `false``;`` ` `    ``return` `true``;``}`` ` `/* Iterative Function to calculate (x^n)%p in``   ``O(logy) */``int` `power(``int` `x, unsigned ``int` `y, ``int` `p)``{``    ``int` `res = 1;     ``// Initialize result`` ` `    ``x = x % p; ``// Update x if it is more than or``    ``// equal to p`` ` `    ``while` `(y > 0)``    ``{``        ``// If y is odd, multiply x with result``        ``if` `(y & 1)``            ``res = (res*x) % p;`` ` `        ``// y must be even now``        ``y = y >> 1; ``// y = y/2``        ``x = (x*x) % p;``    ``}``    ``return` `res;``}`` ` `// Utility function to store prime factors of a number``void` `findPrimefactors(unordered_set<``int``> &s, ``int` `n)``{``    ``// Print the number of 2s that divide n``    ``while` `(n%2 == 0)``    ``{``        ``s.insert(2);``        ``n = n/2;``    ``}`` ` `    ``// n must be odd at this point. So we can skip``    ``// one element (Note i = i +2)``    ``for` `(``int` `i = 3; i <= ``sqrt``(n); i = i+2)``    ``{``        ``// While i divides n, print i and divide n``        ``while` `(n%i == 0)``        ``{``            ``s.insert(i);``            ``n = n/i;``        ``}``    ``}`` ` `    ``// This condition is to handle the case when``    ``// n is a prime number greater than 2``    ``if` `(n > 2)``        ``s.insert(n);``}`` ` `// Function to find smallest primitive root of n``int` `findPrimitive(``int` `n)``{``    ``unordered_set<``int``> s;`` ` `    ``// Check if n is prime or not``    ``if` `(isPrime(n)==``false``)``        ``return` `-1;`` ` `    ``// Find value of Euler Totient function of n``    ``// Since n is a prime number, the value of Euler``    ``// Totient function is n-1 as there are n-1``    ``// relatively prime numbers.``    ``int` `phi = n-1;`` ` `    ``// Find prime factors of phi and store in a set``    ``findPrimefactors(s, phi);`` ` `    ``// Check for every number from 2 to phi``    ``for` `(``int` `r=2; r<=phi; r++)``    ``{``        ``// Iterate through all prime factors of phi.``        ``// and check if we found a power with value 1``        ``bool` `flag = ``false``;``        ``for` `(``auto` `it = s.begin(); it != s.end(); it++)``        ``{`` ` `            ``// Check if r^((phi)/primefactors) mod n``            ``// is 1 or not``            ``if` `(power(r, phi/(*it), n) == 1)``            ``{``                ``flag = ``true``;``                ``break``;``            ``}``         ``}`` ` `         ``// If there was no power with value 1.``         ``if` `(flag == ``false``)``           ``return` `r;``    ``}`` ` `    ``// If no primitive root found``    ``return` `-1;``}`` ` `// Driver code``int` `main()``{``    ``int` `n = 761;``    ``cout << ``" Smallest primitive root of "` `<< n``         ``<< ``" is "` `<< findPrimitive(n);``    ``return` `0;``}`

## Java

 `// Java program to find primitive root of a``// given number n``import` `java.util.*;`` ` `class` `GFG``{`` ` `    ``// Returns true if n is prime``    ``static` `boolean` `isPrime(``int` `n) ``    ``{``        ``// Corner cases``        ``if` `(n <= ``1``)``        ``{``            ``return` `false``;``        ``}``        ``if` `(n <= ``3``) ``        ``{``            ``return` `true``;``        ``}`` ` `        ``// This is checked so that we can skip``        ``// middle five numbers in below loop``        ``if` `(n % ``2` `== ``0` `|| n % ``3` `== ``0``) ``        ``{``            ``return` `false``;``        ``}`` ` `        ``for` `(``int` `i = ``5``; i * i <= n; i = i + ``6``) ``        ``{``            ``if` `(n % i == ``0` `|| n % (i + ``2``) == ``0``) ``            ``{``                ``return` `false``;``            ``}``        ``}`` ` `        ``return` `true``;``    ``}`` ` `    ``/* Iterative Function to calculate (x^n)%p in``    ``O(logy) */``    ``static` `int` `power(``int` `x, ``int` `y, ``int` `p)``    ``{``        ``int` `res = ``1``;     ``// Initialize result`` ` `        ``x = x % p; ``// Update x if it is more than or``        ``// equal to p`` ` `        ``while` `(y > ``0``) ``        ``{``            ``// If y is odd, multiply x with result``            ``if` `(y % ``2` `== ``1``) ``            ``{``                ``res = (res * x) % p;``            ``}`` ` `            ``// y must be even now``            ``y = y >> ``1``; ``// y = y/2``            ``x = (x * x) % p;``        ``}``        ``return` `res;``    ``}`` ` `    ``// Utility function to store prime factors of a number``    ``static` `void` `findPrimefactors(HashSet s, ``int` `n) ``    ``{``        ``// Print the number of 2s that divide n``        ``while` `(n % ``2` `== ``0``) ``        ``{``            ``s.add(``2``);``            ``n = n / ``2``;``        ``}`` ` `        ``// n must be odd at this point. So we can skip``        ``// one element (Note i = i +2)``        ``for` `(``int` `i = ``3``; i <= Math.sqrt(n); i = i + ``2``) ``        ``{``            ``// While i divides n, print i and divide n``            ``while` `(n % i == ``0``) ``            ``{``                ``s.add(i);``                ``n = n / i;``            ``}``        ``}`` ` `        ``// This condition is to handle the case when``        ``// n is a prime number greater than 2``        ``if` `(n > ``2``) ``        ``{``            ``s.add(n);``        ``}``    ``}`` ` `    ``// Function to find smallest primitive root of n``    ``static` `int` `findPrimitive(``int` `n) ``    ``{``        ``HashSet s = ``new` `HashSet();`` ` `        ``// Check if n is prime or not``        ``if` `(isPrime(n) == ``false``) ``        ``{``            ``return` `-``1``;``        ``}`` ` `        ``// Find value of Euler Totient function of n``        ``// Since n is a prime number, the value of Euler``        ``// Totient function is n-1 as there are n-1``        ``// relatively prime numbers.``        ``int` `phi = n - ``1``;`` ` `        ``// Find prime factors of phi and store in a set``        ``findPrimefactors(s, phi);`` ` `        ``// Check for every number from 2 to phi``        ``for` `(``int` `r = ``2``; r <= phi; r++) ``        ``{``            ``// Iterate through all prime factors of phi.``            ``// and check if we found a power with value 1``            ``boolean` `flag = ``false``;``            ``for` `(Integer a : s) ``            ``{`` ` `                ``// Check if r^((phi)/primefactors) mod n``                ``// is 1 or not``                ``if` `(power(r, phi / (a), n) == ``1``) ``                ``{``                    ``flag = ``true``;``                    ``break``;``                ``}``            ``}`` ` `            ``// If there was no power with value 1.``            ``if` `(flag == ``false``)``            ``{``                ``return` `r;``            ``}``        ``}`` ` `        ``// If no primitive root found``        ``return` `-``1``;``    ``}`` ` `    ``// Driver code``    ``public` `static` `void` `main(String[] args) ``    ``{``        ``int` `n = ``761``;``        ``System.out.println(``" Smallest primitive root of "` `+ n``                ``+ ``" is "` `+ findPrimitive(n));``    ``}``}`` ` `/* This code contributed by PrinciRaj1992 */`

## Python3

 `# Python3 program to find primitive root ``# of a given number n ``from` `math ``import` `sqrt`` ` `# Returns True if n is prime ``def` `isPrime( n): `` ` `    ``# Corner cases ``    ``if` `(n <``=` `1``):``        ``return` `False``    ``if` `(n <``=` `3``):``        ``return` `True`` ` `    ``# This is checked so that we can skip ``    ``# middle five numbers in below loop ``    ``if` `(n ``%` `2` `=``=` `0` `or` `n ``%` `3` `=``=` `0``):``        ``return` `False``    ``i ``=` `5``    ``while``(i ``*` `i <``=` `n):``        ``if` `(n ``%` `i ``=``=` `0` `or` `n ``%` `(i ``+` `2``) ``=``=` `0``) :``            ``return` `False``        ``i ``=` `i ``+` `6`` ` `    ``return` `True`` ` `""" Iterative Function to calculate (x^n)%p``    ``in O(logy) */"""``def` `power( x, y, p): `` ` `    ``res ``=` `1` `# Initialize result `` ` `    ``x ``=` `x ``%` `p ``# Update x if it is more ``              ``# than or equal to p `` ` `    ``while` `(y > ``0``): `` ` `        ``# If y is odd, multiply x with result ``        ``if` `(y & ``1``):``            ``res ``=` `(res ``*` `x) ``%` `p `` ` `        ``# y must be even now ``        ``y ``=` `y >> ``1` `# y = y/2 ``        ``x ``=` `(x ``*` `x) ``%` `p `` ` `    ``return` `res `` ` `# Utility function to store prime``# factors of a number ``def` `findPrimefactors(s, n) :`` ` `    ``# Print the number of 2s that divide n ``    ``while` `(n ``%` `2` `=``=` `0``) :``        ``s.add(``2``) ``        ``n ``=` `n ``/``/` `2`` ` `    ``# n must be odd at this po. So we can  ``    ``# skip one element (Note i = i +2) ``    ``for` `i ``in` `range``(``3``, ``int``(sqrt(n)), ``2``):``         ` `        ``# While i divides n, print i and divide n ``        ``while` `(n ``%` `i ``=``=` `0``) :`` ` `            ``s.add(i) ``            ``n ``=` `n ``/``/` `i ``         ` `    ``# This condition is to handle the case ``    ``# when n is a prime number greater than 2 ``    ``if` `(n > ``2``) :``        ``s.add(n) `` ` `# Function to find smallest primitive ``# root of n ``def` `findPrimitive( n) :``    ``s ``=` `set``() `` ` `    ``# Check if n is prime or not ``    ``if` `(isPrime(n) ``=``=` `False``): ``        ``return` `-``1`` ` `    ``# Find value of Euler Totient function ``    ``# of n. Since n is a prime number, the ``    ``# value of Euler Totient function is n-1 ``    ``# as there are n-1 relatively prime numbers.``    ``phi ``=` `n ``-` `1`` ` `    ``# Find prime factors of phi and store in a set ``    ``findPrimefactors(s, phi) `` ` `    ``# Check for every number from 2 to phi ``    ``for` `r ``in` `range``(``2``, phi ``+` `1``): `` ` `        ``# Iterate through all prime factors of phi. ``        ``# and check if we found a power with value 1 ``        ``flag ``=` `False``        ``for` `it ``in` `s: `` ` `            ``# Check if r^((phi)/primefactors)``            ``# mod n is 1 or not ``            ``if` `(power(r, phi ``/``/` `it, n) ``=``=` `1``): `` ` `                ``flag ``=` `True``                ``break``             ` `        ``# If there was no power with value 1. ``        ``if` `(flag ``=``=` `False``):``            ``return` `r `` ` `    ``# If no primitive root found ``    ``return` `-``1`` ` `# Driver Code ``n ``=` `761``print``(``"Smallest primitive root of"``, ``         ``n, ``"is"``, findPrimitive(n))`` ` `# This code is contributed by``# Shubham Singh(SHUBHAMSINGH10)`

## C#

 `// C# program to find primitive root of a``// given number n ``using` `System;``using` `System.Collections.Generic;`` ` `class` `GFG``{`` ` `    ``// Returns true if n is prime``    ``static` `bool` `isPrime(``int` `n) ``    ``{``        ``// Corner cases``        ``if` `(n <= 1)``        ``{``            ``return` `false``;``        ``}``        ``if` `(n <= 3) ``        ``{``            ``return` `true``;``        ``}`` ` `        ``// This is checked so that we can skip``        ``// middle five numbers in below loop``        ``if` `(n % 2 == 0 || n % 3 == 0) ``        ``{``            ``return` `false``;``        ``}`` ` `        ``for` `(``int` `i = 5; i * i <= n; i = i + 6) ``        ``{``            ``if` `(n % i == 0 || n % (i + 2) == 0) ``            ``{``                ``return` `false``;``            ``}``        ``}`` ` `        ``return` `true``;``    ``}`` ` `    ``/* Iterative Function to calculate (x^n)%p in``    ``O(logy) */``    ``static` `int` `power(``int` `x, ``int` `y, ``int` `p)``    ``{``        ``int` `res = 1;     ``// Initialize result`` ` `        ``x = x % p; ``// Update x if it is more than or``        ``// equal to p`` ` `        ``while` `(y > 0) ``        ``{``            ``// If y is odd, multiply x with result``            ``if` `(y % 2 == 1) ``            ``{``                ``res = (res * x) % p;``            ``}`` ` `            ``// y must be even now``            ``y = y >> 1; ``// y = y/2``            ``x = (x * x) % p;``        ``}``        ``return` `res;``    ``}`` ` `    ``// Utility function to store prime factors of a number``    ``static` `void` `findPrimefactors(HashSet<``int``> s, ``int` `n) ``    ``{``        ``// Print the number of 2s that divide n``        ``while` `(n % 2 == 0) ``        ``{``            ``s.Add(2);``            ``n = n / 2;``        ``}`` ` `        ``// n must be odd at this point. So we can skip``        ``// one element (Note i = i +2)``        ``for` `(``int` `i = 3; i <= Math.Sqrt(n); i = i + 2) ``        ``{``            ``// While i divides n, print i and divide n``            ``while` `(n % i == 0) ``            ``{``                ``s.Add(i);``                ``n = n / i;``            ``}``        ``}`` ` `        ``// This condition is to handle the case when``        ``// n is a prime number greater than 2``        ``if` `(n > 2) ``        ``{``            ``s.Add(n);``        ``}``    ``}`` ` `    ``// Function to find smallest primitive root of n``    ``static` `int` `findPrimitive(``int` `n) ``    ``{``        ``HashSet<``int``> s = ``new` `HashSet<``int``>();`` ` `        ``// Check if n is prime or not``        ``if` `(isPrime(n) == ``false``) ``        ``{``            ``return` `-1;``        ``}`` ` `        ``// Find value of Euler Totient function of n``        ``// Since n is a prime number, the value of Euler``        ``// Totient function is n-1 as there are n-1``        ``// relatively prime numbers.``        ``int` `phi = n - 1;`` ` `        ``// Find prime factors of phi and store in a set``        ``findPrimefactors(s, phi);`` ` `        ``// Check for every number from 2 to phi``        ``for` `(``int` `r = 2; r <= phi; r++) ``        ``{``            ``// Iterate through all prime factors of phi.``            ``// and check if we found a power with value 1``            ``bool` `flag = ``false``;``            ``foreach` `(``int` `a ``in` `s) ``            ``{`` ` `                ``// Check if r^((phi)/primefactors) mod n``                ``// is 1 or not``                ``if` `(power(r, phi / (a), n) == 1) ``                ``{``                    ``flag = ``true``;``                    ``break``;``                ``}``            ``}`` ` `            ``// If there was no power with value 1.``            ``if` `(flag == ``false``)``            ``{``                ``return` `r;``            ``}``        ``}`` ` `        ``// If no primitive root found``        ``return` `-1;``    ``}`` ` `    ``// Driver code``    ``public` `static` `void` `Main(String[] args) ``    ``{``        ``int` `n = 761;``        ``Console.WriteLine(``" Smallest primitive root of "` `+ n``                ``+ ``" is "` `+ findPrimitive(n));``    ``}``}`` ` `// This code is contributed by Rajput-Ji`

Output:

```Smallest primitive root of 761 is 6
```

This article is contributed by Niteesh kumar and Sahil Chhabra (akku). If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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