Find M such that GCD of M and given number N is maximum

Last Updated : 01 Oct, 2021

Given an integer N greater than 2, the task is to find an element M such that GCD(N, M) is maximum.

Examples:

Input: N = 10
Output: 5
Explanation:
gcd(1, 10), gcd(3, 10), gcd(7, 10), gcd(9, 10) is 1,
gcd(2, 10), gcd(4, 10), gcd(6, 10), gcd(8, 10) is 2,
gcd(5, 10) is 5 which is maximum.

Input: N = 21
Output: 7
Explanation:
gcd(7, 21) is maximum among all the integers from 1 to 21.

Naive Approach: The simplest approach is to loop through all the numbers in the range [1, N-1] and find GCD of each number with N. The number which given maximum GCD with N is the required result.

Time Complexity: O(N)
Auxiliary Space: O(1)

Efficient Approach: To optimize the above approach, we observe that the GCD of two numbers will be definitely one of its divisors in the range [1, N-1]. And, GCD will be maximum if the divisor is maximum.
Therefore, the idea is to find all the divisors of N and store a maximum of those divisors which is the required result.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the integer M
// such that gcd(N, M) is maximum
int findMaximumGcd(int n)
{
    // Initialize a variable
    int max_gcd = 1;
 
    // Find all the divisors of N and
    // return the maximum divisor
    for (int i = 1; i * i <= n; i++) {
 
        // Check if i is divisible by N
        if (n % i == 0) {
 
            // Update max_gcd
            if (i > max_gcd)
                max_gcd = i;
 
            if ((n / i != i)
                && (n / i != n)
                && ((n / i) > max_gcd))
                max_gcd = n / i;
        }
    }
 
    // Return the maximum value
    return max_gcd;
}
 
// Driver Code
int main()
{
    // Given Number
    int N = 10;
 
    // Function Call
    cout << findMaximumGcd(N);
    return 0;
}

Java

// Java program for the above approach
import java.util.*;
 
class GFG{
 
// Function to find the integer M
// such that gcd(N, M) is maximum
static int findMaximumGcd(int n)
{
     
    // Initialize a variable
    int max_gcd = 1;
 
    // Find all the divisors of N and
    // return the maximum divisor
    for(int i = 1; i * i <= n; i++) 
    {
         
        // Check if i is divisible by N
        if (n % i == 0) 
        {
             
            // Update max_gcd
            if (i > max_gcd)
                max_gcd = i;
 
            if ((n / i != i) && 
                (n / i != n) &&
               ((n / i) > max_gcd))
                max_gcd = n / i;
        }
    }
 
    // Return the maximum value
    return max_gcd;
}
 
// Driver Code
public static void main(String[] args)
{
     
    // Given Number
    int N = 10;
 
    // Function Call
    System.out.print(findMaximumGcd(N));
}
}
 
// This code is contributed by Amit Katiyar

Python3

# Python3 program for the above approach
 
# Function to find the integer M 
# such that gcd(N, M) is maximum
def findMaximumGcd(n):
     
    # Initialize variables 
    max_gcd = 1
    i = 1
     
    # Find all the divisors of N and 
    # return the maximum divisor 
    while (i * i <= n):
         
        # Check if i is divisible by N
        if n % i == 0:
             
            # Update max_gcd 
            if (i > max_gcd):
                max_gcd = i
                 
            if ((n / i != i) and
                (n / i != n) and
               ((n / i) > max_gcd)):
                max_gcd = n / i
        i += 1
         
    # Return the maximum value 
    return (int(max_gcd))
 
# Driver Code 
if __name__ == '__main__':
     
    # Given number 
    n = 10
     
    # Function call
    print(findMaximumGcd(n))
     
# This code is contributed by virusbuddah_

C#

// C# program for the 
// above approach
using System;
class GFG{
 
// Function to find the 
// integer M such that 
// gcd(N, M) is maximum
static int findMaximumGcd(int n)
{    
  // Initialize a variable
  int max_gcd = 1;
 
  // Find all the divisors of 
  // N and return the maximum 
  // divisor
  for(int i = 1; 
          i * i <= n; i++) 
  {
    // Check if i is 
    // divisible by N
    if (n % i == 0) 
    {
      // Update max_gcd
      if (i > max_gcd)
        max_gcd = i;
 
      if ((n / i != i) && 
          (n / i != n) &&
          ((n / i) > max_gcd))
        max_gcd = n / i;
    }
  }
 
  // Return the maximum 
  // value
  return max_gcd;
}
 
// Driver Code
public static void Main(String[] args)
{    
  // Given Number
  int N = 10;
 
  // Function Call
  Console.Write(findMaximumGcd(N));
}
}
 
// This code is contributed by Rajput-Ji

Javascript

<script>
 
// Javascript program to implement
// the above approach
 
// Function to find the integer M
// such that gcd(N, M) is maximum
function findMaximumGcd(n)
{
      
    // Initialize a variable
    let max_gcd = 1;
  
    // Find all the divisors of N and
    // return the maximum divisor
    for(let i = 1; i * i <= n; i++)
    {
          
        // Check if i is divisible by N
        if (n % i == 0)
        {
              
            // Update max_gcd
            if (i > max_gcd)
                max_gcd = i;
  
            if ((n / i != i) &&
                (n / i != n) &&
               ((n / i) > max_gcd))
                max_gcd = n / i;
        }
    }
  
    // Return the maximum value
    return max_gcd;
}
 
    // Driver Code
         
    // Given Number
    let N = 10;
  
    // Function Call
    document.write(findMaximumGcd(N));
 
</script>

Output:

Time Complexity: O(log₂N)
Auxiliary Space: O(1)

Suggest improvement

Finding LCM of more than two (or array) numbers without using GCD

Maximum possible GCD for a pair of integers with product N

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Find M such that GCD of M and given number N is maximum

C++

Java

Python3

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