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Check if a number is a Trojan Number
  • Difficulty Level : Medium
  • Last Updated : 13 May, 2020
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Given a Number N. The task is to check if N is a Trojan Number or not.

Trojan Number is a number that is a strong number but not a perfect power. A number N is known as a strong number if, for every prime divisor or factor p of N, p2 is also a divisor. In other words, every prime factor appears at least twice.

All Trojan numbers are strong. However, not all strong numbers are Trojan numbers: only those that cannot be represented as mk, where m and k are positive integers greater than 1.

Examples:

Input : N = 108
Output : YES

Input : N = 8
Output : NO

The idea is to store the count of each prime factor and check if the count is greater than 2 then it will be a Strong Number.



This part can easily be calculated by prime factorization through sieve.

The next step is to check if the given number cannot be expressed as xy. To check whether a number is perfect power or not refer to this article.

Below is the implementation of above problem:

C++




// CPP program to check if a number is
// Trojan Number or not
  
#include <bits/stdc++.h>
using namespace std;
  
// Function to check if a number
// can be expressed as x^y
bool isPerfectPower(int n)
{
    if (n == 1)
        return true;
  
    // Try all numbers from 2 to sqrt(n) as base
    for (int x = 2; x <= sqrt(n); x++) {
        int y = 2;
        int p = pow(x, y);
  
        // Keep increasing y while power 'p'
        // is smaller than n.
        while (p <= n && p > 0) {
            if (p == n)
                return true;
            y++;
            p = pow(x, y);
        }
    }
    return false;
}
  
// Function to check if a number is Strong
bool isStrongNumber(int n)
{
    unordered_map<int, int> count;
    while (n % 2 == 0) {
        n = n / 2;
        count[2]++;
    }
  
    // count the number for each prime factor
    for (int i = 3; i <= sqrt(n); i += 2) {
        while (n % i == 0) {
            n = n / i;
            count[i]++;
        }
    }
  
    if (n > 2)
        count[n]++;
  
    int flag = 0;
  
    for (auto b : count) {
  
        // minimum number of prime divisors
        // should be 2
        if (b.second == 1) {
            flag = 1;
            break;
        }
    }
  
    if (flag == 1)
        return false;
    else
        return true;
}
  
// Function to check if a number
// is Trojan Number
bool isTrojan(int n)
{
    if (!isPerfectPower(n) && isStrongNumber(n))
        return true;
    else
        return false;
}
  
// Driver Code
int main()
{
    int n = 108;
  
    if (isTrojan(n))
        cout << "YES";
    else
        cout << "NO";
  
    return 0;
}

Java




// Java program to check if a number is
// Trojan Number or not
import java.util.*;
  
class GFG 
{
  
    // Function to check if a number
    // can be expressed as x^y
    static boolean isPerfectPower(int n)
    {
        if (n == 1)
        {
            return true;
        }
  
        // Try all numbers from 2 to sqrt(n) as base
        for (int x = 2; x <= Math.sqrt(n); x++) 
        {
            int y = 2;
            int p = (int) Math.pow(x, y);
  
            // Keep increasing y while power 'p'
            // is smaller than n.
            while (p <= n && p > 0
            {
                if (p == n) 
                {
                    return true;
                }
                y++;
                p = (int) Math.pow(x, y);
            }
        }
        return false;
    }
  
    // Function to check if a number is Strong
    static boolean isStrongNumber(int n) 
    {
        HashMap<Integer, 
                Integer> count = new HashMap<Integer, 
                                             Integer>();
        while (n % 2 == 0
        {
            n = n / 2;
            if (count.containsKey(2)) 
            {
                count.put(2, count.get(2) + 1);
            
            else 
            {
                count.put(2, 1);
            }
        }
  
        // count the number for each prime factor
        for (int i = 3; i <= Math.sqrt(n); i += 2
        {
            while (n % i == 0)
            {
                n = n / i;
                if (count.containsKey(i))
                {
                    count.put(i, count.get(i) + 1);
                }
                else 
                {
                    count.put(i, 1);
                }
            }
        }
  
        if (n > 2)
        {
            if (count.containsKey(n))
            {
                count.put(n, count.get(n) + 1);
            
            else 
            {
                count.put(n, 1);
            }
        }
  
        int flag = 0;
  
        for (Map.Entry<Integer, 
                       Integer> b : count.entrySet()) 
        {
  
            // minimum number of prime divisors
            // should be 2
            if (b.getValue() == 1)
            {
                flag = 1;
                break;
            }
        }
  
        if (flag == 1
        {
            return false;
        
        else 
        {
            return true;
        }
    }
  
    // Function to check if a number
    // is Trojan Number
    static boolean isTrojan(int n) 
    {
        if (!isPerfectPower(n) && isStrongNumber(n))
        {
            return true;
        }
        else 
        {
            return false;
        }
    }
  
    // Driver Code
    public static void main(String[] args)
    {
        int n = 108;
  
        if (isTrojan(n)) 
        {
            System.out.println("Yes");
        
        else 
        {
            System.out.println("No");
        }
    }
  
// This code is contributed by PrinciRaj1992

Python3




# Python 3 program to check if a number 
# is Trojan Number or not
from math import sqrt, pow
  
# Function to check if a number
# can be expressed as x^y
def isPerfectPower(n):
    if n == 1:
        return True
  
    # Try all numbers from 2 to 
    # sqrt(n) as base
    for x in range(2, int(sqrt(n)) + 1):
        y = 2
        p = pow(x, y)
  
        # Keep increasing y while power 
        # 'p' is smaller than n.
        while p <= n and p > 0:
            if p == n:
                return True
            y += 1
            p = pow(x, y)
  
    return False
  
# Function to check if a number 
# is Strong
def isStrongNumber(n):
    count = {i:0 for i in range(n)}
    while n % 2 == 0:
        n = n // 2
        count[2] += 1
  
    # count the number for each
    # prime factor
    for i in range(3,int(sqrt(n)) + 1, 2):
        while n % i == 0:
            n = n // i
            count[i] += 1
  
    if n > 2:
        count[n] += 1
  
    flag = 0
  
    for key,value in count.items():
          
        # minimum number of prime 
        # divisors should be 2
        if value == 1:
            flag = 1
            break
      
    if flag == 1:
        return False
    return True
  
# Function to check if a number
# is Trojan Number
def isTrojan(n):
    return isPerfectPower(n) == False and isStrongNumber(n)
      
# Driver Code
if __name__ == '__main__':
    n = 108
  
    if (isTrojan(n)):
        print("YES")
    else:
        print("NO")
  
# This code is contributed by
# Surendra_Gangwar

C#




// C# program to check if a number is
// Trojan Number or not
using System;
using System.Collections.Generic;
      
class GFG 
{
  
    // Function to check if a number
    // can be expressed as x^y
    static bool isPerfectPower(int n)
    {
        if (n == 1)
        {
            return true;
        }
  
        // Try all numbers from 2 to sqrt(n) as base
        for (int x = 2; x <= Math.Sqrt(n); x++) 
        {
            int y = 2;
            int p = (int) Math.Pow(x, y);
  
            // Keep increasing y while power 'p'
            // is smaller than n.
            while (p <= n && p > 0) 
            {
                if (p == n) 
                {
                    return true;
                }
                y++;
                p = (int) Math.Pow(x, y);
            }
        }
        return false;
    }
  
    // Function to check if a number is Strong
    static bool isStrongNumber(int n) 
    {
        Dictionary<int
                   int> count = new Dictionary<int,
                                               int>();
        while (n % 2 == 0) 
        {
            n = n / 2;
            if (count.ContainsKey(2)) 
            {
                count[2] = count[2] + 1;
            
            else
            {
                count.Add(2, 1);
            }
        }
  
        // count the number for each prime factor
        for (int i = 3; i <= Math.Sqrt(n); i += 2) 
        {
            while (n % i == 0)
            {
                n = n / i;
                if (count.ContainsKey(i))
                {
                    count[i] = count[i] + 1;
                }
                else
                {
                    count.Add(i, 1);
                }
            }
        }
  
        if (n > 2)
        {
            if (count.ContainsKey(n))
            {
                count[n] = count[n] + 1;
            
            else
            {
                count.Add(n, 1);
            }
        }
  
        int flag = 0;
  
        foreach(KeyValuePair<int, int> b in count)
        {
  
            // minimum number of prime divisors
            // should be 2
            if (b.Value == 1)
            {
                flag = 1;
                break;
            }
        }
  
        if (flag == 1) 
        {
            return false;
        
        else
        {
            return true;
        }
    }
  
    // Function to check if a number
    // is Trojan Number
    static bool isTrojan(int n) 
    {
        if (!isPerfectPower(n) && 
             isStrongNumber(n))
        {
            return true;
        }
        else
        {
            return false;
        }
    }
  
    // Driver Code
    public static void Main(String[] args)
    {
        int n = 108;
  
        if (isTrojan(n)) 
        {
            Console.WriteLine("Yes");
        
        else
        {
            Console.WriteLine("No");
        }
    }
}
  
// This code is contributed by Princi Singh
Output:
YES

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