Count numbers from a given range having exactly 5 distinct factors

Given two integers L and R, the task is to calculate the count of numbers from the range [L, R] having exactly 5 distinct positive factors.

Examples:

Input: L = 1, R= 100 
Output: 2 
Explanation: The only two numbers in the range [1, 100] having exactly 5 distinct factors are 16 and 81. 
Factors of 16 are {1, 2, 4, 8, 16}. 
Factors of 81 are {1, 3, 9, 27, 81}.

Input: L = 1, R= 100 
Output: 2 

Naive Approach: The simplest approach to solve this problem is to traverse the range [L, R] and for every number, count its factors. If the count of factors is equal to 5, increment count by 1. 
Time Complexity: (R – L) × ?N 
Auxiliary Space: O(1)
Efficient Approach: To optimize the above approach, the following observation needs to be made regarding numbers having exactly 5 factors. 

Let the prime factorization of a number be p₁^a₁×p₂^a₂ × … ×p_n^a_n. 
Therefore, the count of factors of this number can be written as (a₁ + 1)×(a₂ + 1)× … ×(a_n + 1). 
Since this product must be equal to 5 (which is a prime number), only one term greater than 1 must exist in the product. That term must be equal to 5. 
Therefore, if a_i + 1 = 5 
=> a_i = 4

Follow the steps below to solve the problem:  

• The required count is the count of numbers in the range containing p⁴ as a factor, where p is a prime number.
• For efficiently calculating p⁴ for a large range ([1, 10¹⁸]), the idea is to use Sieve of Eratosthenes to store all prime numbers up to 10^4.5.

Below is the implementation of the above approach:

C++14

// C++ Program to implement
// the above approach

#include <bits/stdc++.h>
using namespace std;

const int N = 2e5;

// Stores all prime numbers
// up to 2 * 10^5
vector<long long> prime;

// Function to generate all prime
// numbers up to 2 * 10 ^ 5 using
// Sieve of Eratosthenes
void Sieve()
{
    prime.clear();
    vector<bool> p(N + 1, true);

    // Mark 0 and 1 as non-prime
    p[0] = p[1] = false;

    for (int i = 2; i * i <= N; i++) {

        // If i is prime
        if (p[i] == true) {

            // Mark all its factors as non-prime
            for (int j = i * i; j <= N; j += i) {
                p[j] = false;
            }
        }
    }

    for (int i = 1; i < N; i++) {

        // If current number is prime
        if (p[i]) {

            // Store the prime
            prime.push_back(1LL * pow(i, 4));
        }
    }
}

// Function to count numbers in the
// range [L, R] having exactly 5 factors
void countNumbers(long long int L,
                  long long int R)
{

    // Stores the required count
    int Count = 0;

    for (int p : prime) {

        if (p >= L && p <= R) {
            Count++;
        }
    }
    cout << Count << endl;
}

// Driver Code
int main()
{
    long long L = 16, R = 85000;

    Sieve();

    countNumbers(L, R);

    return 0;
}

Java

// Java Program to implement
// the above approach
import java.util.*;
class GFG
{
  static int N = 200000;

  // Stores all prime numbers
  // up to 2 * 10^5
  static int prime[] = new int [20000];
  static int index = 0;

  // Function to generate all prime
  // numbers up to 2 * 10 ^ 5 using
  // Sieve of Eratosthenes
  static void Sieve()
  {
    index = 0;
    int p[] = new int [N + 1];
    for(int i = 0; i <= N; i++)
    {
      p[i] = 1;
    }

    // Mark 0 and 1 as non-prime
    p[0] = p[1] = 0;
    for (int i = 2; i * i <= N; i++)
    {

      // If i is prime
      if (p[i] == 1)
      {

        // Mark all its factors as non-prime
        for (int j = i * i; j <= N; j += i)
        {
          p[j] = 0;
        }
      }
    }
    for (int i = 1; i < N; i++)
    {

      // If current number is prime
      if (p[i] == 1)
      {

        // Store the prime
        prime[index++] = (int)(Math.pow(i, 4));
      }
    }
  }

  // Function to count numbers in the
  // range [L, R] having exactly 5 factors
  static void countNumbers(int L,int R)
  {

    // Stores the required count
    int Count = 0;
    for(int i = 0; i < index; i++)
    {
      int p = prime[i];
      if (p >= L && p <= R)
      {
        Count++;
      }
    }
    System.out.println(Count);
  }

  // Driver Code
  public static void main(String[] args)
  {
    int L = 16, R = 85000;
    Sieve();
    countNumbers(L, R);
  }
}

// This code is contributed by amreshkumar3.

Python3

# Python3 implementation of
# the above approach
N = 2 * 100000

# Stores all prime numbers
# up to 2 * 10^5
prime = [0] * N

# Function to generate all prime
# numbers up to 2 * 10 ^ 5 using
# Sieve of Eratosthenes
def Sieve() :
    p = [True] * (N + 1)

    # Mark 0 and 1 as non-prime
    p[0] = p[1] = False
    i = 2
    while(i * i <= N) :

        # If i is prime
        if (p[i] == True) :

            # Mark all its factors as non-prime
            for j in range(i * i, N, i):
                p[j] = False
        i += 1
    for i in range(N):

        # If current number is prime
        if (p[i] != False) :

            # Store the prime
            prime.append(pow(i, 4))

# Function to count numbers in the
# range [L, R] having exactly 5 factors
def countNumbers(L, R) :

    # Stores the required count
    Count = 0
    for p in prime :
        if (p >= L and p <= R) :
            Count += 1
    print(Count)

# Driver Code
L = 16
R = 85000
Sieve()
countNumbers(L, R)

# This code is contributed by code_hunt.

C#

// C# Program to implement
// the above approach
using System;
class GFG
{
  static int N = 200000;

  // Stores all prime numbers
  // up to 2 * 10^5
  static int []prime = new int [20000];
  static int index = 0;

  // Function to generate all prime
  // numbers up to 2 * 10 ^ 5 using
  // Sieve of Eratosthenes
  static void Sieve()
  {
    index = 0;
    int []p = new int [N + 1];
    for(int i = 0; i <= N; i++)
    {
      p[i] = 1;
    }

    // Mark 0 and 1 as non-prime
    p[0] = p[1] = 0;
    for (int i = 2; i * i <= N; i++)
    {

      // If i is prime
      if (p[i] == 1)
      {

        // Mark all its factors as non-prime
        for (int j = i * i; j <= N; j += i)
        {
          p[j] = 0;
        }
      }
    }
    for (int i = 1; i < N; i++)
    {

      // If current number is prime
      if (p[i] == 1)
      {

        // Store the prime
        prime[index++] = (int)(Math.Pow(i, 4));
      }
    }
  }

  // Function to count numbers in the
  // range [L, R] having exactly 5 factors
  static void countNumbers(int L,int R)
  {

    // Stores the required count
    int Count = 0;
    for(int i = 0; i < index; i++)
    {
      int p = prime[i];
      if (p >= L && p <= R)
      {
        Count++;
      }
    }
    Console.WriteLine(Count);
  }

  // Driver Code
  public static void Main(String[] args)
  {
    int L = 16, R = 85000;
    Sieve();
    countNumbers(L, R);
  }
}

// This code is contributed by shikhasingrajput

Javascript

<script>
// javascript program of the above approach

let N = 200000;

  // Stores all prime numbers
  // up to 2 * 10^5
  let prime = new Array(20000).fill(0);
  let index = 0;

  // Function to generate all prime
  // numbers up to 2 * 10 ^ 5 using
  // Sieve of Eratosthenes
  function Sieve()
  {
    index = 0;
    let p = new Array (N + 1).fill(0);
    for(let i = 0; i <= N; i++)
    {
      p[i] = 1;
    }

    // Mark 0 and 1 as non-prime
    p[0] = p[1] = 0;
    for (let i = 2; i * i <= N; i++)
    {

      // If i is prime
      if (p[i] == 1)
      {

        // Mark all its factors as non-prime
        for (let j = i * i; j <= N; j += i)
        {
          p[j] = 0;
        }
      }
    }
    for (let i = 1; i < N; i++)
    {

      // If current number is prime
      if (p[i] == 1)
      {

        // Store the prime
        prime[index++] = (Math.pow(i, 4));
      }
    }
  }

  // Function to count numbers in the
  // range [L, R] having exactly 5 factors
  function countNumbers(L, R)
  {

    // Stores the required count
    let Count = 0;
    for(let i = 0; i < index; i++)
    {
      let p = prime[i];
      if (p >= L && p <= R)
      {
        Count++;
      }
    }
    document.write(Count);
  }

    // Driver Code

    let L = 16, R = 85000;
    Sieve();
    countNumbers(L, R);

</script>

Output:

7

Time Complexity: O(N * log(log(N)) )
Auxiliary Space: O(N)