Sum of largest divisor of numbers upto N not divisible by given prime number P

Given a number N and a prime number P, the task is to find the sum of largest divisors of each number in the range [1, N], which is not divisible by P.
Examples: 

Input: N = 8, P = 2
Output: 22
Explanation: Numbers are in the range [1, 8].
Number                           Largest Divisor not divisible by P = 2
1                                                            1
2                                                            1
3                                                            3
4                                                            1
5                                                            5
6                                                            3
7                                                            7
8                                                            1
Sum of all divisors with given constraint = 22.

Input: N = 10, P = 5
Output: 43
Explanation: Numbers are in the range [1, 8].
Number                           Largest Divisor not divisible by P = 5
1                                                            1
2                                                            2
3                                                            3
4                                                            4
5                                                            1
6                                                            6
7                                                            7
8                                                            8
9                                                            9
10                                                          2
Sum of all divisors with given constraint = 43

Naive Approach: The naive idea is to find the divisors for each number in the range [1, N] and find the largest divisors which is not divisible by P and those numbers. Print the sum of all the those largest divisors. 
Time Complexity: O(N3/2
Auxiliary Space: O(1)

Efficient Approach: The idea is to observe that the largest divisor of a number N not divisible by P would be N itself if N is not divisible by P. Else the required divisor will be the same as that of N/P. Below are the steps:

  1. If N is not divisible by P, then the largest divisor will be N, add this to the final sum..
  2. If N is divisible by P, the required divisor will be the same as that of N/P.
  3. So, find the sum of all numbers which are not divisible by P and add to them divisors of those which are divisible by P separately.
  4. The total sum would be N*(N + 1)/2. Subtract the sum of those which are divisible by P and add their corresponding value by recursively calling the function to find the sum for N/P.

Below is the implementation of the above approach:



C++

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// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
  
// Function to find the sum of largest
// divisors of numbers in range 1 to N
// not divisible by prime number P
int func(int N, int P)
{
    // Total sum upto N
    int sumUptoN = (N * (N + 1) / 2);
    int sumOfMultiplesOfP;
  
    // If no multiple of P exist up to N
    if (N < P) {
        return sumUptoN;
    }
  
    // If only P itself is in the range
    // from 1 to N
    else if ((N / P) == 1) {
        return sumUptoN - P + 1;
    }
  
    // Sum of those that are divisible by P
    sumOfMultiplesOfP
        = ((N / P) * (2 * P + (N / P - 1) * P)) / 2;
  
    // Recursively function call to
    // find the sum for N/P
    return (sumUptoN
            + func(N / P, P)
            - sumOfMultiplesOfP);
}
  
// Driver Code
int main()
{
    // Given N and P
    int N = 10, P = 5;
  
    // Function Call
    cout << func(N, P) << "\n";
  
    return 0;
}

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Java

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// Java program for the above approach
class GFG{
  
// Function to find the sum of largest
// divisors of numbers in range 1 to N
// not divisible by prime number P
static int func(int N, int P)
{
      
    // Total sum upto N
    int sumUptoN = (N * (N + 1) / 2);
    int sumOfMultiplesOfP;
  
    // If no multiple of P exist up to N
    if (N < P) 
    {
        return sumUptoN;
    }
  
    // If only P itself is in the range
    // from 1 to N
    else if ((N / P) == 1)
    {
        return sumUptoN - P + 1;
    }
  
    // Sum of those that are divisible by P
    sumOfMultiplesOfP = ((N / P) * (2 * P + 
                         (N / P - 1) * P)) / 2;
  
    // Recursively function call to
    // find the sum for N/P
    return (sumUptoN + func(N / P, P) - 
            sumOfMultiplesOfP);
}
  
// Driver Code
public static void main(String[] args)
{
      
    // Given N and P
    int N = 10, P = 5;
  
    // Function call
    System.out.println(func(N, P));
}
}
  
// This code is contributed by Rajput-Ji

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C#

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// C# program for the above approach
using System;
  
class GFG{
  
// Function to find the sum of largest
// divisors of numbers in range 1 to N
// not divisible by prime number P
static int func(int N, int P)
{
      
    // Total sum upto N
    int sumUptoN = (N * (N + 1) / 2);
    int sumOfMultiplesOfP;
  
    // If no multiple of P exist up to N
    if (N < P) 
    {
        return sumUptoN;
    }
  
    // If only P itself is in the range
    // from 1 to N
    else if ((N / P) == 1)
    {
        return sumUptoN - P + 1;
    }
  
    // Sum of those that are divisible by P
    sumOfMultiplesOfP = ((N / P) * (2 * P + 
                         (N / P - 1) * P)) / 2;
  
    // Recursively function call to
    // find the sum for N/P
    return (sumUptoN + func(N / P, P) - 
            sumOfMultiplesOfP);
}
  
// Driver Code
public static void Main(String[] args)
{
      
    // Given N and P
    int N = 10, P = 5;
  
    // Function call
    Console.WriteLine(func(N, P));
}
}
  
// This code is contributed by Amit Katiyar

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Output: 

43

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

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Improved By : Rajput-Ji, amit143katiyar