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# Sum of N-terms of geometric progression for larger values of N | Set 2 (Using recursion)

• Difficulty Level : Medium
• Last Updated : 22 Jun, 2021

A Geometric series is a series with a constant ratio between successive terms. The first term of the series is denoted by a and the common ratio is denoted by r. The series looks like this:- The task is to find the sum of such a series, mod M.

Examples:

```Input:  a = 1, r = 2, N = 10000, M = 10000
Output:  8751

Input:  a = 1, r = 4, N = 10000, M = 100000
Output:  12501 ```

Approach:

1. To find the sum of series we can easily take a as common and find the sum of and multiply it with a.
2. Steps to find the sum of the above series.
• Here, it can be resolved that: If we denote, then, and, This will work as our recursive case.

• So, the base cases are:
```Sum(r, 0) = 1.
Sum(r, 1) = 1 + r.```

Below is the implementation of the above approach.

## C++

 `// C++ implementation to``// illustrate the program``#include ``using` `namespace` `std;` `// Function to calculate the sum``// recursively``int` `SumGPUtil(``long` `long` `int` `r,``              ``long` `long` `int` `n,``              ``long` `long` `int` `m)``{``    ` `    ``// Base cases``    ``if` `(n == 0)``        ``return` `1;``    ``if` `(n == 1)``        ``return` `(1 + r) % m;``    ` `    ``long` `long` `int` `ans;``    ``// If n is odd``    ``if` `(n % 2 == 1)``    ``{``        ``ans = (1 + r) *``              ``SumGPUtil((r * r) % m,``                        ``(n - 1) / 2, m);``    ``}``    ``else``    ``{``        ` `        ``// If n is even``        ``ans = 1 + (r * (1 + r) *``             ``SumGPUtil((r * r) % m,``                       ``(n / 2) - 1, m));``    ``}``    ``return` `(ans % m);``}` `// Function to print the value of Sum``void` `SumGP(``long` `long` `int` `a,``           ``long` `long` `int` `r,``           ``long` `long` `int` `N,``           ``long` `long` `int` `M)``{``    ``long` `long` `int` `answer;``    ` `    ``answer = a * SumGPUtil(r, N, M);``    ``answer = answer % M;``    ` `    ``cout << answer << endl;``}` `// Driver Code``int` `main()``{``    ` `    ``// First element``    ``long` `long` `int` `a = 1;``    ` `    ``// Common difference``    ``long` `long` `int` `r = 4;``    ` `    ``// Number of elements``    ``long` `long` `int` `N = 10000;``    ` `    ``// Mod value``    ``long` `long` `int` `M = 100000;` `    ``SumGP(a, r, N, M);` `    ``return` `0;``}` `// This code is contributed by sanjoy_62`

## Java

 `// Java implementation to``// illustrate the program``import` `java.io.*;` `class` `GFG{` `// Function to calculate the sum``// recursively``static` `long` `SumGPUtil(``long` `r, ``long` `n,``                      ``long` `m)``{``    ` `    ``// Base cases``    ``if` `(n == ``0``)``        ``return` `1``;``    ``if` `(n == ``1``)``        ``return` `(``1` `+ r) % m;``    ` `    ``long` `ans;``    ` `    ``// If n is odd``    ``if` `(n % ``2` `== ``1``)``    ``{``        ``ans = (``1` `+ r) *``              ``SumGPUtil((r * r) % m,``                        ``(n - ``1``) / ``2``, m);``    ``}``    ``else``    ``{``        ``// If n is even``        ``ans = ``1` `+ (r * (``1` `+ r) *``             ``SumGPUtil((r * r) % m,``                       ``(n / ``2``) - ``1``, m));``    ``}``    ` `    ``return` `(ans % m);``}` `// Function to prlong the value of Sum``static` `void` `SumGP(``long` `a, ``long` `r,``                  ``long` `N, ``long` `M)``{``    ``long` `answer;``    ``answer = a * SumGPUtil(r, N, M);``    ``answer = answer % M;``    ` `    ``System.out.println(answer);``}` `// Driver Code``public` `static` `void` `main (String[] args)``{``    ` `    ``// First element``    ``long` `a = ``1``;``    ` `    ``// Common difference``    ``long` `r = ``4``;``    ` `    ``// Number of elements``    ``long` `N = ``10000``;``    ` `    ``// Mod value``    ``long` `M = ``100000``;` `    ``SumGP(a, r, N, M);``}``}` `// This code is contributed by sanjoy_62`

## Python3

 `# Python3 implementation to illustrate the program` `# Function to calculate the sum``# recursively``def` `SumGPUtil (r, n, m):``    ` `    ``# Base cases``    ``if` `n ``=``=` `0``: ``return` `1``    ``if` `n ``=``=` `1``: ``return` `(``1` `+` `r) ``%` `m``  ` `    ``# If n is odd``    ``if` `n ``%` `2` `=``=` `1``:``        ``ans ``=` `(``1` `+` `r) ``*` `SumGPUtil(r ``*` `r ``%` `m,``                                  ``(n ``-` `1``)``/``/``2``,``                                  ``m)``    ``else``:``        ``#If n is even``        ``ans ``=` `1` `+` `r ``*` `(``1` `+` `r) ``*` `SumGPUtil(r ``*` `r ``%` `m,``                                          ``n``/``/``2` `-` `1``,``                                          ``m)``  ` `    ``return` `ans ``%` `m` `# Function to print the value of Sum``def` `SumGP (a, r, N, M):``    ` `    ``answer ``=` `a ``*` `SumGPUtil(r, N, M)``    ``answer ``=` `answer ``%` `M``    ``print``(answer)` `#Driver Program``if` `__name__``=``=` `'__main__'``:` `    ``a ``=` `1` `# first element``    ``r ``=` `4` `# common difference``    ``N ``=` `10000` `# Number of elements``    ``M ``=` `100000` `# Mod value` `    ``SumGP(a, r, N, M)`

## C#

 `// C# implementation to``// illustrate the program``using` `System;` `class` `GFG{` `// Function to calculate the sum``// recursively``static` `long` `SumGPUtil(``long` `r, ``long` `n,``                      ``long` `m)``{``    ` `    ``// Base cases``    ``if` `(n == 0)``        ``return` `1;``    ``if` `(n == 1)``        ``return` `(1 + r) % m;``    ` `    ``long` `ans;``    ` `    ``// If n is odd``    ``if` `(n % 2 == 1)``    ``{``        ``ans = (1 + r) *``              ``SumGPUtil((r * r) % m,``                        ``(n - 1) / 2, m);``    ``}``    ``else``    ``{``        ` `        ``// If n is even``        ``ans = 1 + (r * (1 + r) *``             ``SumGPUtil((r * r) % m,``                       ``(n / 2) - 1, m));``    ``}``    ``return` `(ans % m);``}` `// Function to prlong the value of Sum``static` `void` `SumGP(``long` `a, ``long` `r,``                  ``long` `N, ``long` `M)``{``    ``long` `answer;``    ``answer = a * SumGPUtil(r, N, M);``    ``answer = answer % M;``    ` `    ``Console.WriteLine(answer);``}` `// Driver Code``public` `static` `void` `Main()``{``    ` `    ``// First element``    ``long` `a = 1;``    ` `    ``// Common difference``    ``long` `r = 4;``    ` `    ``// Number of elements``    ``long` `N = 10000;``    ` `    ``// Mod value``    ``long` `M = 100000;` `    ``SumGP(a, r, N, M);``}``}` `// This code is contributed by sanjoy_62`

## Javascript

 ``
Output:
`12501`

Time complexity: O(log N)

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