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Nicomachus’s Theorem (Sum of k-th group of odd positive numbers)
  • Last Updated : 10 May, 2018

The positive odd numbers in ascending order as 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, …. and grouped as (1), (3, 5), (7, 9, 11), (13, 15, 17, 19), …. and so on.
Thus, the first group is (1), the second group is (3, 5) and the third group is (7, 9, 11), etc. in general, the kth group contains the next k elements of the sequence.
Given k, find the sum of the kth group.

Examples :

Input : k = 3
Output : 27
3rd group is (7, 9, 11) and the sum is 27.

Input : k = 1
Output : 1

Method 1: O(n)
The idea is to find the first element of the kth group.
For example:

  • The 1st group’s first element is 1, which is the 1st odd number.
  • The 2nd group’s first element is 3, which is the 2st odd number.
  • The 3rd group’s first element is 7, which is the 4st odd number.
  • The 4st group’s first element is 13, which is the 7st odd number.
  • and so on.

In general, the kth group’s first element is the nth odd number, where
n = (1 + 2 + 3 +……+ (k – 1)) + 1.
Now, as we know, the nth odd number is 2n – 1. This gives us the first element of the kth group. We also know that there are k elements in the group, so we can find their sum by simply counting upwards from 2n – 1, by two, k times and adding them all. This gives us a linear time solution.

C++




// CPP program to find sum of k-th group of
// positive odd integers.
#include <bits/stdc++.h>
using namespace std;
  
// Return the sum of k-th group of positive
// odd integers.
int kthgroupsum(int k)
{
    // Finding first element of kth group.
    int cur = (k * (k - 1)) + 1;
    int sum = 0;
  
    // Finding the sum.
    while (k--) {
        sum += cur;
        cur += 2;
    }
  
    return sum;
}
  
// Driver code
int main()
{
    int k = 3;
    cout << kthgroupsum(k) << endl;
    return 0;
}


Java




// Java code to find sum of k-th group
// of positive odd integers.
import java.util.Arrays;
import java.util.Collections;
  
class GFG
    // Return the sum of k-th group 
    // of positive odd integers.
    public static int kthgroupsum(int k)
    {
        // Finding first element of kth group.
        int cur = (k * (k - 1)) + 1;
        int sum = 0;
  
        // Finding the sum.
        while (k-- > 0)
        {
            sum += cur;
            cur += 2;
        }
  
        return sum;
    }
  
    // Driver program to test above methods 
    public static void main(String[] args)
    {
        int k = 3;
        System.out.print(kthgroupsum(k));
    }
}
  
// This code is contributed by Chhavi


Python3




# Python3 code to find sum of k-th group 
# of positive odd integers.
  
# Return the sum of k-th group of 
# positive odd integers.
def kthgroupsum( k ):
      
    # Finding first element of kth group.
    cur = int((k * (k - 1)) + 1)
    sum = 0
      
    # Finding the sum.
    while k:
        sum += cur
        cur += 2
        k=k-1
      
    return sum
      
# Driver code
k = 3
print(kthgroupsum(k))
  
# This code is contributed by "Sharad_Bhardwaj".


C#




// C# code to find sum of k-th group
// of positive odd integers.
using System;
  
class GFG {
      
    // Return the sum of k-th group
    // of positive odd integers.
    public static int kthgroupsum(int k)
    {
          
        // Finding first element of kth group.
        int cur = (k * (k - 1)) + 1;
        int sum = 0;
  
        // Finding the sum.
        while (k-- > 0) {
            sum += cur;
            cur += 2;
        }
  
        return sum;
    }
  
    // Driver program to test above methods
    public static void Main()
    {
        int k = 3;
          
        Console.WriteLine(kthgroupsum(k));
    }
}
  
// This code is contributed by vt_m.


PHP




<?php
// PHP program to find 
// sum of k-th group of
// positive odd integers.
  
// Return the sum of 
// k-th group of positive
// odd integers.
function kthgroupsum($k)
{
      
    // Finding first element
    // of kth group.
    $cur = ($k * ($k - 1)) + 1;
    $sum = 0;
  
    // Finding the sum.
    while ($k--) 
    {
        $sum += $cur;
        $cur += 2;
    }
    return $sum;
}
  
// Driver code
$k = 3;
echo kthgroupsum($k) ;
  
// This code is contributed by anuj_67.
?>



Output :



27

Method 2 : O(1)
An tricky solution is to find k3. One can easily observe, the sum of kth group will be k3.

Below is the implementation of this approach:

C++




// Efficient CPP program to find sum of k-th 
// group of positive odd integers.
#include <bits/stdc++.h>
using namespace std;
  
// Return the sum of kth group of positive
// odd integer.
int kthgroupsum(int k)
{
    return k * k * k;
}
  
// Driven Program
int main()
{
    int k = 3;
    cout << kthgroupsum(k) << endl;
    return 0;
}


Java




// Efficient Java code to find sum of k-th
// group of positive odd integers.
import java.util.Arrays;
import java.util.Collections;
  
class GFG
    // Return the sum of kth group of
    // positive odd integer.
    public static int kthgroupsum(int k)
    {
        return k * k * k;
    }
  
    // Driver program to test above methods 
    public static void main(String[] args)
    {
        int k = 3;
        System.out.print( kthgroupsum(k));
    }
}
  
// This code is contributed by Chhavi


Python3




# Efficient Python code to find sum of  
# k-th group of positive odd integers.
  
# Return the sum of kth group of positive
# odd integer.
def kthgroupsum( k ):
    return k * k * k
  
# Driver code
k = 3
print(kthgroupsum(k))
  
# This code is contributed by "Sharad_Bhardwaj".


C#




// Efficient C# code to find sum of k-th
// group of positive odd integers.
using System;
  
class GFG {
      
    // Return the sum of kth group of
    // positive odd integer.
    public static int kthgroupsum(int k)
    {
        return k * k * k;
    }
  
    // Driver program to test above methods
    public static void Main()
    {
        int k = 3;
          
        Console.WriteLine(kthgroupsum(k));
    }
}
  
// This code is contributed by vt_m.


PHP




<?php
// Efficient PHP program to 
// find sum of k-th group of
// positive odd integers.
  
  
// Return the sum of kth group 
// of positive odd integer.
function kthgroupsum( $k)
{
    return $k * $k *$k;
}
  
// Driven Code
$k = 3;
echo kthgroupsum($k);
  
// This code is contributed by anuj_67.
?>



Output :

27

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