Time Complexity of Loop with Powers - Learnengineeringforu

Time Complexity of Loop with Powers

What is the time complexity of below function?

void fun(int n, int k) {     for (int i=1; i<=n; i++)     {       int p = pow(i, k);        for (int j=1; j<=p; j++)       {           // Some O(1) work       }     } }

Time complexity of above function can be written as 1k + 2k + 3k + … n1k.

Let us try few examples:

k=1
Sum = 1 + 2 + 3 ... n 
    = n(n+1)/2 
    = n2 + n/2

k=2
Sum = 12 + 22 + 32 + ... n12.
    = n(n+1)(2n+1)/6
    = n3/3 + n2/2 + n/6

k=3
Sum = 13 + 23 + 33 + ... n13.
    = n2(n+1)2/4
    = n4/4 + n3/2 + n2/4     

In general, asymptotic value can be written as (nk+1)/(k+1) + Θ(nk)

Note that, in asymptotic notations like Θ we can always ignore lower order terms. So the time complexity is Θ(nk+1 / (k+1))

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above