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 1^k + 2^k + 3^k + … n1^k.

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 (n^k+1)/(k+1) + Θ(n^k)

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

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