# An interesting time complexity question

• Difficulty Level : Easy
• Last Updated : 30 Oct, 2015

What is the time complexity of following function fun()?

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 `int` `fun(``int` `n)``{    ``    ``for` `(``int` `i = 1; i <= n; i++)``    ``{``        ``for` `(``int` `j = 1; j < n; j += i)``        ``{``            ``// Some O(1) task``        ``}``    ``}    ``}`

For i = 1, the inner loop is executed n times.
For i = 2, the inner loop is executed approximately n/2 times.
For i = 3, the inner loop is executed approximately n/3 times.
For i = 4, the inner loop is executed approximately n/4 times.
…………………………………………………….
…………………………………………………….
For i = n, the inner loop is executed approximately n/n times.

So the total time complexity of the above algorithm is (n + n/2 + n/3 + … + n/n)

Which becomes n * (1/1 + 1/2 + 1/3 + … + 1/n)

The important thing about series (1/1 + 1/2 + 1/3 + … + 1/n) is, it is equal to Θ(Logn) (See this for reference). So the time complexity of the above code is Θ(nLogn).

As a side note, the sum of infinite harmonic series is counterintuitive as the series diverges. The value of is ∞. This is unlike geometric series as geometric series with ratio less than 1 converges.