Complexity Analysis

The outer loop

for (int i = 2; i * i <= N; i++)

Runs for $\sqrt{N}$ values but the number of iterations of the inner loop is dependent on $i$.

for (int j = i + i; j <= N; j += i)

Also, note that the inner loop only runs for values when $i$ is prime.

$i$	Inner loop iterations
2	$\frac{N}{2}$
3	$\frac{N}{3}$

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