There are some cases where it is not so simple to decide which function grows faster asymptotically.

For example, in the following cases, why (intuitively) $g(n)$ should grow faster than $f(n)$, or vice-versa?

$$ \begin{array}{c|c} f(n) & g(n)\\ \hline 10 \log(n) & \log(n^2)\\ \frac{n^2}{\log(n)} & n(\log(n))^2\\ n^{0.1} & \log(n)^{10}\\ \log(n)^{\log(n)} & \frac{n}{\log(n)}\\ \sqrt n & \log(n)^3\\ n\,2^n & 3^n\\ \log(n)^{\log(n)} & 2^{(\log_2 n)^2} \end{array} $$

It is easy to type some values on a calculator or to plot the functions to see which one grows faster, but during an exam I cannot do this of course.

  • 1
    $\begingroup$ Another helpful rule of thumb: $n^k$ grows faster than $\log(n)^l$ for any fixed positive exponents $k,l$. $\endgroup$ Aug 3 '15 at 13:14
  • $\begingroup$ @KlausDraeger This is not really a rule of thumb, because it holds in any case $\endgroup$
    – tired
    Aug 3 '15 at 15:41

Ultimately, it suffices to evaluate the limit $$ \lim_{n \to \infty} \frac{g(n)}{f(n)} $$ if it's $\infty$, then $g$ grows faster. If it's $0$, then $f$ grows faster. If it's anything else, then they grow the same (up to some constant), i.e. $f = \Theta(g)$.

As far as intuition goes, it helps to rewrite things, or sometimes to divide/multiply both sides by the same term.

  1. Rewrite $\log(n^2) = 2 \log(n)$. It should now be obvious.

  2. Divide both sides by $\frac{n}{\log(n)}$. We're now comparing $n$ to $\log(n)^3$. $n$ is asymptotically larger since $n^\alpha$ will always beat $\log(n)^\beta$ for any $\alpha,\beta > 0$ ("polynomial beats log"). Similarly, $e^{n^\alpha}$ will always beat $n^\beta$ for any $\alpha,\beta > 0$ ("exponential beats polynomial").

  3. Same rule as last time. The left side is larger.

  4. Rewrite $\log(n)^{\log(n)} = n^{\log(\log(n))}$ (how?). Perhaps it is now clear that the left side beats $n$, let alone $\frac{n}{\log(n)}$.

  5. Polynomial always beats log (see 2.). Left is larger.

  6. Divide both sides by $2^n$. Now, the right side is larger because exponential always beats polynomial (see 2.). Right is larger.

  7. Rewrite the right side as $n^{\log_2(n)}$ (how?). This beats $n^{\log(\log(n))}$ (why)? So, the right side is larger.

Note on 4: we have $$ \log(n)^{\log(n)} = [e^{\log(\log(n))}]^{\log(n)} = e^{\log(n)\log(\log(n))} = [e^{\log(n)}]^{\log(\log(n))} = n^{\log(\log(n))} $$

  • 6
    $\begingroup$ @nbro: $\tfrac{3^n}{2^n}=\left(\tfrac{3}{2}\right)^n$ $\endgroup$
    – Matěj G.
    Aug 3 '15 at 13:49

You can use known limits such has $\lim _{n\to \infty} \frac {k^n}{n^h}=\infty \quad \forall k>1$ and $h\in \mathbb{R}$ or $\lim _{n\to \infty} \frac {n^h}{\log^k{n}}=\infty \quad \forall k> O,h\in \mathbb{R}$. For example if you compute $\frac{f(n)}{g(n)}$ from the second line you obtain $\frac{n}{\log^3{n}}$ which has infinite as limit so $f(n)$ grows faster.


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