I read an example that said explain what "$f(n)$ is $n^{O(1)}$" means.
I can't interpret the $n^{O(1)}$ syntax. I know what Big $O$ notation is, its just that this example looks odd to me.
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$\begingroup$ This should answer your question: en.m.wikipedia.org/wiki/Big_O_notation#Multiple_usages $\endgroup$ – ryan Aug 12 '17 at 15:31
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1$\begingroup$ You are right to be confused; few authors actually define the notation. $\endgroup$ – Raphael♦ Aug 12 '17 at 17:21
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1$\begingroup$ Such notation is commonplace in the literature, and everybody know exactly what it means, despite Raphael's misgivings. $\endgroup$ – Yuval Filmus Aug 12 '17 at 21:38
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$\begingroup$ @YuvalFilmus And yet, every time such a question comes up, people who "know exactly what it means" struggle to explain what that is, exactly. $\endgroup$ – Raphael♦ Aug 13 '17 at 11:33
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It's short-hand for "$n^{f(n)}$ for some function $f(n)\in O(1)$". In other words, the function is at most $n^c$ for some constant $c$.
You can see this by directly substituting the definition of $O(1)$ in the expression. $g(n)=n^{O(1)}$ if there's constant $c$ such that, for all large enough $n$, $f(n)\leq n^{c\cdot 1} = n^c$.
This includes all polynomially-bounded functions. For example, for $n>0$, $$n^2+3n = n^2(1+\tfrac3n) = n^{2+\log(1+3/n)/\log n} = n^{O(1)}\,,$$ since, for $n\geq 3$, we have $1<1+\tfrac3n\leq 2$ and $\log n>1$, so the exponent is between $2+\log 2$ and $2$.
answered Aug 12 '17 at 15:36
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1$\begingroup$ I recall this thread where a disagreement became clear. Just to clarify: you are proposing that $n^{O(1)}$ is the set of all polynomially bounded functions, correct? $\endgroup$ – Raphael♦ Aug 12 '17 at 17:24
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$\begingroup$ @Raphael, wouldn't it just be the set of all monomial functions with coefficient $1$? e.g. $n^3 + n^2 \notin n^{O(1)}$ and $3n^2 \notin n^{O(1)}$ because they don't follow the form of $n$ to some power $O(1)$. Is this correct? $\endgroup$ – ryan Aug 12 '17 at 17:41
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$\begingroup$ @ryan: $3n^2 = n^{f(n)}$ where f (n) = 2 + log 3 / log n. $\endgroup$ – gnasher729 Aug 12 '17 at 17:46
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1$\begingroup$ Aaaaand we once again have proof that such notation is incomprehensible and should be avoided. :'D $\endgroup$ – Raphael♦ Aug 12 '17 at 18:13
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1$\begingroup$ @Raphael Yes, polynomially bounded -- edited to clarify. $\endgroup$ – David Richerby Aug 12 '17 at 21:21
