Give two functions that is in O(n^2) but not in o(n^2).

The answer is nlogn and n,But I think a lot but can't understand why answer are them.I doubt answers is wrong.

And this question,Give two functions that is in big-omega(n^2) not in little-omega(n^2)?

The answer is n^2logn and n^3.But I think a lot but can't understand why answer are them.

  • $\begingroup$ Hint: Study the definitions, and the lemmas given here. $\endgroup$
    – Raphael
    Aug 24, 2017 at 18:30

2 Answers 2


Where have you got those answers?

1. Answers are wrong if that was the task. The simplest answer is $n^2$. Why? Because this function satisfies restructions: $\lim_{n\to\infty}\frac{f(x)}{n^2}>0$ (not in $o(n^2)$ requirement) and $\lim_{n\to\infty}\frac{n^2}{f(x)}>0$ (in $O(n^2)$ requirement).

2. Exactly the same answer.

  • $\begingroup$ I got answer from textbook,but I don't understand why question 2 is correct answer. $\endgroup$
    – ben
    Aug 24, 2017 at 17:09
  • $\begingroup$ What do you mean by "why question 2 is correct answer"? You don't understand why answer is the same? $\endgroup$
    – rus9384
    Aug 24, 2017 at 17:18
  • $\begingroup$ I thought correct answer is my answer.I have understood your meaning. $\endgroup$
    – ben
    Aug 24, 2017 at 17:21

The answers you give are wrong. It is easy to verify that $n, n\log n \in o(n^2)$, and similarly $n^2 \log n, n^3 \in ω(n^2)$.

Give two functions that is in $O(n^2)$ but not in $o(n^2)$.

Looking at the definitions, we see that there are two ways to come up with such a function:

  • Pick any function from $\Theta(n^2)$.

    Besides the obvious candidates of the form $cn^2 + o(n^2)$, there are also more obscure ones like, for instance $ 2^{\frac{\sin n}{\log n}} \cdot n^2$.

  • Pick any function $f \in O(n^2)$ for which $\lim_{n \to \infty} f(n) \cdot n^{-2}$ does not exist.

    Simple examples are $(2 + \sin(n)) \cdot n^2$ or $2^{\sin n} \cdot n^2$ or

    $\qquad \displaystyle f(n) = \begin{cases}n^2, &n \in 2\mathbb{N};\\n, &n \in 2\mathbb{N}+1.\end{cases}$

    There are many more.

I'll leave the proofs to you as an exercise, as well as the symmetric case with $\Omega$ vs $\omega$.

  • $\begingroup$ Forgot about limitless functions (in fact $\limsup$ still must work). $\endgroup$
    – rus9384
    Aug 24, 2017 at 19:25
  • $\begingroup$ @rus9384 It sure does! One can use the lim sup definitions to solve the exercise, too. $\endgroup$
    – Raphael
    Aug 24, 2017 at 19:58
  • $\begingroup$ I think it's no more $\Theta(n^2)$, as $\frac{3n^2}{\log n}$ is smaller. Ans this is a supremum. $\endgroup$
    – rus9384
    Aug 25, 2017 at 5:34
  • $\begingroup$ @rus9384 Damn, you are completely right. I wanted the limit to exist, which turns out to be slightly more involved. Got one now! I hope. $\endgroup$
    – Raphael
    Aug 25, 2017 at 6:36

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