Tanguy Andreani

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# "Big Oh": Is my reasoning right?

Apologies for bad notation. Also, I'm not a native speaker.

I am currently reading the first chapter, which is about time complexity / asymptotic notation.

So I'm told that for an algorithm whose running time is described by the function 𝑇(𝑛) = 3𝑛³ + 2n² we have a worst-case complexity of O(𝑛³). Fine.

I'm told that I can easily prove that T(n) ≤ 5𝑛³. Fine. (I'm pretty confident in my highschool maths skills.)

Basically, I ended up with T(𝑛) ≤ 3.0...01𝑛³ for 𝑛 tends to +∞. A different value of 𝑐.

It might sound dumb that 3𝑛³ + something could be smaller than 3𝑛³ but I found this using limits.

My reasoning was that, after manipulating the inequality T(n) ≤ 𝑥𝑛³, one could end up with the following statement:

3 + 𝑛-2 ≤ 𝑥

Now 𝑛 is just a positive integer constant. So let it be 1; smallest input. We end up with 𝑥 = 5. The value provided by the book.

But let's say that 𝑛 gets bigger and bigger; and eventually tends to +∞. Since 𝑛-2 would then tend to 0, we have 𝑥 tends to 3.

So T(𝑛) ≤ 𝑥𝑛³ with 𝑥 a real number that tends to 3 from upper values but is not exactly 3. In fact, it wouldn't work with 3.

### In GeoGebra

C is the point at which 𝑥𝑛³ gets bigger than T(𝑛).

### My question

First of all, is my reasoning correct? Can I use this kind of reasoning when dealing with algorithms complexity? I'm pretty confident that it "holds" mathematically speaking but does it apply here?

I understand that it is not really important since we end up with the same O() but still, just curious.

Thank you.

Håvard Krogstie

Your understanding of limits is correct. As n approaches infinity, c can get closer and closer to 3. n³ is bigger than n² by a factor of n, which means the 2n² part gets closer and closer to negligibly.

Now, the definition of big O states:
T(n) belongs to O(n³) if there exists some finite constants c and n_0 for which every n >= n_0 satisfies T(n) <= c*n³

If n_0 is 1, then c has to be 5, as you showed.

As n_0 gets bigger, c can get closer to 3, as you also showed.

The important part is that there exists some pair of n_0 and c satisfying the requirement.

In fact, if one such pair exists, infinite pairs exist!

Hope this helped, cheers

Tanguy Andreani

Ok I think I get it now.

Thank you very much!^

Håvard Krogstie

If you want some practice, do have a look at a challenge I uploaded

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The O(n²) solution is trivial, but see if you can come up with one that's O(nlog n) (Hint: sorting). I have posted a python solution in the comments, with explanations. Good luck!

Curtis Fenner • Edited

There's a much more succinct and formal way of putting what I think your reasoning is:

if the limit of f(n) / g(n) is a finite number as n goes to infinity, then f is O(g).

So for example, if you're comparing `3𝑛³ + 2n²` to `5𝑛³`, the limit of the ratio is `3/5`, which is a finite number, so `3𝑛³ + 2n²` is `O(5𝑛³)`.

On the other hand, if we compare `10n²` and `n`, the limit is +infinity. So that means `10n²` is NOT `O(n)`.

Tanguy Andreani

Thank you for this. It doesn’t exactly help regarding my reasoning but I think your method is easier to prove that f is O(g). I didn’t know about it so thanks for introducing it to me^