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According to the book "Introduction to Algorithms" a function that has the following form
belong to $O(n^2)$ , and that this can be easily proven if we set
$c = a +|b|$
But I don't get it, it still seems to belong to $O(n)$.

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Note big O is an upper asymptomic bound, not strict bound. So it will also fit to $O(n^5)$, $O(n^{99})$, and so on. – Studentmath Jun 11 '14 at 15:41
It belongs in $O(n)$ and because $O(n)⊂O(n^2)$ , it belongs in $O(n^2)$ as well?<br/>If Big-O were strict it would belong in $O(n)$ right? – QxFFu Jun 11 '14 at 15:44
$O(n)\subset O(n^2)$ , but I am pretty sure you mean that so yes. – Studentmath Jun 11 '14 at 15:46
@QxFFu You meant $O(n)\subset O(n^2)$. – Andreas Blass Jun 11 '14 at 15:46
@AndreasBlass Yes thats what I meant, thanks all for your fast answers. – QxFFu Jun 11 '14 at 15:48
up vote 0 down vote accepted

Everything in $O(n)$ is also $O(n^2)$. Recall that a function f(n) is O(n) if $f(n) \leq cn$ for sufficiently large $n$ and some constant $c>0$. Well, if $f(n) \leq cn$, then it's also the case that $f(n) \leq c n^2$ for all appropriately large $n$, so f(n) is $O(n^2)$ as well.

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To be strict: $\forall n>N \ an +b <an^2 +b n^2 = (a+b)n^2 = O(n^2)$ hence the result, a very loose one of course.

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