Sunday, February 10, 2013

The Big $O$

I need to teach myself this stuff, so I'm going to make this post as a place where I can prove a bunch of elementary facts about Big O notation, (& maybe some related topics).

Starring Rarity as Roger Smith
Definition:
$f(x) = O(g(x)) \mbox{ as  } x \rightarrow \infty$ 
if and only if
$\exists \, M, x_0 \in \mathbb{R}^+ : |f(x)| \leq M|g(x)| \, \forall x > x_0$

Some basic observations:
-The equality sign here is an odd convention, because this is more of an order relation.

-This isn't a 'total ordering' when we look at all $f$ and $g$; $e^xsin(x)$ and $x^2$ are incomparable.

-There's an obvious analogue of this definition for sequences (those discrete things) and all the results discussed here apply equally to them.

Basic result(s) come after the break.