When I was doing the Probability homework, I found this asymptotic density is kind of fun. It defines some finite-additively measure, and it is not an algebra.

**Definition:** is a map from subsets of to , where

Consider the collection , it is obvious that the collection has the property of finite additivity. However, it is easy to verify that is not an algebra.

How do you add equations in your blog? Is it a feature of wordpress?

do not use your google blog, just use sinaapp or wordpress, that is perfect with latex ,there is a plugin for latex.

see yihei.sinaapp.com, or you may register one wordpress for yourself.