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.

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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.