A Web Site to Check the English Definition and Usage

http://www.wordreference.com/definition/

When doubted about the usage of words such as

as follows or as following
base on or based on
disability or disabled, etc.

Just type in the words and see the result in discussion forum.
There are many English experts there.

Table of mathematical symbols

This is useful source of information about mathematical notations or symbols reading and usage.

http://en.wikipedia.org/wiki/Table_of_mathematical_symbols