what does an upsdie down a mean in math code example

Example: discrete math differnce between a and upside down a

Date: 04/08/97 at 21:35:35
From: Doctor Keith
Subject: Re: Math Symbols

Hi,

Most math books put an appendix with notation or mention it in the 
preface, to avoid this problem.  Here are a couple of basics:

1. upside down A  means  "for all"

2. backward E    means  "there exists"

3. epsilon (looks like c with horizontal line in middle)
                 means  "an element of" or "in" of "belongs to"

4. backwards epsilon
                 means  "such that"

5. three dots triangle, point up
                 means  "therefore" or "thus"

6. three dots triangle, point down
                 means  "because" or "due to"


These basic four cover most of the cases that you will find but of
course many others exist.  You can always go to a library and check 
out a book by another author, who includes the symbol definitions.  

There's also a link to the page that compiles information about the 
first uses of some math symbols at the bottom of our Dr. Math FAQ:

  http://mathforum.org/dr.math/faq/   

-Doctor Keith,  The Math Forum
 Check out our web site!  http://mathforum.org/dr.math/