You guys are asking for trouble putting special characters directly into the text (using CharMap or by typing the 3-4 digit Alt+Numlock sequences) because those are the numeric character codes specific to a particular font. The characters will show up when somebody else reads the message only if their browser is set to the same character set and the same font is installed. You can see what might happen if you change your own browser's character set (FireFox: View--Character Encoding; IE: View--Encoding) and watch the special characters disappear or turn into something else.

The one and only certain way to get special characters in HTML text is to use what HTML defines as entities. Most people are be familiar with two of them: <> produced by the sequence &lt;&gt;. Entities begin with an ampersand and end with a semicolon, and the name in the middle allegedly has mnemonic value. It's possible to compose an entity using a numeric symbol value like &950;, but that defeats the whole purpose of entities, which is to make the symbols independent of number.

There are hundreds of entities defined, and quite a few are applicable to displaying math. Most come from mapping the Symbol font onto standard entity names. Here's a reference to symbol entities useful in math. Some examples:

&alpha; = α
&beta; = β
&ge; = ≥
&Psi; = &Psi;
&equiv; = &equiv;
&exist; = &exist;
Σ

Here's an ambitious example that leverages the SUP and SUB tags and uses style to set font size and some relative positioning:

Σ4327

<span style="font-size:60px;">&Sigma;^{^{<span style="font-size:14px">43</span>}}<span style="font-size:14px;position:relative;left:-12px;">27</span></span>

You could compose more complicated equations using a table to get a precise layout.

Bottom line: use anything other then entities, and you're likely to read complaints that other people can't read your post.