Can I use special characters in the post title? (Like a math symbol))

Yes, but it will not show up in every browser. Even if (and you should) use code for the symbol, ∞ (ampersand#8734semicolon) not everyone in the world could read it. Other disadvantage, sometimes feed picks up the code.

So try it but check.

Maybe here. http://webdesign.about.com/od/localization/l/blhtmlcodes-math.htm

You are welcome. You can click on member to see the blogs I play around with.