It's not too tricky - really!
What I'm typing into the 'Your Reply' box is literally "π ∞ √"
By the magic of HTML that then gets converted by your own browser into "π & √"
(oh, in case you're wondering, the way I got "π" not to turn into "π" is to cheat, by typing in "π" instead - "&" gives an ampersand "&" character.)
As for character map - I'm using the Windows XP character map, searching for the right characters (in 'advanced' view) then copying into the 'Your Reply' box. I'm using Mozilla - perhaps your browser doesn't support Unicode properly?