In Ubuntu the alt+numpad ACSII codes don’t work—this was a major stumbling block for me since I am annoyingly picky about typography.

After spending way too much time Googling I found that these steps work:

• Hold Ctrl+Shift+U
• Release the “U” key (while still holding Ctrl+Shift)
• Type the Unicode code for the special character you’d like to use
• Find unicode characters by searching on Wolfram Alpha

Another great unicode character reference for math and science can be found at Penn State’s website. Of course, if you don’t want to remember these unicode characters, CopyPasteCharacter.com is a good ACSII character resource.