A much easier way to enter symbols missing in your keyboard layout in Linux is using Compose key. With Compose key you may remember easy mnemonics rather than obscure codes and enter special symbols as sequences.
For example:
Compose, c, o gives ? (and Compose, c, u gives ?) Compose, ", o gives ö Compose, `, o gives ò Compose, ', o gives ó Compose, ~, o gives õ Compose, =, E gives € Compose, a, e gives æ Compose, ", < and Compose, ", > give “ and ” respectively Compose, -, -, - gives — (em-dash) Compose, -, -, . gives – (en-dash) Compos, 1, 2 gives ½ Compose, O, C gives © Compose, (, digit, ) gives ?, ?, ? etc. Compose, o, o gives ° (like in °C) Compose, x, x gives × Compose, :, - gives ÷ Compose, +, - gives ± Compose, ^, 2 gives ² (superscript) Compose, _, 2 gives ? (subscript) and a lot more... you can even define you own sequences. And yes, you may choose which physical key is your Compose (right-Win-key in my case).
See also this blog post: Compose key magic
In Gnome and KDE you may enable Compose key from Keyboard layout settings dialog.
Windows users may try AllChars programme if they would like to have Compose key too.