Wow, I have discovered that it is possible to write directly from the keyboard such cool and useful character as:
« » → ⇒ ≤ ≥ ∞ × ½ …
and a long et cetera. This can easily done in Linux, especially in Gnome or KDE.
Check this:
https://help.ubuntu.com/community/GtkComposeTable
and
https://help.ubuntu.com/community/ComposeKey
Using the Compose Key is rather intuitive.