Up Arrowemoji

To type up ↑,down ↓, right →, and left ← arrows, make sure activate Num Lock then Press Alt, while pressing the Alt button type 24, 25, 26, or 27 respectively and release the Alt button and the arrows will appear.

Arrowsymbol

Installing this small indicator will let you easily insert characters like this without having to memorize many key combinations:

Open your keyboard settings (system settings → keyboard settings) and go to the shortcuts tab. Under the "typing" section you'll find a "Compose Key" setting. Set this to your compose key (I advice right alt to keep your know shortcuts)

Decrease symbol

Downarrow

There is a much simpler answer than all the other answers given here, the upwards arrow is directly available : AltGr+Shift+U gives ↑. You can visually check all the keyboard layout in the language setting menu.

So apparently an up arrow (↑) is already defined -- for ALT GR+SHIFT+U. If not, you can copy the file to a directory, edit it at will, name the profile somehow (say "foo" instead of "latin") and simply run

Log out and log back in. To quickly summon your symbols press the following keys in sequence Compose Key → 1 → 2 → 3. Just replace 1,2,3,4,5 etc etc with whatever keys you want to use.

Up arrowsymbol

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to the end of ~/.profile. If xim isn't working for you you'll have have to install uim (sudo apt-get install uim) and replace the xim in ~/.profile with uim and logout and log back in.

The four strings assign to each key give the four characters that are produced by pressing the key, key with shift, key with altgr and key with algr+shift. The names of the entities (keysyms) that xkb can recognize can be found here.