You can see a pretty big list at https://cgit.freedesktop.org/xorg/lib/libX11/plain/nls/en_US.UTF8/Compose.pre.
Math
See the XCOMM Characters from "Mathematical Operators"
section of the compose key list.
Sequence  Result  Comment 

8 8  ∞  Infinity 
:   ÷  Division sign 
x x  ×  Multiplication sign 
 _  − 
MINUS SIGN, not to be confused with “” (HYPHENMINUS). 
I can’t find ways to enter:

“⋅” (DOT OPERATOR) (It can be entered with US, Symbolic with AltGr+Shift+9)
.  inputs “·” (MIDDLE DOT), which is different.
Misc
Some fractions are just entered with the numerator and denominator.
For example, “¼” is entered using Compose 1 4.
I can’t find a way to easily input a “FRACTION SLASH
” (i.e. “⁄”) though.
Sequence  Result  Comment 

  .  –  En dash 
    —  Em dash 
space space  “ ”  Nobreak space (nonbreaking space). 
" < or < "  “  Left double quote 
" > or > "  ”  Right double quote 
Emacs
Emacs has a compose key input method (called “compose
”).
Pretty handy along with activatetransientinputmethod
(Cx \) for entering accents.
Where are compose sequences defined?
 It seems like it’s in libX11.
 GTK includes its own copy of the X11 Compose file