c-/ enter a fraction
c-9 enter math mode
c-7 to stack objects
c-6 for superscript (caret)
c-- for subscript (underscore)
c-<space> pops subscript/superscript mode
c-0 exit math mode

c-A select all
c-sh-{ open selected cells
c-sh-} close selected cells
c-sh-T traditional math notation
c-sh-N normal (Mathematica) notation
c-sh-E shows currently selected cell; repeat to exit

alt-4 = section (Format -> Style -> Section)

For menu of shorcuts, do:  Insert -> Typsetting -> menu

<esc>G<esc> = greek Gamma
<esc> < <esc> = angle bracket
<esc> - - > <esc> = long right arrow