Group division G/N is defined if N is normal

G/N = cosets of N in G.
example: D3/C3

forall g,g1,g2 in G
NN=N => (gN)N=gN
NgN=gN
g1Ng2N=g1g2N

(gN)^=g^N

example: C3 subset of D3 normal
D3/C3= {N,bN} ~= C2

Sn/An ~= C2 forall n