The alternating group is generated by three-cycles 
Prove that, for $n \geq 3$, the three-cycles generate the alternation group $A_n$

Proof:  We multiply on the left by 3-cycles to "reduce" an even permutation $p$ to the identity, using induction on the number of indices fixed by a permutation. How the indices are numbered is irrelevant. If $p$ contains a $k$-cycle with $k \geq 3$, we may assume that it has the form $p=(123\dots k)\dots$ Multiplying on the left by $(321)$ gives $$p'= (321)(123 \dots k)\dots=(1)(2)(3\dots k)\dots$$ More fixed indices. 
What do you think ?

The idea of the proof is that we will show that any product of two transpositions is a product of 3-cycles ; and ,if so, then since every member of the An is a product of even number of transpositions, we are done. 
To begin with lets us suppose that we have a product of transpositions a1 and a2  where both have a common number a which belongs to the set {1,2,.....}- say they both move a. Then we have 
a1  of the form (a b) 
and a2 of the form (a c) . 
In this case , we have 
a1 a2 =
 (a b)(a,c)= (a,c,b) and we are done. 
Now suppose that a1 and 
a2 move different numbers .
a1 = (a b)
a2 =  (c d)
Then 
a1a2 = 
(a,b)(c,d) = ( d a c ) ( a b d ). 
And we are done. 
