- let L reg
- Sing L reg, L satisfy PL
- let PL constatnt n
- then choose w = w(p)
- by PL w can be factorized into xyz where |xy| ≤ n, |x| ≥ 0, |y| > 0
- for all i ≥ 0 xyz \in L
- so we can let y = n(k)
- consider i = i(k, n)
- show contradiction which are not in L that is x^i y z^i