how can i prove that this grammar is ambiguous?

Question

S -> bA|aB
A -> a|aS|bAA
B -> b|bS|aBB

Any easy method other than trying to find a string that would generate two parse trees ?

Can someone please give me a string that can prove this.

Answer 1

There is a string though: bbaaba

S -> bA -> bbAA -> bbaA -> bbaaS -> bbaabA -> bbaaba
S -> bA -> bbAA -> bbaSA -> bbaaBA -> bbaabA -> bbaaba

Answer 2

没有简单的方法来证明无上下文语法模糊不清 - 实际上，通过简化Post对应问题 ， 这个问题是不可判定的 。