Prerequisite: Basic knowledge of pushdown automata.
Problem :
Design a non deterministic PDA for accepting the language L = {an bn | n>=1}, i.e.,
L = {ab, aabb, aaabbb, aaaabbbb, ......}
In each of the string, the number of a's are followed by equal number of b's.
Explanation
Here, we need to maintain the order of a’s and b’s. That is, all the a's are coming first and then all the b's are coming. Thus, we need a stack along with the state diagram. The count of a’s and b’s is maintained by the stack. We will take 2 stack alphabets:
𝛤= { a, z }
Where, 𝛤= set of all the stack alphabet, z = stack start symbol
Approach used in the construction of PDA
As we want to design a NPDA, thus every time 'a' comes before 'b'. When ‘a’ comes then push it in stack and if again ‘a’ comes then also push it.
After that, when ‘b’ comes then pop one 'a' from the stack each time. So, at the end if the stack becomes empty then we can say that the string is accepted by the PDA.
Stack transition functions
𝝳 (q0, a, z) ├ (q0, az) [ push a on empty stack]
𝝳 (q0, a, a) ├ (q0, aa) [push a's ]
𝝳 (q0, b, a) ├ (q1, 𝝐 ) [pop a when b comes(state change)]
𝝳 (q1, b, a) ├ (q1, 𝝐 ) [pop a for each b]
𝝳 (q1, 𝝐 , z) ├ (qf, z) [final state]
Where, q0 = Initial state qf = Final state
So, this is our required non deterministic PDA for accepting the language L = { an bn | n>=1}