Pushdown Automata A pushdown automaton (PDA) is a finite automaton equipped with a stack-based memory. Each transition is based on the current input symbol and the top of the stack, optionally …

We can push a finite sequence of characters to the stack by introducing a finite chain of states which pushes them one at a time, so we will typically act as if the codomain of δ is P(Q × Γ∗). The idea is …

Given an input symbol, current state, and stack symbol, the automaton can follow a transition to another state, and optionally manipulate (push or pop) the stack. If at most one such transition action is …

NPDA can simulate any DPDA, but not vice versa. Therefore, NPDA is more powerful than DPDA.

The pushdown model adds a pushdown store, that can be conceptualized as a tape that has a beginning but is infinitely long. It, too, is divided into cells, and each cell that is in use contains a …

Every finite automaton can be viewed as a pushdown automaton that never operates on its stack. Proof. Let M = (Q, Σ, δ, q0, F ) be a finite automaton. Define M ‘ = (Q, Σ, Γ, δ’, q0, F ), where ...

Examples of Pushdown Automata. The key idea is to count the number of a’s using the stack. Repeatedly push X onto the stack for each a. Repeatedly pop X from the stack for each b. Now, we …