This document discusses pushdown automata (PDA). It defines a PDA as an ε-NFA with a stack, where transitions accommodate stack operations. Unlike DFAs which can only remember a finite amount of information, PDAs can remember an infinite amount using their stack. The document then formally defines the components of a PDA as a finite set of states Q, input symbols S, stack alphabet G, transition function δ, start state q0, start stack symbol Z0, accepting states F. It notes PDA diagrams graphically represent states as nodes, the start state with an arrow, accepting states as doubly circled, and transitions as edges labeled with input symbols, stack symbols, and replacement strings.

2.  Push Down Automata
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3.  Koushik das (152-35-1270)
 Md. Sohag ali (152-35-1271)
 Md. Mehedi hasan (152-35-1274)
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4.  Informally:
◦ A PDA is an ε-NFA with a stack.
◦ Transitions are modified to accommodate stack
operations.
 A DFA can “remember” only a finite amount
of information, whereas a PDA can
“remember” an infinite amount of (certain
types of) information.
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Q: A finite set of states.
S : A finite set of input symbols.
G: A finite stack alphabet.
d: The transition function with input:
qi is a state in Q.
a is a symbol in S or a = e (the empty string).
X is a stack symbol, X  G.
and the output is a finite set of pairs:
p the new state.
γ is the string of stack symbols that replaces X at the top of the
stack.
q0: The start state.
Z0 : Initially, the PDA’s stack consists this symbol and nothing else.
F : The set of accepting states.

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1. The nodes correspond to the states of the PDA.
2. An arrow labeled Start indicates the unique start state.
3. Doubly circled states are accepting states.
4. Edges correspond to transitions in the PDA as follows:
An edge labeled a, X/ α from state q to state p means that
d(q, a, X) contains the pair (p, α), perhaps among other pairs.
The only other thing that the diagram does not tell us is which
stack symbol is the start symbol. Conventionally, it is Z0,
unless we indicate otherwise.

15. Thank you
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