Lec 20 : Computation Theory
Two way DFA (2 DFA)
1
Two way DFA (2 DFA)
In comparison, DFA and 2DFA differ in that an input can be read only
once from left to right by a DFA, whereas A 2DFA can read the input
back and front with no limit on how many times an input symbol can be
read.
Definition A 2DFA over ∑ is a system A=(Q,,q0,F) as in DFA with the
difference that now  is a function from
Q×∑ into Q×D where D={L,R,S}
Example:
Design A 2DFA that accept “101” and go back to the beginning of the
tape
First Example TM
y\y,R
n n
L={0 1 }
B\B,R
q5
q3
0\0,R
y\y,R
q0
0\x,R
q1
1\y,L
x\x,R
y\y,L
q2
q0
x\x,R
0\0,L
0\0,L