I am trying to understand the intersection of 2 DFA's. I got the following link:

Intersection of DFA

I can't understand why they have not used the symbol 'b':

For M', we have...

- E' = {a}//Here
- Q' = {s0, s1}
- q0' = s0
- A' = {s0}
- f'(s0, a) = s1, f'(s1, a) = s0

For M''', we get...

- E''' = {a}
- Q''' = {(s0, t0), (s0, t1), (s0, t2), (s1, t0), (s1, t1), (s1, t2)}
- q0''' = (s0, t0)
- A''' = {(s0, t0)} for intersection, {(s0, t0), (s0, t1), (s0, t2), (s1, t0)} for union, {(s0, t1), (s0, t2)} for difference.
- f'''((s0, t0), a) = (s1, t1), f'''((s1, t1), a) = (s0, t2), f'''((s0, t2), a) = (s1, t0), f'''((s1, t0), a) = (s0, t1), f'''((s0, t1), a) = (s1, t2), f'''((s1, t2), a) = (s0, t0).

Somebody please guide me.

Zulfi.