b = 1 , Doesn't makes sense.
T(n) = T(n) + f(n) ,
Did you get what this recurrence relation is doing?
T(n) in each step doesn't divide or reduce the problem size. This will lead to infinite loop.
When b > 1
T(n) = T(n/2) + c
Problems size is reduced at each step here.
Hopefully you got it.