Oscillatory and Bistable output

Consider three inverters connected like in the figure. The output is fed back to the input. This circuit has no primary inputs. To understand the working of the loop consider that at t=0 there is a noise induced dynamic '1'. This will propagate through the gates and give an output of dynamic 0 at t= 2t1 + t2 . T1 and t2 are propagation delays of the gates. This will go on endlessly. Thus the output of any gate will be oscillatory. An oscillatory output is also called unstable.

If we have even number of gates we can be see that the output can only be 0 or 1