The characteristic equation of a servo system is given by
a0s4 + a1s3 + a2s2 + a3s + a4 = 0
Now, consider the following conditions required to be satisfied by the coefficient of the above characteristic equation for the system to be stable:
1. a1 > 0 and a4 > 0
2. a1 < 0 and a4 > 0
3. a1a2< a0a3
4. a1a2 > a0a3
Which of the above conditions holds true for the given system to be stable?