1. Chapter V
Interacting Fields
Lecture 2
Books Recommended:
Lectures on Quantum Field Theory by Ashok
Das
Advanced Quantum Mechanics by Schwabl

2. Perturbation Theory
Eq. of motion for the time evolution operator in
Interaction picture
----(1)
To solve above, use initial condition
-----(2)
Thus, from (1), we write
----(3)

3. Using iterative method, we write
---(4) Or
----(5)

4. Using time ordering product we can write
---(6)
Above equation is known as Dyson series.

5. If at t0, system is in state |i> then the probability of finding it in state |f> at later time t will be
----(7)
Transition rate
----(8)