Introduction to quantum integrability, factorized S-matrix theory and TBA

In this series of lectures given at KIAS in Nov 2018, I briefly introduce the concepts of integrability, at classical and quantum level. Then I elaborate on the factorised scattering theory emerging in 1+1 integrable QFT. The Thermdynamic Bethe Ansatz (TBA) is a well known method to get exact information from the scattering theory, in order to identify the underlying QFT. Here I illustrate how to get TBA in the case of the prototypical Sine-Gordon model, particularly interesting to show the techniques when the S-matrix is not diagonal.