I am reading about non-standard models of peano arithmetic, and came across a theorem by Friedman that states the following.
Every non-standard countable model of (peano) arithmetic is isomorphic to an initial segment of itself.
There seems to be little available to non-academics on this subject, and although I can accept that the theorem is true, I cannot think of why it is true, nor of a concrete example of an isomorphism between a countable non-standard model and an initial segment of itself.
Question: What would such an isomorphism look like, and is there a nice and/or intuitive way of thinking of this?