# Understanding the Su-Schrieffer-Heeger (SSH) model and Topological insulators regarding invariants

I'm studying the topic of Topological insulators, I'm having a very hard time understanding what is the relationship between the fact that topological invariants are different from $$0$$ and the presence of edge states.

For instance in the Su Schrieffer model, all lecture notes and books I've read just limit themselves to calculate the topological invariants at different phases and then they just show that at this parameters it happens that there are edge states (example https://arxiv.org/abs/1509.02295).

This explanation to me just happens to show a 'coincidence', whenever there is a non zero topological invariant,there is presence of edge states. But then I don't understand the connection. Could someone clear up this for me? Or at least give links to literature that explain clearly this connection?