Sustainability of Collusion

I think it is generally not possible in static situations. Here, there is always an incentive to undercut the current price/competitor? Are there non-standard assumptions under which it does become possible?
In dynamic settings, it seems likely there is a rich combination of conditions under which collusion is possible. 
* Differentiate between explicit and tacit collusion?
* Collusion in auctions
=== Some refs: ===
https://ec.europa.eu/competition/mergers/studies_reports/the_economics_of_tacit_collusion_en.pdf