Another paper which should be added to the list of references here is Derksen-Weyman, On the number of subrepresentations of a general quiver representation. They generalize the Intersection Multiplicity = Tensor Product Multiplicity equation to a general statement about quivers, and prove it in a very elegant manner.

I have long had a vague feeling that Mukhon-Tarasov-Varchenko, Belkale and Derksen-Weyman are describing the same construction in different languages. I haven't been able to confirm this but, if so, I recommend Derksen-Weyman's version as particularly readable.

Another paper which should be added to the list of references here is Derksen-Weyman, On the number of subrepresentations of a general quiver representation. They generalize the Intersection Multiplicity = Tensor Product Multiplicity equation to a general statement about quivers, and prove it in a very elegant manner.

I have long had a vague feeling that Mukhin-Tarasov-Varchenko, Belkale and Derksen-Weyman are describing the same construction in different languages. I haven't been able to confirm this but, if so, I recommend Derksen-Weyman's version as particularly readable.