Feasible successor question

As you can tell from the lack of response you will have to work this out for yourself really. The thing is that the only way to understand it is to figure it all out.

I have never bothered.

This "rule" as I understand it comes from a mathematical analysts of the network layout and costs. This reduces at the end of the day to the simple rule of thumb which has been published. There may be many other rules of arbitrary complexity to figure out that a topology is loop free, I don't supopse that there is anything magic about this one, it is just the one that cisco settled on that works well enough and that we network designers and managers can follow.

I would guess that eigrp came out of some academic work in a university and that Cisco picked up on it later. Don't know though.

Have you seen -

The above documents says: "If Router Three accepts all of these routes, it results in a routing loop."

You will just have to follow through the various routing decisions that each router makes and see if you can follow.

Good luck.