If using the nodal analysis for a certain circuit one usually chooses a node as reference node such that one can reach as many other nodes as possible. Unfortunately 99% of the literature I found on this subject seems to assume that one can alway find such a node that reaches every other node within a single branch.
One can see that obviously it's not possible to reach every node within a single branch, even if we change the reference node.
My text book states that one would need to use a super node in such a situation, as also shown in the example with node #4.
I assume this is due to our equations not being linear independent if we would consider #4 a "normal" node.
Please provide me some insight in why we have to use a super node here and if my assumption is correct. I'd highly appreciate to see some kind of mathematical proof.