I know that it is easy to prove that diagonally dominant matrices are regular (non-singular) by the gershgorin circle theorem. But the theorem that diagonally dominant matrices are regular was discovered earlier, e.g. by Minkowski in 1900, so am looking for one of those original proofs.
Can you help me to find this one? Is there any free eBook or skript? Also, I am not looking for the proof by Levy who proved this for real matrices only, but another proof e.g. by Desplanques would be nice to see as well.
I'm sorry that this is not the complete answer, but I found in Matrix Iterative Analysis by Varga this (see item 1.5 in Bibliography and Discussion section of Chapter 1):
Reference:
O. Taussky, A recurring theorem on determinants. Amer. Math. Monthly 56, 672--676, 1949.
I don't have access there but it seems that the paper is accessible via JStor.