I know that it is easy to proof 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 find this one? Is there any free eBook or skript? Also, I am not looking for the proof by Levy who proofed this for real matrices only, but another proof e.g. by Desplanques would be nice to see as well.