Reading this "proof" gave me a headache, and at first I thought it's because I'm not familiar enough with formal inductive logic to precisely follow the conversion from statements of reasoning to shorthand notation.
Then I realized the aim is to trick you by playing a bit fast and loose with that convention, and hoping you don't notice. e.g. I stumbled at Step 4, and if you click on the details for it the authour admits k is ill-defined. That comes back to bite you when you hit the fallacious step.
I'm fascinated how some very old works by ancient physicists and mathematicians are written using plain (if verbose) language and diagrams, and you didn't need to learn a bunch of shorthand conventions specific to the field in order to participate. Does anyone know any good books on Quantum Mechanics that don't require you to learn Dirac notation first?
Many books that just give an overview of QM don't use dirac notation; look for books with "Modern Physics" or "Introduction" in their names. However, Dirac notation is used so ubiquitously in QM that your question is a bit like asking if there are linear algebra books that don't require you to learn matrix notation. Sure you can do linear algebra without matrices, but that would be a bit eccentric today.
Then I realized the aim is to trick you by playing a bit fast and loose with that convention, and hoping you don't notice. e.g. I stumbled at Step 4, and if you click on the details for it the authour admits k is ill-defined. That comes back to bite you when you hit the fallacious step.
I'm fascinated how some very old works by ancient physicists and mathematicians are written using plain (if verbose) language and diagrams, and you didn't need to learn a bunch of shorthand conventions specific to the field in order to participate. Does anyone know any good books on Quantum Mechanics that don't require you to learn Dirac notation first?