First of all: Thank you very much for this proof. I'll link to it form my post.
I just have one question. It's probably just an unknown notation on my side but, first you say "it implies a bijection f" and then you speak of functions
. What does it mean in this context?
Since A and A hat have the same cardinality there exists a bijective f from A to A hat.
It is defined as follows
Ah I see. Thank you. :)