>>14418384Collatz is well defined on the integers. For that to be true, the converse of the Collatz function has to be well defined as well.
Let's go through the proofs for these two cases.
Assume first that Collatz is well defined for the naturals. As I mentioned above, Collatz is not well defined for the rationals, and we also have to define the converse function of Collatz. The converse of Collatz for a positive integer is
Similarly, the converse of Collatz for a negative integer is
Note that we can't write the converse of Collatz as
because that would not define a function for negative integers.
I think it's now a good time to make an axiom that Collatz is well defined on the naturals. The converse of Collatz is defined as for positive integers and as for negative integers.
It is now possible to prove that 4n is not of the form 4n