I have a question about the first part of the proof for the statement "If $x$ and $y$ are odd integers, then $xy$ is odd" if we are using a direct proof.
Now i've gotten the proof roughly correct but I have an issue with what we write at the beginning of the proof. If we were to put the statement into first order logic, wouldn't we start off with two universal quantifiers $\forall x \forall y \alpha$ where $\alpha$ is the rest of the formula?
If this is the case, shouldn't we use universal instantiation at the beginning of the proof? Is it okay to start off with "For any arbitrary $x$ and $y$ integers, suppose $x$ and $y$ are odd integers." Or is that incorrect since, in essence, we have assumed something about the integers, namely that they are odd, and thus they cannot be arbitrary?
All the answers to the basic proof which i've found on the net that use a direct proof approach, all just say "suppose $x$ and $y$ are odd integers", so I'm just wondering if adding the line about $x$ and $y$ being arbitrary is fine.