/sci/ - Science & Math

When you need to seperate two sets of numbers from each other in a proof, how do you express them?

For even and odd numbers, it's pretty easy, since it would just be

<span class="math"> 2k [/spoiler] vs. <span class="math"> 2k + 1[/spoiler],

but what if you only want to express numbers which aren't multiples of 4, for example?

I know it's easy stuff, but I'm only slowly getting into number theory and proofs, so bear with me.