I have some definitions that would apply to these, but I'm not sure whether or not they are very concrete, and I would also like to confirm that they are correct. Here are the sets and my current answers.

a)The set of odd positive numbers.

If , then

b)The positive integer powers of 3.

If , then

c)The positive multiples of 4.

If , then

Are these all accurate, or would they benefit from more detailed answers?