How do you say "It is" and "It is not" in Korean?

Just a simple sentence I want to add into my scrap paper for math. How would you say, "It is a function" and "It is not a function" in Korean?

Steven2017-02-01T14:09:09Z

It is a function: (그것은) 함수이다.
It is not a function: (그것은) 함수가 아니다.