Use idea of a paper to prove new result I'm a phd student of Mathematic. I have following question. Recently, I proved something that I got main idea from some paper. To be honest, I don't like my proof, because I use the proof of  paper, then I changed a bit, and proved new result.  Though, when I show that my result to my supervisor he really likes. I told him that I used idea of some paper. In fact, 90% of proof is the same, but he said, it's okay, if someone didn't before. 

Since that's my first paper, it is very important for me. I don't know what I can do. Do I neglect it? Do I need to mention that I use that paper? Is it possible referee reject my paper?
Since it's your first paper, you can in general assume that your supervisor knows better than you if it is worthwhile publishing. There are many publications out there - in math and other fields - that takes an idea or formalism from one paper, and applies it to a slightly different area. This is how progress in research works.

And yes, you should definitely cite the other paper. The fact that you've been inspired by something else does not disqualify your paper. In fact, quite the opposite can be the case, if it is not obvious that the idea from the other paper can be applied to a new area.