Submiting a two page paper to a math journal I recently posted my two-page paper to arxiv and sent it to the editor of a good journal. The editor responded:


  "Your result is not substantial enough" and so it was rejected."


I cannot say that I totally disagree with the editor, the paper does not make any significant progress on number theory but I can't say that my paper is totally without value. It is the first time I have tried to publish a paper, and I am confused:


Was my paper rejected because it is too short?
Should I submit it to another journal?


I would really appreciate if someone gave me some advice on how to proceed.
I am no expert in number theory, so I can't really help you with the last question (if you should submit your paper to another journal). However, it sounds like the editor stated the reason for rejection pretty clear: In his impression the results are not substantial enough. This is a valid (and frequent) reason for rejection. So in this respect you are in good company. If the editor did not state explicitly that he finds the paper "too short" than there is no reason to believe that this was the reason for rejection.

To get good feedback on how to proceed with your paper and your research you should ask your advisor (or find one).