A few months ago I came up with a proof for an old theorem. After being excited for a moment, I then tried to find my proof in the literature. Since I did not find it, then I started to wonder if it was worth publishing it.
I asked a few people about journals that could publish something like this, and they gave me two recommendations:
(1) The Mathematical Gazette, http://www.m-a.org.uk/the-mathematical-gazette
(2)The Plus Magazine, https://plus.maths.org/content/about-plus
First I submitted to the Mathematical Gazette, and my article was rejected because according to the reviewer I was trying to prove something very simple using something much more complex (although I just used undergraduate level math).
Then I submitted to Plus, and it was also rejected by the editors (it probably doesn't fit well with their magazine).
Do you have any suggestions? Thanks.
Best Answer
If the old theorem is something commonly seen in an undergraduate math class (with the old demonstration), then this might be appropriate as a "Note" in the American Mathematical Monthly.
What could happen if you submit it? They may publish it. The referee may give you a reference for it. They may respond in the same way as the Gazette.
What if the old theorem is not commonly seen in an undergraduate math course? When you write a textbook on that area of math, you can include your new proof. But if you think it unlikely you will write a textbook on this, then probably there is little prospect for publishing this. Maybe if you make it known to the experts* then some day one of them may include it in their new textbook.
*Perhaps by posting somewhere on-line...