Can I email a professor from a different university about a proposed course project? I am currently taking a math class and saw that a professor from a different university had some ideas/open questions for final course research projects on his page. I began working on one of these project ideas but I ran into a problem that I couldn't overcome (basically a certain type of equation doesn't seem to exist for the object proposed in his question). 

I asked my math professor about it but they weren't sure. Would it be inappropriate to email the professor from a different university about it and ask them if this is the correct direction to work in for the problem? It would just be a one time email, not like expecting him to answer all my questions, but I understand how he could take it as such.
I would say go ahead. Just make sure that you ask your question clearly: describe the approaches you tried and what you have stumbled upon. Professors are usually delighted to see the interest in their subject, and moreover, such questions from students help to improve their courses. The professor might not necessarily reply to your email due to lack of time, but you lose nothing by trying.