How much does one have to contribute to be considered a co-author (and therefore listed as such)?
This differs widely by field. The general principle is (or should be) that co-authors contribute to the intellectual content of the paper. The actual ideas that make it worth publishing.
But there are fields, such as high energy physics where nothing can get done without the participation of a lot (lot) of technicians. They sometimes get listed as &quot;authors&quot;. But people in that field understand the intent. The first few authors came up with the ideas and framework. The rest actualized it in some sense. There are a few papers in which the list of authors is longer than the paper itself.
If you are asking general mathematical questions of experts to help you understand, then it is unlikely that they would warrant authorship. This remains true even if the questions are quite deep. If you ask them to help you prove the theorems or come up with important applications that you will include, then they probably do.