Is my Major Professor Counted as a Co-Author? According to the APA style manual, the editorial &quot;we&quot; is used to refer to the author and co-authors but should not be used if there is a single author. However, in writing my mathematics thesis, I am working closely with a professor to do the research. He won't be listed as an author as far as I'm aware, but it feels natural to use the pronoun &quot;we&quot; to talk about results found during or after a proof. It also has always seemed more a reference to the author and reader together but perhaps this has only been my own erroneous imagination. Here is the link to the APA style reference I mentioned: https://apastyle.apa.org/style-grammar-guidelines/grammar/first-person-pronouns
Should I avoid using the pronoun &quot;we?&quot;
Using &quot;we&quot; as you suggest is fine.  As to the APA, I say a mathematician needs a guide to psychology writing like a fish needs a bicycle.  We with a single author is good enough for Terry Tao 1,2 so you can do it as well.
There are many specific style issues in mathematics.  There are many guides. Here is one 3.
By the way, the best time to discuss authorship issues with your professor is back when you started this project.  The second best time is today.