When do we write a mathematical expression on a separate line, and when do we write it inline with the text?
I believe that the answer is the same for researching papers and Ph.D. theses, or maybe even textbooks; but please correct me if I am wrong.
Here I am referring to a paper/thesis on mathematics. When one writes a mathematical paper, sometimes one writes a mathematical expression in a separate line such as in the following example:
We know that the identity
holds true if ....
While sometimes one writes the mathematical expression inline with the text, such as in the following example:
Let f:X->Y be a one-to-one map, ...
My impression is the longer the expression is, the more likely it is written in a separate line. But is there any (possibly unwritten) rule for it?