This is a soft question on writing math for research or publication purposes.
When do we number an equation? Or, when should we not number an equation?
Obviously, we number equations when we wish to refer back to them at some later point. But I have observed in textbooks and many publications that the numbering seems to be inconsistent.
For example: Can anyone explain why the top two equations are not numbered, but the bottom ones are? https://arxiv.org/pdf/1201.6656.pdf
Is there a stylistic guide as to when a equation should or should not be numbered?