It's a [nosism][1] (because *weism* is too close to bathroom humor), specifically [the author's **we**][2].

 > Similar to the editorial "we", *pluralis modestiae* is the practice common in mathematical and scientific literature of referring to a generic third person by we (instead of the more common one or the informal you).

  [1]: https://en.wikipedia.org/wiki/Nosism
  [2]: https://en.wikipedia.org/wiki/Nosism#The_author.27s_.22we.22_or_pluralis_modestiae