The subjunctive is here used in its archaic function to express commands or requests – sort of an imperative to the third person singular, which is the only person for which this still works in some cases, namely the following:
- Recipe-style instructions like manuals, mathematical definitions or exam tasks.
- In mostly fixed figures of speech. One such example is »man denke«, where the reader is addressed in the third person singular as man and which only works with this an similar verbs. Two examples:
Man denke nur, jeder täte dies. – Just think about (what would happen) if everybody did that.
Man stelle sich vor, die Bombe wäre nicht entdeckt worden. – Imagine the bomb had not been discovered.
Though in a mathematical text, your example is of the second kind and not the first – it is a motivatory text, no definitions or similar instructions are being made.
Using such a construction in a case that is neither covered by neither of the aforementioned cases is so over-the-top archaic, snobby and feudal, that it is only used ironically (and may not be understood by everybody):
Man reiche mir den Salzstreuer. – Can someone please hand me the salt cellar? [Not translating the archaic, snobby and feudal tone.]