It is the old function of the optative which the Konjunktiv 1 has lost as productive feature between 1000 and 1500 AD in German (coniunctivus optativus). It's preserved in recipes, proverbs and some phrases:
Man nehme ein Ei und schlage es sich kräftig gegen den Kopf.
It is preserved in other languages, too:
God save the Queen.
Vive la France.
Requiescat in pace! – Er ruhe in Frieden!
The relict use is restricted to the third person. All other forms of sei- are real imperative forms, so this sei- of the third person could be set into imperative form paradigm, but the problem is that this use is strictly limited to the sound of recipes and, of course, mathematics. It has a long tradition there, because the optativus is used in logics and philosophy for a very long time.
We (Belles Lettres) have an article and a video-tutorial about this (in German).