I've been tasked with proofreading an Engineering/Mathematics thesis paper. I was always told that numbers under 10 should be spelled out (one, two, three, ...) but I was wondering if this rule holds in math and science fields as well. This paper understandably uses a lot of numbers in it and switching back and forth between numbers written out in words and numerals seems inconsistent at best and somewhat confusing at worst.
Generally speaking you should try to distinguish between an English number and a mathematical number. As in "Over the next five chapters we will prove that 5 is a prime number." Never spell out a number which is a subject of study, like the "5" in that sentence. However ordinals like in "Section 4" and "Lemma 2" are always written with digits. "There are three results in Theorem 3." 


Also in engineering watch for units... "The two rods had diameter 5 mm." "A 6volt battery was used." 


I believe that the rule here is to spell out numbers used for counting (unless they require three or more words to be spelled out), and use digits otherwise. Just check the examples in the answers above to see that they all fit this rule. As one more example: "There are five theorems in the manuscript labeled Theorem 5". 

