Summary

The number theory in Books VII-IX. Between them, they have 102 propositions, prefaced by 22 definitions in Book VII.  The material in these books is probably older than that in Books V and VI and some results that were proved there for magnitudes are proved in these books for numbers.
Pay particular attention to the Definitions in Book VII, as well as Propositions 2, 16 and 19; Book VIII, Propositions 8 and 23, and Book IX, Propositions 20, 35 and 36.

Reading

Euclid, Elements VII-IX.  Heath's version is on reserve, or you may look at David Joyce's Elements on the web.

Homework

Euclidean Number Theory handout.