Definition: 7.03: Proper Divisor

A number is part of a(nother) number, the lesser of the greater, when it measures the greater.

Euclid requires a "part" to be a number strictly lesser then the number \( n > 1\) it measures.
Albeit some commentaries to Euclid's “Elements” state that the modern term of the "part" is the "divisor", this is not a correct interpretation, because Euclid never uses the term "part" for the trivial divisors \(\pm 1\) and \(\pm n\). Instead, in these cases he uses the notions "is measured by a unit" for \(1\mid n\) or "is equal to" for \(n\mid n\).
Therefore, the modern term most close to the meaning of a "part" is "proper divisor".
Please note that there is no definition in Euclid's “Elements” corresponding to the modern definition of a divisor.

Definitions: 1 2
Proofs: 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Propositions: 20 21 22 23 24 25 26 27 28 29