One has no proper factors.
"Proper factors" are all of a number's factors except one and the number itself.
The proper factors of a number do not include one and the number itself.
The proper factors are the regular factors without one and the number itself.
Proper factors don't include one and the number itself.
Proper factors don't include one and the number itself.
Proper factors do not include one and the number itself.
Proper factors don't include one and the number itself.
The set of factors includes one and the number itself. Proper factors do not include those two.
Proper factors do not include one and the number itself. The only one left is 2.
That is one definition of the set of proper factors.
That is one definition of the set of proper factors.