Chapter 04
Number theory
Number theory is the part of the course that sorts numbers into kinds. A question often names the kind it wants rather than the number, and knowing what the name means is half of answering it.
Kinds of number
Number theory is about the different kinds of numbers and how they behave.
- Positive numbers are above zero: 1, 2, 3, 4, 5.
- Negative numbers are below zero: −1, −2, −3, −4, −5.
- Zero is neither positive nor negative.
- Even numbers divide by 2 with nothing left over: 2, 4, 6, 8, 10.
- Odd numbers do not: 1, 3, 5, 7, 9.
- Prime numbers have only two factors, 1 and themselves: 2, 3, 5, 7, 11, 13.
Comparing two numbers
3 > −2
3 is greater than negative 2.
Where this leads
- Integers and negative numbers
- The arithmetic rules for the numbers below zero named here.