# SAT Study Guide/Part 2 - The Math Section/General Numerical Concepts

## Number properties[edit]

There are several sets of numbers:

- Counting or Natural numbers are numbers that can be used to count real things: 1, 2, 3,...
- Integers (including 0) include the natural numbers and their opposites: -3, -2, -1, 0, 1, 2, 3
- Even integers are divisible by 2, and can be given by 2n, where n is any integer: 0, 2, 4, 6,...
- Odd integers are not divisible by 2, and can be given by 2n-1, where n is any integer: -1, 1, 3, 5,...

- Rational numbers can be expressed as a fraction: -2, -1/2, 3/5, 7/15,...
- Irrational numbers are non-repeating, non-terminating decimals: π,
*e*, √2 - Real numbers include all of the above.

Numbers can be *prime*. That is, they are divisible by themselves and one, only. 1 is not considered a prime number, and 2 is the only even prime number. The first eight prime numbers are: 2, 3, 5, 7, 11, 13, 17, 19,...