Arithmetic pages:

Learn how to calculate fast
Multiply with 11 fast
The square of a number that ends in 5

Irrational numbers
A Proof that the Square Root of 2 is Irational

Special numbers in Arithmetic
142857
Fibonacci number

Divisibility tests
Test if a number is divisible by 3
Test if a number is divisible by 6
Test if a number is divisible by 9
Test if a number is divisible by a power of 2

Prime numbers
The definition of prime numbers
There are an infinite number of prime numbers

Calculate sums
The sum of the first n natural numbers
The sum of the first n odd natural numbers
The sum of the squares of the first n natural numbers
The sum of the squares of the first n odd natural numbers
The sum of the cubes of the first n natural numbers