Here are 10 facts about the number 9:
- Nine is the natural number following 8 and preceding 10.
- It is the fourth composite number and the first odd composite number.
- The shape of the digit 9 possibly derives from the Arabic letter "waw" (و).
- 9 is the only square number that is the sum of two consecutive positive cubes: 32=9=13+233^2=9=1^3+2^332=9=13+23.
- 9 is the largest single-digit number in the decimal system.
- The number 9 is a refactorable number.
- Casting out nines is a traditional quick method to check calculations of sums, differences, products, and quotients in decimal.
- There are nine Heegner numbers, which are special square-free positive integers related to unique factorization in imaginary quadratic fields.
- The number 9 can be handwritten with two strokes and sometimes appears similar to a raised lowercase letter "q".
- The nine dots puzzle is a popular mathematical puzzle involving connecting nine points with four straight lines without lifting the pen.
If facts about the historical or event significance related to the number 9 (such as 9/11) are desired, that can be provided separately.