The number 9 is the natural number following 8 and preceding 10. It has rich historical, mathematical, and cultural significance. Here are some key facts about the number 9:

• It is the highest single-digit number in the decimal system.
• The shape of the digit 9 has evolved over centuries from ancient Indian numerals and may be derived from the Arabic letter "waw."
• Mathematically, 9 is the fourth composite number and the first odd composite number.
• It is unique as the only square number that is the sum of two consecutive cubes: 9=32=13+239=3^2=1^3+2^39=32=13+23.
• A quick check for calculations involving 9 exists called "casting out nines," known since the 12th century.
• The sum of the digits of any multiple of 9 is always 9. Also, rearranging the digits in a number divisible by 9 produces another number divisible by 9.
• Number 9 originated in the Indian subcontinent around 3000 B.C. and is considered a complete and divine number as it completes the decimal cycle.
• There are cultural references such as the "nine dots puzzle" and sayings like a cat having nine lives.
• Additionally, 9 was once the number of planets in the solar system before Pluto was reclassified as a dwarf planet.
• The element with atomic number 9 is fluorine, a highly reactive and toxic gas.

These facts showcase the mathematical properties, history, and cultural significance of the number 9.