9 (nine) is the natural number following 8 and preceding 10.
9 is a composite number, its proper divisors being 1 and 3. It is 3 times 3 and hence the third square number. Nine is a Motzkin number. It is the first composite lucky number, along with the first composite odd number and only single-digit composite odd number.
9 is the only positive perfect power that is one more than another positive perfect power, by Mihăilescu's Theorem.
9 is the highest single-digit number in the decimal system. It is the second non-unitary square prime of the form (p2) and the first that is odd. All subsequent squares of this form are odd.
Since 9 = 321, 9 is an exponential factorial.
A polygon with nine sides is called a nonagon or enneagon. A group of nine of anything is called an ennead.
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