9

9 (nine) is the natural number following 8 and preceding 10. The ordinal adjective is ninth. Nine is a composite number, its proper divisors being 1 and 3. It is 3 times 3 and hence the third square number. 9 is a Motzkin number. It is the first composite lucky number. Nine is the highest single-digit number in the decimal system. It is the second non-unitary square prime of the form (p) and the first that is odd. All subsequent squares of this f... more

9 (nine) is the natural number following 8 and preceding 10. The ordinal adjective is ninth. Nine is a composite number, its proper divisors being 1 and 3. It is 3 times 3 and hence the third square number. 9 is a Motzkin number. It is the first composite lucky number. Nine is the highest single-digit number in the decimal system. It is the second non-unitary square prime of the form (p) and the first that is odd. All subsequent squares of this form are odd. It has a unique aliquot sum 4 which is itself a square prime. 9 is; and can be, the only square prime with an aliquot sum of the same form. The aliquot sequence of 9 has 5 members (9,4,3,1,0) this number being the second composite member of the 3-aliquot tree. There are nine Heegner numbers. Since , 9 is an exponential factorial. 8 and 9 form a Ruth-Aaron pair under the second definition that counts repeated prime factors as often as they occur. A polygon with nine sides is called a nonagon or enneagon. A group of nine of anything... less