9 (nine) is the natural number following 8 and preceding 10. In mathematics 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. 9 is the second non-unitary square prime (3^2). It has a unique aliquot sum 4 which is itself a square prime. 9 is 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 = 3^, 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 an enneagon (technically) or nonagon (in common usage). A group of nine of anythi

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # TAGS