4

4 (four) is a number, numeral, and glyph. It is the natural number following 3 and preceding 5. Four is the smallest composite number, its proper divisors being 1 and 2. Four is also a highly composite number. The next highly composite number is 6. Four is the second square number, the second centered triangular number. 4 is the smallest squared prime (p) and the only even number in this form. It has an aliquot sum of 3 which is itself prime. The... more

4 (four) is a number, numeral, and glyph. It is the natural number following 3 and preceding 5. Four is the smallest composite number, its proper divisors being 1 and 2. Four is also a highly composite number. The next highly composite number is 6. Four is the second square number, the second centered triangular number. 4 is the smallest squared prime (p) and the only even number in this form. It has an aliquot sum of 3 which is itself prime. The aliquot sequence of 4 has 4 members (4,3,1,0) and is accordingly the first member of the 3-aliquot tree. Only one number has an aliquot sum of 4 and that is squared prime 9 The prime factorization of four is two times two. Four is the smallest composite number that is equal to the sum of its prime factors. (As a consequence of this, it is the smallest Smith number). However, it is the largest (and only) composite number n for which is false. It is also a Motzkin number. In addition, . Continuing the pattern in Knuth's up-arrow notation, ,... less