21 (number)

21 (twenty-one) is the natural number following 20 and preceding 22.

In mathematics

Twenty-one is a Fibonacci number, a Harshad number, a Motzkin number, a triangular number and an octagonal number, as well as a composite number, its proper divisors being 1, 3 and 7.

Related Topics:
Fibonacci number - Harshad number - Motzkin number - Triangular number - Octagonal number - Composite number - Divisor - 1 - 3 - 7

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