# 21

## Nearby Numbers

Positive, nonzero integers within three units

18
Eightteen
19
Nineteen
20
Twenty
22
Twenty-Two
23
Twenty-Three
24
Twenty-Four

## Scientific Notation

Scientific notation expresses a quantity as the product of its significand with 10 raised to an integer exponent.

## Reciprocal

A number multiplied with its reciprocal is one.

### `0.047619047619047616`

The reciprocal of 21.

## Palindrome?

A numerical palindrome has the same value when all of its digits are reversed.

The number 21 is not a palindrome.

## Not A Prime Number

A prime number is a positive integer that is divisible only by itself and one.

Twenty-one is a composite number with 4 total factors (including 1 and itself).   See primes in Base 10 Decimal

## A Composite

Composites have more than just these two factors.

Twenty-one is a composite number with 4 total factors (including 1 and itself).

## Prime Factors

The prime factors of a positive integer are the integers that divide it exactly and are also prime.

The number twenty-one has the following 2 prime factors:

3
Three
7
Seven

## Prime Factorization

The prime factorization of a positive integer is the unique list of prime factors together with their multiplicities

## Base Conversions

The number twenty-one in 35 different bases