### About the Number 21 (twenty-one)

This nBAB was created on the 75th anniversary of the ratification of the 21st Amendment. The 21st Amendment repealed the 18th Amendment (Prohibition).

```   MathBabbler Number Analyst (MBNA) output:
=========================================
21 is a natural, whole, integer
21 is odd
21 proper divisors are: 1,3,7,
21 is deficient (sum of divisors is 11)
21 is unhappy
21 is a Harshad number
21 is not prime
21 is undulating
21 has the prime factors: 3*7
21 in octal is 025
21 in hexadecimal is 0x15
21 in binary is 10101 (is odious)
21 nearest square numbers: -5...4 (16 ...25 )
sqrt(21) = 4.58258
ln(21) = 3.04452
log(21) = 1.32222
21 reciprocal is .04761904761904761904761904761904
21! is 5.10909e+19
21 is 6.68451 Pi years
21 is 1 score and 1 year
```

`21` written as a Roman numeral is `XXI`.

`21` is semi-prime.

`[wikipedia]` `21` is a Fibonacci number.

`[wikipedia]` `21` is the sum of the first six integers.

This nBAB was created in the `21`st century.

Added the song title "`21`st Century Life" to Songs With Numbers In Their Titles.

##### Update::2013.02.21

This update was made on February `21`st when Pi Day 2013 was `21` days away. This nBAB was created in 2008 and the following is MBNA output for the number `21` on `21` February 2013.

```   MathBabbler Number Analyst (MBNA) output:
=========================================
21 is a natural, whole, integer
21 is odd
21 proper divisors are: 1,3,7,
21 is deficient (sum of divisors is 11)
21 is unhappy
21 is a Squarefree Number
21 is a Harshad number
21 is not prime
21 has the prime factors: 3*7 (sum=10)
21 is semiprime (biprime or 2-almost prime)
21 is undulating
21 is a Fibonacci number
21 is a Padovan number(12+9)
21 is a lucky number
21 is triangular
21 in octal is 025
21 in hexadecimal is 0x15
21 in binary is 10101 (is odious)
21 nearest square numbers: -5...4 (16...25 )
sqrt(21) = 4.58258
ln(21) = 3.04452
log(21) = 1.32222
21 reciprocal is .04761904761904761904761904761904
21! is 5.10909e+19
21 is 6.68451 Pi years
21 is 1 score and 1 year
21 written as a Roman numeral is XXI
21 = 11^2 - 10^2 and 11 + 10 = 21
```

Creator: Gerald Thurman [gthurman@gmail.com]
Created: 05 December 2008 