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 The 21st Amendment repealed the 18th Amendment (Prohibition).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 [4]...25 [5])
   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 21st century.

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

Update::2013.02.21

This update was made on February 21st 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)
   3...7...answer: 10
   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 [5])
   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

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This work is licensed under a Creative Commons Attribution 3.0 United States License.