Senary
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A senary numeral system is a base-six numeral system.
All prime numbers besides 2 and 3 end in either 1 or 5.
2,3,5,11,15,21,25,31,35,45,51,101,105,111,115,125,....
All known perfect numbers besides 6 itself end in 44.
Base 6 can be counted on hands by using each hand as a digit (0 to 5). The 0 digit is represented by a fist (no fingers out).
The senary number 3333 is equal to decimal 777.
Fractions
Six is the product of the first two prime numbers and is adjacent to the next two prime numbers. This makes many senary fractions simple.
Decimal Senary 1/2 1/2 = 0.3 1/3 1/3 = 0.2 1/4 1/4 = 0.13 1/5 1/5 = 0.1111 recurring 1/6 1/10 = 0.1 1/7 1/11 = 0.05050505 recurring 1/8 1/12 = 0.043 1/9 1/13 = 0.04 1/10 1/14 = 0.03333 recurring 1/12 1/20 = 0.03 1/14 1/22 = 0.023232323 recurring 1/15 1/23 = 0.022222222 recurring 1/16 1/24 = 0.0213 1/18 1/30 = 0.02 1/20 1/32 = 0.014444444 recurring
See also: octal, decimal, duodecimal and hexadecimal.fr:Système senaire