In mathematics, a senary numeral system is a base-6 numeral system. The name heximal is also valid for such a numeral system, but is deprecated to avoid confusion with the more often used hexadecimal number base, colloquially known as 'hex'.
Senary may be considered useful in the study of prime numbers since all primes, when expressed in base-six, other than 2 and 3 have 1 or 5 as the final digit. Writing out the prime numbers in base-six (and using the subscript 6 to denote that these are senary numbers), the first few primes are
That is, for every prime number with , one has the modular arithmetic relations that either or : the final digits is a 1 or a 5. Furthermore, all known perfect numbers besides 6 itself have 44 as the final two digits.
Each regular human hand may be said to have six unambiguous positions; a fist, one finger (or thumb) extended, two, three, four and then all five extended.
If the right hand is used to represent a unit, and the left to represent the 'sixes', it becomes possible for one person to represent the values from zero to 55senary (35decimal) with their fingers, rather than the usual ten obtained in standard finger counting. e.g. if three fingers are extended on the left hand and four on the right, 34senary is represented. This is equivalent to 3 × 6 + 4 which is 22decimal.
Which hand is used for the 'sixes' and which the units is down to preference on the part of the counter, however when viewed from the counter's perspective, using the left hand as the most significant digit correlates with the written representation the same senary number.
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