Base six

A base six numeral system (also known as senary, heximal, or seximal) has six as its base. It has been adopted independently by a small number of cultures. Like decimal, it is a semiprime, though it is unique as the product of the only two consecutive numbers that are both prime (2 and 3). As six is a superior highly composite number, many of the arguments made in favor of the duodecimal system also apply to senary.

Mathematical Properties

When expressed in senary, all prime numbers other than 2 and 3 have 1 or 5 as the final digit. In senary, the prime numbers are written:

2, 3, 5, 11, 15, 21, 25, 31, 35, 45, 51, 101, 105, 111, 115, 125, 135, 141, 151, 155, 201, 211, 215, 225, 241, 245, 251, 255, 301, 305, 331, 335, 345, 351, 405, 411, 421, 431, 435, 445, 455, 501, 515, 521, 525, 531, 551, ... (sequence A004680 in the OEIS)

That is, for every prime number p greater than 3, one has the modular arithmetic relations that either p ≡ 1 or 5 (mod 6) (that is, 6 divides either p − 1 or p − 5); the final digit is a 1 or a 5. This is proved by contradiction.

For any integer n:

If n ≡ 0 (mod 6), 6 | n If n ≡ 2 (mod 6), 2 | n If n ≡ 3 (mod 6), 3 | n If n ≡ 4 (mod 6), 2 | n

Additionally, since the smallest four primes (2, 3, 5, 7) are either divisors or neighbors of 6, senary has simple divisibility tests for many numbers.

Furthermore, all even perfect numbers besides 6 have 44 as the final two digits when expressed in senary, which is proven by the fact that every even perfect number is of the form 2p – 1(2p – 1), where 2p − 1 is prime.

Senary is also the largest number base r that has no totatives other than 1 and r − 1, making its multiplication table highly regular for its size, minimizing the amount of effort required to memorize its table. This property maximizes the probability that the result of an integer multiplication will end in zero, given that neither of its factors do.

If a number is divisible by 2, then the final digit of that number, when expressed in senary, is 0, 2, or 4. If a number is divisible by 3, then the final digit of that number in senary is 0 or 3. A number is divisible by 4 if its penultimate digit is odd and its final digit is 2, or its penultimate digit is even and its final digit is 0 or 4. A number is divisible by 5 if the sum of its senary digits is divisible by 5 (the equivalent of casting out nines in decimal). If a number is divisible by 6, then the final digit of that number is 0. To determine whether a number is divisible by 7, one can sum its alternate digits and subtract those sums; if the result is divisible by 7, the number is divisible by 7, similar to the "11" divisibility test in decimal.

Natural Languages

Despite the rarity of cultures that group large quantities by 6, a review of the development of numeral systems suggests a threshold of numerosity at 6 (possibly being conceptualized as "whole", "fist", or "beyond five fingers"), with 1–6 often being pure forms, and numerals thereafter being constructed or borrowed.

The Ndom language of Indonesian New Guinea is reported to have senary numerals. Mer means 6, mer an thef means 6 × 2 = 12, nif means 36, and nif thef means 36 × 2 = 72.

Another example from Papua New Guinea are the Yam languages. In these languages, counting is connected to ritualized yam-counting. These languages count from a base six, employing words for the powers of six; running up to 66 for some of the languages. One example is Komnzo with the following numerals: nibo (61), fta (62 [36]), taruba (63 [216]), damno (64 [1296]), wärämäkä (65 [7776]), wi (66 [46656]).

Some Niger-Congo languages have been reported to use a senary number system, usually in addition to another, such as decimal or vigesimal.

Proto-Uralic has also been suspected to have had senary numerals, with a numeral for 7 being borrowed later, though evidence for constructing larger numerals (8 and 9) subtractively from ten suggests that this may not be so.

Sumerians and Babylonians used a base of 60 with six and 10 as sub-bases.