Whereas the importance of a prime number in metrology is influenced by such objective features as the ease with which a division into that prime number of pieces can be constructed, determined by laws of mathematical geometry, the optimally efficient bases for certain metrological procedures including measurement and the specification of values to the greatest accuracy balanced with the minimum number of steps or symbols, and subjective or human cognitive capabilities such as limits of subitisation, the importance of a prime number as a factor can be quantified mathematically as the expected number of times that the prime number appears in a randomly selected number.
For example, the prime number two appears at least once in all of the even numbers or half of all whole numbers. Furthermore, the prime factor two appears again in numbers divisible by four or a quarter of all whole numbers, and yet more frequently in numbers divisible by eight, and so on. The sum of these expected frequencies is the converging limit of the infinite series
1/2 + 1/4 + 1/8 + … + 1/2^n
which is a geometric series of common ratio 1/2 between adjacent terms. The sum is given by the first term 1/2 divided by the subtraction of the common ratio 1/2 from unity, which works out to be one. This means that if numbers were selected at random, the expected average number of times the prime number two would appear as a factor would be once.
Similarly, the expected number of times the prime number three occurs in randomly selected numbers is given by the sum of the infinite series
1/3 + 1/3^2 + 1/3^3 + … 1/3^n
and this works out to be a half or 1/2, so the expected frequency of the prime number three in randomly selected numbers is a half.
In general, the sum of the infinite series for a prime number p is (1/p)/(1 – 1/p) = 1/(p – 1). So, the expected frequency of the prime number five would be 1/(5 – 1) = 1/4, a quarter. Likewise, the expected frequency for the prime number seven turns out to be a sixth. [The author completed this reasoning and derived this formula for this purpose of the quantification of the importance of prime numbers by the month June of the year 2017]
From these calculations, and specifying the importance of a prime number as being directly proportional to its expected frequency of occurance in randomly selected numbers, we see that the prime number two is twice as important as the prime number three, which in turn is twice as important as the prime number five.
To choose or design a base of numeration to have in its prime factorisation an appropriate balance of prime factors according to their importances so defined mathematically, we ought to select the prime factorisation to be a power of the form
[2^1 * 3^(1/2) * 5^(1/4) * 7^(1/6) * … * p^(1/(1-p))]^n,
choosing some value of n such that the exponents of the prime numbers are whole numbers.
Since the number of prime numbers is infinite and they cannot all be included as prime factors in a usable base, the infinite product must be truncated after the first few prime numbers, such that only the most important prime numbers in order are prioritised. Truncation after just the smallest prime number produces the binary-type bases of the form 2^n, such as two, four, and the square of four.
Truncation after the second smallest prime number three produces the number twelve or its powers as the base.
If the prime factor five is insisted upon, truncation to exclude the prime number seven and larger primes gives the number 2^4 * 3^2 * 5^1 = 720, which happens to be six factorial or the product of the first six whole numbers, as the smallest possible example that maintains the correct frequencies of the three smallest prime factors in accordance with their mathematically expected occurrances.
For a practical base of numeration, the number 720 is far too large. Those who desired the prime number five at any cost would therefore have to decrease the frequency in the prime factorisation of the smaller and objectively more important prime numbers two or three, while preferably still maintaining that the exponent of a less important prime number will not be greater than that of a more important prime number, resulting in such numbers as
2^3 * 3^2 * 5^1 = 360
2^4 * 3^1 * 5^1 = 240
2^2 * 3^2 * 5^1 = 180
2^3 * 3^1 * 5^1 = 120
2^2 * 3^1 * 5^1 = 60
2^1 * 3^1 * 5^1 = 30.
The number 360 was used as the number of angular degrees in a full circular turn. The number 60 was used as the base of the sexagesimal system of numeration, now retained in the numbers of minutes in an hour, seconds in a minute, and for subdivisions of angular degrees. Others have emphasised that these numbers as bases remain too large for convenient computations from the viewpoint of the limitations of the long-term human memory for multiplication tables. However, the inconvenience of large numbers as bases of numeration can also be argued from less subjective or more objective reasons of mathematics and consequently metrology.
Consider the smallest of the above numbers 30 as a base to include the prime factor five. Since this number has a disproportionately large exponent of the prime factor five, the smaller and more important prime factors two and three are under-represented. Consequently, numbers as the denominators of unit fractions which are expected to contain more of the prime number two which occur more often would be represented by the same number of digital value places as much less frequent unit fractions having much larger denominators as powers of the less important prime number five. This is because the number of digital value places in the digital form of a fraction written in a base B is determined by the prime factors of that base B and the prime factors in the denominator of the fraction in its simplest form. So, in the case of base thirty, the fraction 1/8, an eighth, where eight is two to the power of three, which is expected to appear frequently compared to fractions with larger denominators, would have three digital value places, because the power of the prime number two in the base thirty is only one. In base thirty, the digital form of a fraction such as 1/5^3, which is decimally 1/125, or 1/3^3, which is decimally 1/27, would also have three digits after the fractional point. However, we should have preferred for rather the numbers that appear more frequently to be represented by fewer digital value places or significant figures at the expense of those that are less frequent. In contrast, a base such as dozenal having a larger and more proportionate exponent of the smallest prime numbers would represent the fraction 1/8 in digital form by fewer than three digital value places.
This mathematical reasoning becomes relevant to the representation of numbers and their measurement in metrology. In a base such as thirty, where the number of figures after the fractional point required to represent the fraction 1/8 digitally is three, the amount of precision by obligation to attain this number of significant figures is phenomenal, because thirty to the power of three is twenty-seven thousand. A requirement to reach such precision or one part in so many thousands in order to express a fraction so simple and frequent as an eighth or one part in eight is unwarranted and heavily burdensome. A result of such excessive precision is many irrelevant steps or degrees of the scale at each value position or place in the digital representation or on the ruler or implement of measurement. [This reasoning by the author on the base thirty was done on Tuesday 18th December 2018]
A base such as the square of four, being a binary power, would represent a commonplace fraction such as an eighth using fewer digits. However, a base with only the prime number two in its factorisation would then not represent the frequent ternary fractions with a judicious amount of concision. Only the base twelve and its square represent the fractions of the most important prime numbers in the optimal way, until the number 720 which is too large as a base for the reason that it would demand too much precision at each positional digit even for the representation of the simplest and most commonly used fractions.
In order to discriminate a unit fraction such as an eighth, 1/8, at its level of precision it is required to be discernable as distinct from a seventh, 1/7, and a ninth, 1/9. The difference to be discriminated or resolved between 1/8 and 1/7 is their difference 1/56. The resolution needed to separate 1/8 from 1/9 is their difference 1/72. The average of these differences 1/56 and 1/72 is 1/63, which is near 1/64. In general, for resolution of a unit fraction 1/n, it must be distinguished from 1/(n – 1) and 1/(n + 1), by differences of 1/(n(n – 1)) and 1/(n(n + 1)). The average of n(n – 1) and n(n + 1) is n^2, so the required precision is about 1/n^2. This means that if the fraction to be resolved is 1/n and the base is n, then there is a requirement to go to the next digital value place after those needed to just specify the fraction to ensure that there is a following zero.
Fractions whose denominators have prime factors that are not factors of the base of numeration by which those fractions are represented do not have terminating digital forms. Their number of significant figures would be endless. Such non-terminating digital expressions are not convenient either for mathematical calculations where their necessary truncation would cause the accumulation of rounding errors or for metrological measurement where they would require an unattainable infinite number of steps in procedure. Numbers such as a third in decimal as 0.333… are of this type, where the digital representation never ends but does repeat. The number of digits between repetitions depends on whether the denominator of the fraction shares a factor with one of the numbers beside or adjacent to the base or a power of the base. For a fraction to have fewer digits before they repeat is slightly better than having a longer stretch of digits. However, non-terminating digital representations remain a nuisance no matter how short the cycle of repetition because of the errors and inexactness that they introduce into calculations and recorded values, such that their use is preferably avoided altogether. In the metrological sense, satisfactorily representating fractions with long periods of repetition, such as a seventh has in decimal, does not practically occur, especially for large bases, because the necessary precision is physically unrealistic. This means that, for example, in ordinary circumstances it is not practically feasible for a quantity to be measured with the decimal base in order to recognise whether it was intended to be exactly a seventh.
From the viewpoint of metrology, there is an inconvenience in a base having too many different kinds of prime factor. Before the metric system, haphazard agglomerations of units of measurement gave rise to irregular and unpredictable multiples of units to make up to the next named unit in the same type of measureable quantity. Irregularities such as these make calculations with them quite inconvenient. For example, the number of feet in a mile has an erratic prime factor, and there is no predictable way to extend the system of units to larger or smaller extremes or incorporate this into a sensible pattern. Having three different prime factors as the number thirty has would produce a similar effect of irregularity within its subdivisions and multiples between its powers. Subdivisions and multiples between powers become practically necessary where the base is too large to be subitised.
While historically, powers of two were used for convenience in metrology such as for denominations of weights and coins of currency, in the modern world with the advent of digital electronic computers and their storage of information in binary format, powers of base two have become even more important. In the decimal base to which society became accustomed, a fraction such as 1/2^6 = 1/64 having its digital decimal form as 0.015625 requires a ludicrous number of significant figures and consequent amount of precision. In contrast, the base twelve in representing the same fraction would require only half as many digits after the fractional point, because the power of two in the prime factorisation of the number twelve is twice that in the base ten. This means that base twelve is twice as suitable in the modern era than decimal for the representation of binary powers.
In summary, base twelve is the only practically sized base suitable for optimally representing fractions according to their mathematical distributions.