Unit Fractions

The most interesting concept described in the Rhind papyrus is the treatment of fractions since, of the 87 problems it contains, all but six deals with fractions. All Egyptian Fractions are unit fractions (in the form 1/n), which is a way of working unique to that culture. There was a special symbol for 2/3.

The Egyptians mainly used two scripts; the familiar hieroglyphics and the simpler hieratic which was the handwritten form where the scribe used a reed brush and ink. So hieratic is a corrupt hieroglyphic.

Now, what is so remarkable about the use of unit fractions is the fact that they are in fact used to reduce a division between two numbers to the sums of unit fractions. An example would be dividing 2 by 43:

You will notice that no fraction is allowed to appear twice, but is not known how these particular unit fractions were derived. It is evident that there are many possible solutions, but it is not so evident why some solutions would be preferential to others.

It is interesting to note that the first section of Rhind Papyrus is a table of the division of 2 by every odd integer from 3 to 101, and it may be that the Egyptian scribes realised that the result of multiplying by 2 is the same as that of dividing 2 by n.

In fact, it may be that the decomposition for fractions of the form was the only necessary decomposition since Egyptians used the dyadic multiplication. This means that, instead of using times tables (as we do); they used two operations to multiply, doubling and adding.

Horus was Egyptian God who fought the forces of darkness (in the form of a boar - a pig) and won. His eye is a symbol for Egyptian Unit Fractions. Each part of the eye is a part of the whole. All the parts of eye, however, don't add up to the whole. This, some Egyptologists

think, is the sign that the knowledge can never be total, and that one part of the knowledge is not possible to describe or measure.

Eye of Horus fraction

These numerals are written in hieroglyphic script. Ancient Egyptians also used hieratic script. In both cases, they wrote from right to left, as Arabs do today.

Egyptians used different notation for their fractions (unit fractions).

In hieroglyphic script the fraction was given by placing a symbol for ro over an integer. Since ro is at the same time a smallest unit for grain -- a mouthful of grain, the symbol is a mouth.

For example, the fraction 1/12 in hieroglyphic is which uses the symbol for 10 and two strokes for the integer 2.

In hieratic this would be

Here the symbol for 10 has become angular. The position is read from right to left. Since the position of the dot does not cover the whole fraction, care has to be taken in reading hieratic so that should be read as 1/12 and not 1/10 + 2.

The special hieroglyph for 2/3 is

Or sometimes

Horus, Sesheta and Thoth

Horus is the ultimate god of mathematics, as his eye represents the basis of all Egyptian mathematics as we know it - the unit fractions.

An Egyptian fraction is the sum of distinct unit fractions, such as . That is, each fraction in the expression has a numerator equal to 1 and a denominator that is a positive integer, and all the denominators differ from each other. The value of an expression of this type is a positive rational number a/b; for instance the Egyptian fraction above sums to 43/48. Every positive rational number can be represented by an Egyptian fraction. Sums of this type, and similar sums also including 2/3 and 3/4 as summands, were used as a serious notation for rational numbers by the ancient Egyptians, and continued to be used by other civilizations into medieval times. In modern mathematical notation, Egyptian fractions have been superseded by vulgar fractions and decimal notation. However, Egyptian fractions continue to be an object of study in modern number theory and recreational mathematics, as well as in modern historical studies of ancient mathematics

Ancient Egypt

Egyptian fraction notation was developed in the Middle Kingdom of Egypt, altering the Old Kingdom's Eye of Horus numeration system. Five early texts in which Egyptian fractions appear were the Egyptian Mathematical Leather Roll, the Moscow Mathematical Papyrus, the Reisner Papyrus, the Kahun Papyrus and the Akhmim Wooden Tablet. A later text, the Rhind Mathematical Papyrus, introduced improved ways of writing Egyptian fractions. The Rhind papyrus was written by Ahmes and dates from the Second Intermediate Period; it includes a table of Egyptian fraction expansions for rational numbers 2/n, as well as 84 word problems. Solutions to each problem were written out in scribal shorthand, with the final answers of all 84 problems being expressed in Egyptian fraction notation. 2/n tables similar to the one on the Rhind papyrus also appear on some of the other texts. However, as the Kahun Papyrus shows, vulgar fractions were also used by scribes within their calculations.

Notation

To write the unit fractions used in their Egyptian fraction notation, in hieroglyph script, the Egyptians placed the hieroglyph

(Or, "[one] among" or possibly re, mouth) above a number to represent the reciprocal of that number. Similarly in hieratic script they drew a line over the letter representing the number. For example:

The Egyptians had special symbols for 1/2, 2/3, and 3/4 that were used to reduce the size of numbers greater than 1/2 when such numbers were converted to an Egyptian fraction series. The remaining number after subtracting one of these special fractions was written using as a sum of distinct unit fractions according to the usual Egyptian fraction notation.

The Egyptians also used an alternative notation modified from the Old Kingdom and based on the parts of the Eye of Horus to denote a special set of fractions of the form 1/2k (for k = 1, 2, ..., 6) and sums of these numbers, which are necessarily dyadic rational numbers. These "Horus-Eye fractions" were used in the Middle Kingdom in conjunction with the later notation for Egyptian fractions to subdivide a hekat, the

primary ancient Egyptian volume measure for grain, bread, and other small quantities of volume, as described in the Akhmim Wooden Tablet. If any remainder was left after expressing a quantity in Eye of Horus fractions of a hekat, the remainder was written using the usual Egyptian fraction notation as multiples of a ro, a unit equal to 1/320 of a hekat.

Calculation methods

Modern historians of mathematics have studied the Rhind papyrus and other ancient sources in an attempt to discover the methods the Egyptians used in calculating with Egyptian fractions. In particular, study in this area has concentrated on understanding the tables of expansions for numbers of the form 2/n in the Rhind papyrus. Although these expansions can generally be described as algebraic identities, the methods used by the Egyptians may not correspond directly to these identities. Additionally, the expansions in the table do not match any single identity; rather, different identities match the expansions for prime and for composite denominators, and more than one identity fits the numbers of each type:

For small odd prime denominators p, the expansion 2/p = 2/ (p + 1) + 2/p (p + 1) was used.

For larger prime denominators, an expansion of the form 2/p = 1/A + (2A − p)/Ap was used, where A is a number with many divisors (such as a practical number) between p/2 and p. The remaining term (2α − p)/αp was expanded by representing the number (2α − p)/Ap as a sum of divisors of A and forming a fraction d/Ap for each such divisor d in this sum (Hultsch 1895, Bruins 1957). As an example, Ahmes' expansion 1/24 + 1/111 + 1/296 for 2/37 fits this pattern with A = 24 and (2A − p)/Ap = 11 = 3 + 8, as 1/24 + 1/111 + 1/296 = 1/24 + 3/ (24 × 37) + 8/(24 × 37). There may be many different expansions of this type for a given p; however, as K. S. Brown observed, the expansion chosen by the Egyptians was often the one that caused the largest denominator to be as small as possible, among all expansions fitting this pattern.

For composite denominators, factored as p×q, one can expand 2/pq using the identity 2/pq = 1/aq + 1/apq, where a = (p+1)/2. For instance, applying this method for pq = 21 gives p = 3, q = 7, and a = (3+1)/2 = 2, producing the expansion 2/21 = 1/14 + 1/42 from the Rhind papyrus. Some authors have preferred to write this expansion as 2/A × A/pq, where A = p+1 (Gardner 2002); replacing the second term of this product by p/pq + 1/pq, applying the distributive law to the product, and simplifying leads to an expression equivalent to the first expansion described here. This method appears to have been used for many of the composite numbers in the Rhind papyrus (Gillings 1982, Gardner 2002), but there are exceptions, notably 2/35, 2/91, and 2/95 (Knorr 1982).

One can also expand 2/pq as 1/pr + 1/qr, where r = (p+q)/2. For instance, Ahmes expands 2/35 = 1/30 + 1/42, where p = 5, q = 7, and r = (5+7)/2 = 6. Later scribes used a more general form of this expansion, n/pq = 1/pr + 1/qr, where r = (p + q)/n, which works when p + q is a multiple of n (Eves 1953).

For some other composite denominators, the expansion for 2/pq has the form of an expansion for 2/q with each denominator multiplied by p. For instance, 95=5×19, and 2/19 = 1/12 + 1/76 + 1/114 (as can be found using the method for primes with A = 12), so 2/95 = 1/ (5×12) + 1/ (5×76) + 1/ (5×114) = 1/60 + 1/380 + 1/570 (Eves 1953). This expression can be simplified as 1/380 + 1/570 = 1/228 but the Rhind papyrus uses the unsimplified form.

The final (prime) expansion in the Rhind papyrus, 2/101, does not fit any of these forms, but instead uses an expansion 2/p = 1/p + 1/2p + 1/3p + 1/6p that may be applied regardless of the value of p. That is, 2/101 = 1/101 + 1/202 + 1/303 + 1/606. A related expansion was also used in the Egyptian Mathematical Leather Roll for several cases.

Medieval mathematics

Egyptian fraction notation continued to be used in Greek times and into the Middle Ages (Struik 1967), despite complaints as early as Ptolemy's Almagest about the clumsiness of the notation compared to alternatives such as the Babylonian base-60 notation. An important text of medieval mathematics, the Liber Abaci (1202) of Leonardo of Pisa (more commonly known as Fibonacci), provides some insight into the uses of Egyptian fractions in the middle ages, and introduces topics that continue to be important in modern mathematical study of these series.

The primary subject of the Liber Abaci is calculations involving decimal and vulgar fraction notation, which eventually replaced Egyptian fractions. Fibonacci himself used a complex notation for fractions involving a combination of a mixed radix notation with sums of fractions. Many of the calculations throughout Fibonacci‘s book involve numbers represented as Egyptian fractions, and one section of this book (Sigler 2002, chapter II.7) provides a list of methods for conversion of vulgar fractions to Egyptian fractions. If the number is not already a unit fraction, the first method in this list is to attempt to split the numerator into a sum of divisors of the denominator; this is possible whenever the denominator is a practical number, and Liber Abaci includes tables of expansions of this type for the practical numbers 6, 8, 12, 20, 24, 60, and 100.

The next several methods involve algebraic identities such as For instance,

Fibonacci represents the fraction by splitting the numerator into a sum of two numbers, each of which

divides one plus the denominator: Fibonacci applies the algebraic identity above to each

these two parts, producing the expansion Fibonacci describes similar methods for denominators that are two or three less than a number with many factors.

In the rare case that these other methods all fail, Fibonacci suggests a greedy algorithm for computing Egyptian fractions, in which one repeatedly chooses the unit fraction with the smallest denominator that is no larger than the remaining fraction to be expanded: that is, in more modern notation, we replace a fraction x/y by the expansion

where represents the ceiling function.

Fibonacci suggests switching to another method after the first such expansion, but he also gives examples in which this greedy expansion was iterated until a complete Egyptian fraction expansion was

constructed: and

As later mathematicians showed, each greedy expansion reduces the numerator of the remaining fraction to be expanded, so this method always terminates with a finite expansion. However, compared to ancient Egyptian expansions or to more modern methods, this method may produce expansions that are quite long, with large denominators, and Fibonacci himself noted the awkwardness of the expansions produced by this method. For instance, the greedy method expands

While other methods lead to the much better expansion

Sylvester's sequence 2, 3, 7, 43, 1807 ... can be viewed as generated by an infinite greedy expansion of

this type for the number one, where at each step we choose the denominator instead

of , and sometimes Fibonacci ‘s greedy algorithm is attributed to Sylvester.

After his description of the greedy algorithm, Fibonacci suggests yet another method, expanding a

fraction by searching for a number c having many divisors, with , replacing by

, and expanding as a sum of divisors of , similar to the method proposed by Hultsch and Bruins to explain some of the expansions in the Rhind papyrus.

Modern number theory

Modern number theorists have studied many different problems related to Egyptian fractions, including problems of bounding the length or maximum denominator in Egyptian fraction representations, finding expansions of certain special forms or in which the denominators are all of some special type, the termination of various methods for Egyptian fraction expansion, and showing that expansions exist for any sufficiently dense set of sufficiently smooth numbers.

The Erdős–Graham conjecture in combinatorial number theory states that, if the unit fractions are partitioned into finitely many subsets, then one of the subsets can be used to form an Egyptian fraction representation of one. That is, for every r > 0, and every r-coloring of the integers greater than one, there is a finite monochromatic subset S of these integers such that

The conjecture was proven in 2003 by Ernest S. Croot, III.

Znám's problem and primary pseudo perfect numbers are closely related to the existence of Egyptian fractions of the form

For instance, the primary pseudo perfect number 1806 is the product of the prime numbers 2, 3, 7, and 43, and gives rise to the Egyptian fraction 1 = 1/2 + 1/3 + 1/7 + 1/43 + 1/1806.

Egyptian fractions are normally defined as requiring all denominators to be distinct, but this requirement can be relaxed to allow repeated denominators. However, this relaxed form of Egyptian fractions does not allow for any number to be represented using fewer fractions, as any expansion with repeated fractions can be converted to an Egyptian fraction of equal or smaller length by repeated application of the replacement

If k is odd, or simply by replacing 1/k+1/k by 2/k if k is even. This result was first proven by Takenouchi (1921).

Graham and Jewett (Wagon 1991) and Beeckmans (1993) proved that it is similarly possible to convert expansions with repeated denominators to (longer) Egyptian fractions, via the replacement

This method can lead to long expansions with large denominators, such as

Botts (1967) had originally used this replacement technique to show that any rational number has Egyptian fraction representations with arbitrarily large minimum denominators.

Any fraction x/y has an Egyptian fraction representation in which the maximum denominator is bounded by

(Tenenbaum & Yokota 1990) and a representation with at most

Terms (Vose 1985).

Graham (1964) characterized the numbers that can be represented by Egyptian fractions in which all denominators are nth powers. In particular, a rational number q can be represented as an Egyptian fraction with square denominators if and only if q lies in one of the two half-open intervals

Martin (1999) showed that any rational number has very dense expansions, using a constant fraction of the denominators up to N for any sufficiently large N.

Engel expansion, sometimes called an Egyptian product, is a form of Egyptian fraction expansion in which each denominator is a multiple of the previous one:

In addition, the sequence of multipliers ai is required to be non-decreasing. Every rational number has a finite Engel expansion, while irrational numbers have an infinite Engel expansion.

Anshel & Goldfeld (1991) study numbers that have multiple distinct Egyptian fraction representations with the same number of terms and the same product of denominators; for instance, one of the examples they supply is

Unlike the ancient Egyptians, they allow denominators to be repeated in these expansions. They apply their results for this problem to the characterization of free products of Abelian groups by a small number of numerical parameters: the rank of the commutator subgroup, the number of terms in the free product, and the product of the orders of the factors.Open problem

Some notable problems remain unsolved with regard to Egyptian fractions, despite considerable effort by mathematicians.

The Erdős–Straus conjecture concerns the length of the shortest expansion for a fraction of the form 4/n. Does an expansion exist for every n? It is known to be true for all n < 1014, and for all but a vanishingly small fraction of possible values of n, but the general truth of the conjecture remains unknown.

It is unknown whether an odd greedy expansion exists for every fraction with an odd denominator. If Fibonacci’s greedy method is modified so that it always chooses the smallest possible odd denominator, under what conditions does this modified algorithm produce a finite expansion? An obvious necessary condition is that the starting fraction x/y has an odd denominator y, and it is conjectured but not known that this is also a sufficient condition. It is known (Breusch 1954; Stewart 1954) that every x/y with odd y has an expansion into distinct odd unit fractions, constructed using a different method than the greedy algorithm.

It is possible to use brute-force search algorithms to find the Egyptian fraction representation of a given number with the fewest possible terms (Stewart 1992) or minimizing the largest denominator; however, such algorithms can be quite inefficient. The existence of polynomial time algorithms for these problems, or more generally the computational complexity of such problems, remain Guy (2004)

2 * (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11) = 132, an even sum.since smallest prime applicable in "prime sum" is 3 (and not 2, the only even prime), with 11 boxes in between to house an ODD prime each, but sum of 11 odds = odd =/ 132