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Concept
Egyptian fraction
Summary
An Egyptian fraction is a finite 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 ; for instance the Egyptian fraction above sums to . Every positive rational number can be represented by an Egyptian fraction. Sums of this type, and similar sums also including and 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 In modern mathematical notation, Egyptian fractions have been superseded by vulgar fractions and decimal notation.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.
Beyond their historical use, Egyptian fractions have some practical advantages over other representations of fractional numbers.
For instance, Egyptian fractions can help in dividing food or other objects into equal shares. For example, if one wants to divide 5 pizzas equally among 8 diners, the Egyptian fraction
means that each diner gets half a pizza plus another eighth of a pizza, for example by splitting 4 pizzas into 8 halves, and the remaining pizza into 8 eighths. Exercises in performing this sort of fair division of food are a standard classroom example in teaching students to work with unit fractions.
Egyptian fractions can provide a solution to rope-burning puzzles, in which a given duration is to be measured by igniting non-uniform ropes which burn out after a unit time. Any rational fraction of a unit of time can be measured by expanding the fraction into a sum of unit fractions and then, for each unit fraction , burning a rope so that it always has simultaneously lit points where it is burning. For this application, it is not necessary for the unit fractions to be distinct from each other.
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Ontological neighbourhood
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