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A **unit fraction** is a rational number written as a fraction where the numerator is one and the denominator is a positive integer. A unit fraction is therefore the reciprocal of a positive integer, 1/*n*. Examples are 1/1, 1/2, 1/3, 1/4, 1/5, etc.

Multiplying any two unit fractions results in a product that is another unit fraction:

However, adding, subtracting, or dividing two unit fractions produces a result that is generally not a unit fraction:

Unit fractions play an important role in modular arithmetic, as they may be used to reduce modular division to the calculation of greatest common divisors. Specifically, suppose that we wish to perform divisions by a value *x*, modulo *y*. In order for division by *x* to be well defined modulo *y*, *x* and *y* must be relatively prime. Then, by using the extended Euclidean algorithm for greatest common divisors we may find *a* and *b* such that

from which it follows that

or equivalently

Thus, to divide by *x* (modulo *y*) we need merely instead multiply by *a*.

Any positive rational number can be written as the sum of unit fractions, in multiple ways. For example,

The ancient Egyptian civilisations used sums of distinct unit fractions in their notation for more general rational numbers, and so such sums are often called Egyptian fractions. There is still interest today in analyzing the methods used by the ancients to choose among the possible representations for a fractional number, and to calculate with such representations.^{[1]} The topic of Egyptian fractions has also seen interest in modern number theory; for instance, the Erdős–Graham conjecture and the Erdős–Straus conjecture concern sums of unit fractions, as does the definition of Ore's harmonic numbers.

In geometric group theory, triangle groups are classified into Euclidean, spherical, and hyperbolic cases according to whether an associated sum of unit fractions is equal to one, greater than one, or less than one respectively.

Many well-known infinite series have terms that are unit fractions. These include:

- The harmonic series, the sum of all positive unit fractions. This sum diverges, and its partial sums
- The Leibniz formula for π is
- The Basel problem concerns the sum of the square unit fractions:
- The binary geometric series is

The Hilbert matrix is the matrix with elements

It has the unusual property that all elements in its inverse matrix are integers.^{[2]} Similarly, Richardson (2001) defined a matrix with elements

where *F*_{i} denotes the *i*th Fibonacci number. He calls this matrix the Filbert matrix and it has the same property of having an integer inverse.^{[3]}

Two fractions and (in lowest terms) are called **adjacent** if , which implies that their difference is a unit fraction. For instance, and are adjacent: and . However, some pairs of fractions whose difference is a unit fraction are not adjacent in this sense: for instance, and differ by a unit fraction, but are not adjacent, because for them . The terminology comes from the study of Ford circles, circles that are tangent to the number line at a given fraction and have the squared denominator of the fraction as their diameter: fractions and are adjacent if and only if their Ford circles are tangent circles.^{[4]}

In a uniform distribution on a discrete space, all probabilities are equal unit fractions. Due to the principle of indifference, probabilities of this form arise frequently in statistical calculations.^{[5]} Additionally, Zipf's law states that, for many observed phenomena involving the selection of items from an ordered sequence, the probability that the *n*th item is selected is proportional to the unit fraction 1/*n*.^{[6]}

The energy levels of photons that can be absorbed or emitted by a hydrogen atom are, according to the Rydberg formula, proportional to the differences of two unit fractions. An explanation for this phenomenon is provided by the Bohr model, according to which the energy levels of electron orbitals in a hydrogen atom are inversely proportional to square unit fractions, and the energy of a photon is quantized to the difference between two levels.^{[7]}

Arthur Eddington argued that the fine-structure constant was a unit fraction, first 1/136 then 1/137. This contention has been falsified, given that current estimates of the fine structure constant are (to 6 significant digits) 1/137.036.^{[8]}

**^**Guy, Richard K. (2004), "D11. Egyptian Fractions",*Unsolved problems in number theory*(3rd ed.), Springer-Verlag, pp. 252–262, ISBN 978-0-387-20860-2.**^**Choi, Man Duen (1983), "Tricks or treats with the Hilbert matrix",*The American Mathematical Monthly*,**90**(5): 301–312, doi:10.2307/2975779, MR 0701570.**^**Richardson, Thomas M. (2001), "The Filbert matrix" (PDF),*Fibonacci Quarterly*,**39**(3): 268–275, arXiv:math.RA/9905079, Bibcode:1999math......5079R**^**Ford, L. R. (1938), "Fractions",*The American Mathematical Monthly*,**45**(9): 586–601, doi:10.1080/00029890.1938.11990863, JSTOR 2302799, MR 1524411**^**Welsh, Alan H. (1996),*Aspects of statistical inference*, Wiley Series in Probability and Statistics, vol. 246, John Wiley and Sons, p. 66, ISBN 978-0-471-11591-5.**^**Saichev, Alexander; Malevergne, Yannick; Sornette, Didier (2009),*Theory of Zipf's Law and Beyond*, Lecture Notes in Economics and Mathematical Systems, vol. 632, Springer-Verlag, ISBN 978-3-642-02945-5.**^**Yang, Fujia; Hamilton, Joseph H. (2009),*Modern Atomic and Nuclear Physics*, World Scientific, pp. 81–86, ISBN 978-981-283-678-6.**^**Kilmister, Clive William (1994),*Eddington's search for a fundamental theory: a key to the universe*, Cambridge University Press, ISBN 978-0-521-37165-0.

- Weisstein, Eric W., "Unit Fraction",
*MathWorld*