A continued fraction is a mathematical expression written as a fraction whose denominator contains a sum involving another fraction, which may itself be a simple or a continued fraction. If this iteration (repetitive process) terminates with a simple fraction, the result is a finite continued fraction; if it continues indefinitely, the result is an infinite continued fraction are positive integers, is referred to as a simple (or regular) continued fraction. Any positive rational number can be expressed as a finite simple continued fraction, and any positive irrational number can be expressed as an infinite simple continued fraction.

Different areas of mathematics use different terminology and notation for continued fractions. In number theory, the unqualified term continued fraction usually refers to simple continued fractions, whereas the general case is referred to as generalized continued fractions. In complex analysis and numerical analysis, the general case is usually referred to by the unqualified term continued fraction.

The numerators and denominators of continued fractions can be sequences of numbers or functions.

Formulation

where the () are the partial numerators, the are the partial denominators, and the leading term is called the integer part of the continued fraction.

The successive convergents of the continued fraction are formed by applying the fundamental recurrence formulas where is the numerator and is the denominator, called continuants, of the th convergent. They are given by the three-term recurrence relation

If the sequence of convergents approaches a limit, the continued fraction is convergent and has a definite value. If the sequence of convergents never approaches a limit, the continued fraction is divergent. It may diverge by oscillation (for example, the odd and even convergents may approach two different limits), or it may produce an infinite number of zero denominators .

History

The story of continued fractions begins with the Euclidean algorithm, a procedure for finding the greatest common divisor of two natural numbers and . That algorithm introduced the idea of dividing to extract a new remainder and then dividing by the new remainder repeatedly.

Nearly two thousand years passed before devised a technique for approximating the roots of quadratic equations with continued fractions in the mid-sixteenth century. Now the pace of development quickened. Just 24 years later, in 1613, Pietro Cataldi introduced the first formal notation for the generalized continued fraction. with the dots indicating where the next fraction goes, and each representing a modern plus sign.

Late in the seventeenth century John Wallis introduced the term "continued fraction" into mathematical literature. New techniques for mathematical analysis (Newton's and Leibniz's calculus) had recently come onto the scene, and a generation of Wallis' contemporaries put the new phrase to use.

In 1744 Euler published De fractionibus continuis dissertatio [An essay on continued fractions]. (A very readable English translation is available.)

In 1748 Euler published a theorem showing that a particular kind of continued fraction is equivalent to a certain very general infinite series. Euler's continued fraction formula is still the basis of many modern proofs of convergence of continued fractions.

In 1761, Johann Heinrich Lambert gave the first proof that is irrational, by using the following continued fraction for :

Continued fractions can also be applied to problems in number theory, and are especially useful in the study of Diophantine equations. In the late eighteenth century Lagrange used continued fractions to construct the general solution of Pell's equation, thus answering a question that had fascinated mathematicians for more than a thousand years. Lagrange's discovery implies that the canonical continued fraction expansion of the square root of every non-square integer is periodic and that, if the period is of length , it contains a palindromic string of length .

In 1813 Gauss derived from complex-valued hypergeometric functions what are now called Gauss's continued fractions. They can be used to express many elementary functions and some more advanced functions (such as the Bessel functions), as continued fractions that are rapidly convergent almost everywhere in the complex plane.

Notation

The long continued fraction expression displayed in the introduction is easy for an unfamiliar reader to interpret. However, it takes up a lot of space and can be difficult to typeset. So mathematicians have devised several alternative notations. Gottfried Leibniz sometimes used notation like or sometimes with the plus signs typeset to vertically align with the denominators but not under the fraction bars,

Pringsheim

Carl Friedrich Gauss evoked the more familiar infinite product Here, the "" stands for , the German word for "continued fraction". This is probably the most compact and convenient way to express continued fractions; however, it is not widely used by English typesetters. (Usually the should be the same size as used for a sum or used for a product, but within this article appears much smaller than those due to technical limitations.)

Some elementary considerations

Here are some elementary results that are of fundamental importance in the further development of the analytic theory of continued fractions.

Partial numerators and denominators

If one of the partial numerators is really just a finite continued fraction with fractional terms, and therefore a rational function of to and to . Such an object is of little interest from the point of view adopted in mathematical analysis, so it is usually assumed that all . There is no need to place this restriction on the partial denominators .

The determinant formula

When the is expressed as a simple fraction we can use the determinant formula to relate the numerators and denominators of successive convergents and to one another. The proof for this can be easily seen by induction.

Base case

Inductive step

The equivalence transformation

If is any infinite sequence of non-zero complex numbers where equality is understood as equivalence, which is to say that the successive convergents of the continued fraction on the left are exactly the same as the convergents of the fraction on the right.

The equivalence transformation is perfectly general, but two particular cases deserve special mention. First, if none of the are zero, a sequence where , , , and in general .

Second, if none of the partial denominators are zero we can use a similar procedure to choose another sequence where and otherwise .

These two special cases of the equivalence transformation are enormously useful when the general convergence problem is analyzed.

Notions of convergence

converges if the sequence of convergents part of the fraction by , instead of by 0, to compute the convergents. The convergents thus obtained are called modified convergents. We say that the continued fraction converges generally is then called an exceptional sequence for the continued fraction. See Chapter 2 of for a rigorous definition.

There also exists a notion of absolute convergence for continued fractions, which is based on the notion of absolute convergence of a series: a continued fraction is said to be absolutely convergent are the convergents of the continued fraction, converges absolutely. The Śleszyński–Pringsheim theorem provides a sufficient condition for absolute convergence.

Finally, a continued fraction of one or more complex variables is uniformly convergent in an open neighborhood when its convergents converge uniformly on ; that is, when for every there exists such that for all

Even and odd convergents

It is sometimes necessary to separate a continued fraction into its even and odd parts. For example, if the continued fraction diverges by oscillation between two distinct limit points and , then the sequence must converge to one of these, and must converge to the other. In such a situation it may be convenient to express the original continued fraction as two different continued fractions, one of them converging to , and the other converging to .

is a continued fraction, then the even part and the odd part respectively. More precisely, if the successive convergents of the continued fraction are , then the successive convergents of as written above are , and the successive convergents of are .

Conditions for irrationality

If and are positive integers with for all sufficiently large converges to an irrational limit.

Fundamental recurrence formulas

The partial numerators and denominators of the fraction's successive convergents are related by the fundamental recurrence formulas

These recurrence relations are due to John Wallis (1616–1703) and Leonhard Euler (1707–1783). These recurrence relations are simply a different notation for the relations obtained by Pietro Antonio Cataldi (1548-1626).

As an example, consider the simple continued fraction in canonical form that represents the golden ratio φ

Applying the fundamental recurrence formulas we find that the successive numerators are and the successive denominators are , the Fibonacci numbers. Since all the partial numerators in this example are equal to one, the determinant formula assures us that the absolute value of the difference between successive convergents approaches zero quite rapidly.

Linear fractional transformations

A linear fractional transformation (LFT) is a complex function where is a complex variable, and are arbitrary complex constants such that . An additional restriction that is customarily imposed, to rule out the cases in which is a constant. The linear fractional transformation, also known as a Möbius transformation, has many fascinating properties. Four of these are of primary importance in developing the analytic theory of continued fractions.

• If the LFT has one or two fixed points which is clearly a quadratic equation in . The roots of this equation are the fixed points of . If the discriminant is zero the LFT fixes a single point; otherwise it has two fixed points.

• If the LFT is an invertible conformal mapping of the extended complex plane such that for every point in the extended complex plane, and both and preserve angles and shapes at vanishingly small scales. From the form of we see that is also an LFT.

• The composition of two different LFTs for which is itself an LFT for which . In other words, the set of all LFTs for which is closed under composition of functions. The collection of all such LFTs, together with the "group operation" composition of functions, is known as the automorphism group of the extended complex plane.

• If which is a very simple meromorphic function of with one simple pole (at ) and a residue equal to . (See also Laurent series.)

The continued fraction as a composition of LFTs

Here we use to represent each simple LFT, and we adopt the conventional circle notation for composition of functions. We also introduce a new symbol to represent the composition of transformations where the last partial denominator in the finite continued fraction is understood to be . And, since , the image of the point under the iterated LFT is indeed the value of the finite continued fraction with

A geometric interpretation

Defining a finite continued fraction as the image of a point under the iterated linear fractional transformation leads to a geometric interpretation of infinite continued fractions.

can be understood by rewriting and in terms of the fundamental recurrence formulas

In the first of these equations the ratio tends toward as tends toward zero. In the second, the ratio tends toward as tends to infinity. This leads us to our first geometric interpretation. If the continued fraction converges, the successive convergents are eventually arbitrarily close together. Since the linear fractional transformation is a continuous mapping, there must be a neighborhood of that is mapped into an arbitrarily small neighborhood of . Similarly, there must be a neighborhood of the point at infinity which is mapped into an arbitrarily small neighborhood of . So if the continued fraction converges the transformation maps both very small and very large into an arbitrarily small neighborhood of , the value of the continued fraction, as gets larger and larger.

For intermediate values of where is a constant, introduced for convenience. But then, by substituting in the expression for so that even the intermediate values of (except when ) are mapped into an arbitrarily small neighborhood of , the value of the continued fraction, as gets larger and larger. Intuitively, it is almost as if the convergent continued fraction maps the entire extended complex plane into a single point.

Notice that the sequence lies within the automorphism group of the extended complex plane, since each is a linear fractional transformation for which . And every member of that automorphism group maps the extended complex plane into itself: not one of the can possibly map the plane into a single point. Yet in the limit the sequence defines an infinite continued fraction which (if it converges) represents a single point in the complex plane.

When an infinite continued fraction converges, the corresponding sequence of LFTs "focuses" the plane in the direction of , the value of the continued fraction. At each stage of the process a larger and larger region of the plane is mapped into a neighborhood of , and the smaller and smaller region of the plane that's left over is stretched out ever more thinly to cover everything outside that neighborhood.

For divergent continued fractions, we can distinguish three cases:

1. The two sequences and might themselves define two convergent continued fractions that have two different values, and . In this case the continued fraction defined by the sequence diverges by oscillation between two distinct limit points. And in fact this idea can be generalized: sequences can be constructed that oscillate among three, or four, or indeed any number of limit points. Interesting instances of this case arise when the sequence constitutes a subgroup of finite order within the group of automorphisms over the extended complex plane.

2. The sequence may produce an infinite number of zero denominators while also producing a subsequence of finite convergents. These finite convergents may not repeat themselves or fall into a recognizable oscillating pattern. Or they may converge to a finite limit, or even oscillate among multiple finite limits. No matter how the finite convergents behave, the continued fraction defined by the sequence diverges by oscillation with the point at infinity in this case.

3. The sequence may produce no more than a finite number of zero denominators . while the subsequence of finite convergents dances wildly around the plane in a pattern that never repeats itself and never approaches any finite limit either.

where is any real number such that .

Euler's continued fraction formula

Euler proved the following identity:

Euler's formula connecting continued fractions and series is the motivation for the , and also the basis of elementary approaches to the convergence problem.

Examples

Transcendental functions and numbers

Here are two continued fractions that can be built via Euler's identity.

Here are additional generalized continued fractions:

This last is based on an algorithm derived by Aleksei Nikolaevich Khovansky in the 1970s.

Example: the natural logarithm of 2 (= ≈ 0.693147...):

Here are three of 's best-known generalized continued fractions, the first and third of which are derived from their respective arctangent formulas above by setting and multiplying by 4. The Leibniz formula for converges slowly, requiring roughly terms to achieve correct decimal places.

The series derived by Nilakantha Somayaji also converges quite slowly, requiring nearly 50 terms for five decimals and nearly 120 for six. Both converge sublinearly.

converges linearly, adding at least three digits of precision per four terms, a pace slightly faster than the arcsine formula for which adds at least three decimal digits per five terms.

• Note: this continued fraction's rate of convergence tends to , hence tends to , whose common logarithm is . The same (the silver ratio squared) also is observed in the unfolded general continued fractions of both the natural logarithm of 2 and the nth root of 2 (which works for any integer ) if calculated using . For the folded general continued fractions of both expressions, the rate convergence , hence , whose common logarithm is , thus adding at least three digits per two terms. This is because the folded GCF folds each pair of fractions from the unfolded GCF into one fraction, thus doubling the convergence pace. The Manny Sardina reference further explains "folded" continued fractions.

• Note: Using the continued fraction for cited above with the best-known Machin-like formula

Roots of positive numbers

The nth root of any positive number can be expressed by restating

The square root of is a special case with and which can be simplified by noting that

The square root can also be expressed by a periodic continued fraction, but the above form converges more quickly with the proper and .

Example 1

The cube root of two (21/3 or ≈ 1.259921...) can be calculated in two ways:

Firstly, "standard notation" of , , and

Secondly, a rapid convergence with , and

Example 2

Pogson's ratio (1001/5 or ≈ 2.511886...), with , and

Example 3

The twelfth root of two (21/12 or

Example 4

Equal temperament's perfect fifth (27/12 or ≈ 1.498307...), with :

A rapid convergence with , , and

More details on this technique can be found in General Method for Extracting Roots using (Folded) Continued Fractions.

Higher dimensions

Another meaning for generalized continued fraction is a generalization to higher dimensions. For example, there is a close relationship between the simple continued fraction in canonical form for the irrational real number , and the way integer lattice points in two dimensions lie to either side of the line . Generalizing this idea, one might ask about something related to lattice points in three or more dimensions. One reason to study this area is to quantify the mathematical coincidence idea; for example, for monomials in several real numbers, take the logarithmic form and consider how small it can be. Another reason is to find a possible solution to Hermite's problem.

There have been numerous attempts to construct a generalized theory. Notable efforts in this direction were made by Felix Klein (the Klein polyhedron), Georges Poitou and George Szekeres.

See also

• Convergence problem

• Gauss's continued fraction

• Infinite compositions of analytic functions

• Lentz's algorithm

• Padé table

• Solving quadratic equations with continued fractions

Notes

References

• (Covers both analytic theory and history.)

  (Covers primarily analytic theory and some arithmetic theory.)

External links

• The first twenty pages of Steven R. Finch, Mathematical Constants, Cambridge University Press, 2003, , contains generalized continued fractions for and the golden mean.

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