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From Euler to Appell and Past—Wolfram Weblog


Hypergeometric Functions: From Euler to Appell and Beyond

Hypergeometric collection appeared within the mid-seventeenth century; since then, they’ve performed an essential position within the growth of mathematical and bodily theories. A lot of the elementary and particular features are members of the big hypergeometric class.

Hypergeometric features have been part of Wolfram Language since Model 1.0. The next plot reveals the implementation timeline of various hypergeometric features throughout the evolution of our system:

Evolution of hypergeometric functions in Wolfram Language

The Gauss hypergeometric 2F1, Kummer hypergeometric 1F1 and confluent hypergeometric 0F1 features have been carried out in Wolfram Language Model 1.0, and in Variations 3.0, 4.0 and 7.0, highly effective updates have been made that carried out 4 very common features: the generalized hypergeometric pFq operate, the “monster” superfunction MeijerG, the AppellF1 operate and the so-called q-hypergeometric operate, carried out as QHypergeometricPFQ. All these common features considerably elevated the combination, summation and different symbolic manipulation capabilities of Wolfram Language.

Over the past three years, we have now made a powerful effort to implement the remaining computable hypergeometric features. Three Appell features (AppellF2, AppellF3 and AppellF4) have been carried out in Model 13.3; additional generalization of MeijerG—the FoxH operate—was carried out somewhat earlier, in Model 12.3; and, lastly, for Model 14.0, we’re presenting the doubly infinite hypergeometric operate of 1 variable—the so-called bilateral hypergeometric operate—as BilateralHypergeometricPFQ.

A Little bit of Historical past

The time period “hypergeometric collection” seems to have first been utilized by John Wallis in his 1655 e book Arithmetica Infinitorum, after which these hypergeometric collection have been handled by Leonhard Euler.

Ranging from the works of Carl Gauss and persevering with with Ernst Kummer, Bernhard Riemann, Paul Appell and different nice students, these features have been systematically studied, together with the differential equations they fulfill and their huge functions in several engineering, bodily and different functions.

J. Wallis, L. Euler, C. Gauss, E. Kummer, B. Riemann, P. Appell

Hypergeometric Collection

A hypergeometric collection is an influence collection , the place the ratio of successive coefficients is a rational operate of n (, the place A(n) and B(n) are polynomials in n).

Let’s check out the Taylor collection of the exponential operate:

Taylor series of the exponential function

Calculate the ratio of successive coefficients (this may be completed through DiscreteRatio):

DiscreteRatio

This ratio is clearly a rational operate of n, and for this case A(n) = 1, B(n) = n + 1, therefore the Taylor collection of Exp is hypergeometric.

In actual fact, numerous well-known collection are hypergeometric, so having a complete idea of such series-based features is fascinating in addition to very helpful in several areas of science. So let’s swap to the category of hypergeometric features and begin with the main one—the generalized hypergeometric operate pFq—after which transfer on to the well-known Kummer 1F1 and Gauss 2F1 hypergeometric features that continuously come up in several bodily and mathematical functions.

The Generalized Hypergeometric Perform

The primary operate of the hypergeometric class is the generalized hypergeometric operate pFq, which is outlined by the next collection:

Generalized hypergeometric function defined

the place (ai)n is the Pochhammer image or the rising factorial.

The ratio of successive phrases of pFq is clearly rational:

Ratio of successive terms

The generalized hypergeometric operate pFq is carried out in Wolfram Language as HypergeometricPFQ[a;b;z]. Right here, the variety of parameters within the a and b lists is just not mounted; they could even be empty lists.

The q-analog of pFq is the fundamental hypergeometric operate rΦs, which has the collection growth

Basic hypergeometric function series expansion

the place (a;q)n is the q-Pochhammer image. The essential hypergeometric operate rΦs, carried out in Wolfram Language as QHypergeometricPFQ, turns into the generalized hypergeometric operate pFq within the restrict q → 1.

pFq performs an essential position within the idea of differential equations. A big set of odd differential equations (ODEs) will be solved when it comes to pFq features (we discuss with such equations as hypergeometric ODEs). Following, we current such an ODE that’s solved when it comes to pFq features:

DSolveValue

pFq has a well-developed idea and numerous elementary functions in science (one would possibly check out the Purposes part of the HypergeometricPFQ reference web page).

One other outstanding software instance is the trinomial equation xnx + t = 0 that, within the common type, is solved when it comes to pFq features:

x[m_,n_Integer,t_]

The trinomial equation has n roots. Let’s generate one among them for, say, n = 5 and t = 2:

x[3,5,2]

Now we generate a desk of 5 options and test that they actually clear up the trinomial equation:

Desk[

pFq is extensively used for integration and summation as well as for symbolic expression simplification. For example, here is a seemingly simple integration example:

Integrate[Exp

And here is an example of an infinite sum:

Sum[ (2k

Other hypergeometric functions can be written in terms of HypergeometricPFQ:

Hypergeometric2F1

The following table shows some special cases of pFq:

funclist = Inactivate

Although pFq is a very general and important function, its special cases are even more popular. They significantly affected mathematical and physical theories of the nineteenth and twentieth centuries. Two of the most famous special cases are the Gauss hypergeometric function 2F1 and the Kummer confluent hypergeometric function 1F1.

Gauss Hypergeometric Function

The well-known 2F1 function is defined by the following series:

Gauss hypergeometric function defined

It is a solution of the Gauss differential equation, which is a singular second-order linear ODE:

DSolveValue

ComplexPlot3D demonstrates the pole of 2F1 at the singular point 1:

ComplexPlot3D

Why is this function of fundamental importance? Because every second-order linear ODE with three regular singular points can be transformed to it, hence the Gauss differential equation is the “basic” ODE with three singular points.

Second-order linear ODEs with a low number of singularities (the majority of ODEs that describe some physical phenomenon) can often be treated as special or limiting cases of the Gauss hypergeometric equation. This means that the powerful 2F1 incorporates most of the known special functions as special cases, including the famous Bessel functions, Legendre polynomials and others.

More information about the second-order linear ODEs, their solutions and their singularities is available in the author’s earlier blog post, titled “From Sine to Heun,” as well as a comprehensive tutorial on Wolfram Language’s DSolve function.

Aside from its mathematical importance, the Gauss hypergeometric function has various applications in physics, statistics and other areas of science. The twentieth-century quantum mechanical potentials can typically be solved in terms of hypergeometric functions.

Some of the applications are presented on the reference page of Hypergeometric2F1.

Kummer Confluent Hypergeometric Function

The confluent hypergeometric function 1F1 is defined by the following series:

Kummer hypergeometric function defined

It is a solution of the Kummer confluent differential equation xy“(x) + (bx)y‘(x) – ay(x) = 0. This differential equation can be obtained from the Gauss differential equation for 2F1 via the complex procedure of merging two regular singularities (coalescence).

The radial wavefunction for the continuous spectrum for the hydrogen atom is written in terms of the 1F1 function:

ψ [

Here is a plot of the solution:

Plot [

Plotting the solution in 3D gives more insight about the behavior of the radial wavefunction for the hydrogen atom:

Plot3D

Finally, here is a differential equation that can be solved in terms of 1F1:

DSolve

Hypergeometric Functions of Two Variables

So far, we’ve talked about hypergeometric functions of one variable. pFq is a very general function with an unlimited number of parameters, but it has only one argument. What if we turn to hypergeometric functions of two or more arguments? Does that make sense?

The answer is yes. Further extensions to two or more variables are possible and yes, they open some new possibilities.

The first class is the Appell hypergeometric functions of two variables, named after French mathematician Paul Émile Appell.

Paul Émile Appell

Appell was a remarkable French mathematician who contributed to various fields of mathematics (projective geometry, algebraic functions, differential equations, complex analysis, etc.). Appell polynomials and Appell’s equations of motion in mechanics are named after him. Appell hypergeometric functions were introduced by him in 1880, and in 1926 he authored a treatise on these functions with another famous French mathematician, Joseph Kampé de Fériet:

"Fonctions hypergéométriques et hypersphériques : polynomes d'Hermite"

There are four Appell functions. These functions have the following double series definitions around the origin (presented here with their convergence regions):

The four Appell functions with their convergence regions

Appell functions reduce to Hypergeometric2F1 when x = 0 or y = 0.

As noted earlier, AppellF1 was introduced in Wolfram Language 4.0 back in 1999, while we’ve implemented the AppellF2, AppellF3 and AppellF4 functions only in 2023 in Wolfram Language 13.3.

Here are plots of a family of AppellF2 functions:

Plot [Table

The series expansions of Appell functions can be written in Hypergeometric2F1 functions:

Series [ AppellF4

As with HypergeometricPFQ, we use AppellF1 for integration:

Integrate [

And here is another general example of a whole class of integrands:

Integrate [

All four Appell functions solve the corresponding Horn PDEs with polynomial coefficients (we might think about these PDEs as a generalization of Gauss hypergeometric ODEs). This is the PDE that AppellF3 solves:

pde =
(pde /

And as for HypergeometricPFQ, many elementary and special functions are to be considered as special cases of the Appell functions:

funclist = Inactivate

Even More General Hypergeometric Functions

The Appell functions are the first four functions in the set of 34 Horn hypergeometric functions of two variables.

The Appell functions are special cases of the Kampé de Fériet function, which is the general hypergeometric function of two variables. The Kampé de Fériet function can be used to represent the derivatives of pFq with respect to parameters and multiple integrals of the Meijer G-function.

Further hypergeometric generalizations to n dimensions include the Lauricella functions, which are very general and very complex. For n = 2, they reduce to the Appell F1F4 functions, while for n = 1 we get the 2F1 Gauss hypergeometric function.

Bilateral or Doubly Infinite Hypergeometric Series

Another generalization of the hypergeometric pFq function is the doubly infinite hypergeometric function (the bilateral hypergeometric function). It is written as

Bilateral hypergeometric function

with a very similar definition to pFq except that for the bilateral series, the sum is computed from negative infinity to infinity. This function is available in Wolfram Language 14.0 as BilateralHypergeomtricPFQ.

There are two completely different subcases of the bilateral hypergeometric function: the “good” case when
 p = q (i.e. 2H2) and the “bad” case when pq.

For the first case, we can think about the bilateral function as a sum of two ordinary generalized hypergeometric functions. For example:

Bilateral function as a sum of two ordinary generalized hypergeometric functions

In the following, we calculate the value of 2H2 (1/2, 3/4; 1/4, 1/3; 5.4) and plot this function:

BilateralHypergeometricPFQ
ReImPlot

And for this “good” case of BilateralHypergeometricPFQ, simplifications are possible:

BilateralHypergeometricPFQ
BilateralHypergeometricPFQ

For the second case, where pq, the bilateral hypergeometric series is divergent. Usually for the calculation of such sums, various regularization methods are used. The blog post “The ABCD of Divergent Series” gives comprehensive information about this topic.

For calculation of the bilateral hypergeometric function, we use the Borel regularization technique:

BilateralHypergeometricPFQ

Following is the series expansion for BilateralHypergemoetricPFQ at the origin:

Series

The bilateral hypergeometric series has its unique and important role: it can be used for summing doubly infinite series:

Sum [Pochhammer

So to sum a doubly infinite series, we internally first sum it to BilateralHypergeometricPFQ (as in the previous example) and then, where possible, simplify it—as in the following example:

Sum [Pochhammer

The use of BilateralHypergeometricPFQ gives a huge speedup in the summation of doubly infinite hypergeometric series. As an example, the summation of the previous series in Wolfram Language 13.3 (without using BilateralHypergeometricPFQ) took more than 46 seconds, but now we’re able to reduce the calculation time by a factor of 1,000!

Closing Words

Hypergeometric functions have been at the core of Wolfram Language since the first version was launched more than 35 years ago. We constantly improve them, along with implementing new ones.

Version 14.0 contains the whole set of hypergeometric functions of one variable; the four Appell functions; the bilateral hypergeometric function and related ones (the monster superfunctions MeijerG/FoxH and others); and the q-analog of pFq—the basic hypergeometric function rΦs.

It seems that we now have an almost complete “hypergeometric” infrastructure needed by researchers. This infrastructure includes powerful symbolic and numeric computational abilities as well as documentation that is being updated in almost every new version of Wolfram Language.

To close this blog post, we would like to thank all the Wolfram Research developers that contributed to this huge project.

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