In
mathematics, Riemann's differential equation, named after
Bernhard Riemann, is a generalization of the
hypergeometric differential equation, allowing the
regular singular points
In mathematics, in the theory of ordinary differential equations in the complex plane \Complex, the points of \Complex are classified into ''ordinary points'', at which the equation's coefficients are analytic functions, and ''singular points'', at ...
to occur anywhere on the
Riemann sphere
In mathematics, the Riemann sphere, named after Bernhard Riemann, is a model of the extended complex plane: the complex plane plus one point at infinity. This extended plane represents the extended complex numbers, that is, the complex number ...
, rather than merely at 0, 1, and
. The equation is also known as the Papperitz equation.
The
hypergeometric differential equation is a second-order linear differential equation which has three regular singular points, 0, 1 and
. That equation admits two linearly independent solutions; near a singularity
, the solutions take the form
, where
is a local variable, and
is locally holomorphic with
. The real number
is called the exponent of the solution at
. Let ''α'', ''β'' and ''γ'' be the exponents of one solution at 0, 1 and
respectively; and let ''α''', ''β''' and ''γ''' be those of the other. Then
:
By applying suitable changes of variable, it is possible to transform the hypergeometric equation: Applying
Möbius transformations
Moebius, Möbius or Mobius may refer to:
People
* August Ferdinand Möbius (1790–1868), German mathematician and astronomer
* Theodor Möbius (1821–1890), German philologist
* Karl Möbius (1825–1908), German zoologist and ecologist
* Paul ...
will adjust the positions of the regular singular points, while other transformations (see below) can change the exponents at the regular singular points, subject to the exponents adding up to 1.
Definition
The differential equation is given by
:
::
The regular singular points are , , and . The exponents of the solutions at these regular singular points are, respectively, , , and . As before, the exponents are subject to the condition
:
Solutions and relationship with the hypergeometric function
The solutions are denoted by the ''Riemann P-symbol'' (also known as the ''Papperitz symbol'')
:
The standard
hypergeometric function may be expressed as
:
The P-functions obey a number of identities; one of them allows a general P-function to be expressed in terms of the hypergeometric function. It is
:
In other words, one may write the solutions in terms of the hypergeometric function as
:
The full complement of
Kummer Kummer is a German surname. Notable people with the surname include:
*Bernhard Kummer (1897–1962), German Germanist
*Clare Kummer (1873—1958), American composer, lyricist and playwright
*Clarence Kummer (1899–1930), American jockey
* Christo ...
's 24 solutions may be obtained in this way; see the article
hypergeometric differential equation for a treatment of Kummer's solutions.
Fractional linear transformations
The P-function possesses a simple symmetry under the action of
fractional linear transformations known as
Möbius transformation
In geometry and complex analysis, a Möbius transformation of the complex plane is a rational function of the form
f(z) = \frac
of one complex variable ''z''; here the coefficients ''a'', ''b'', ''c'', ''d'' are complex numbers satisfying ''ad' ...
s (that are the
conformal remappings of the Riemann sphere), or equivalently, under the action of the group . Given arbitrary
complex number
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the for ...
s , , , such that , define the quantities
:
and
:
then one has the simple relation
:
expressing the symmetry.
Exponents
If the Moebius transformation above moves the singular points but does not change the exponents,
the following transformation does not move the singular points but changes the exponents:
:
See also
*
Method of Frobenius
*
Monodromy
In mathematics, monodromy is the study of how objects from mathematical analysis, algebraic topology, algebraic geometry and differential geometry behave as they "run round" a singularity. As the name implies, the fundamental meaning of '' ...
Notes
References
* Milton Abramowitz and Irene A. Stegun, eds., ''
Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables'' (Dover: New York, 1972)
*
Chapter 15Hypergeometric Functions
*
Riemann's Differential Equation
{{Bernhard Riemann
Hypergeometric functions
Ordinary differential equations
Bernhard Riemann