Confluent hypergeometric function: Difference between revisions

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{{Short description|Solution of a confluent hypergeometric equation}}

In [[mathematics]], a '''confluent [[hypergeometric function]]''' is a solution of a '''confluent hypergeometric equation''', which is a degenerate form of a [[hypergeometric differential equation]] where two of the three [[regular singular point|regular singularities]] merge into an [[irregular singularity]]. (The term "[[Confluence|confluent]]" refers to the merging of singular points of families of differential equations; "confluere" is Latin for "to flow together".) There are several common standard forms of confluent hypergeometric functions:

[[File:Plot of the Kummer confluent hypergeometric function 1F1(a;b;z) with a=1 and b=2 and input z² with 1F1(1,2,z²) in the complex plane from -2-2i to 2+2i with colors created with Mathematica 13.1.svg|alt=Plot of the Kummer confluent hypergeometric function 1F1(a;b;z) with a=1 and b=2 and input z² with 1F1(1,2,z²) in the complex plane from -2-2i to 2+2i with colors created with Mathematica 13.1|thumb|Plot of the Kummer confluent hypergeometric function 1F1(a;b;z) with a=1 and b=2 and input z² with 1F1(1,2,z²) in the complex plane from -2-2i to 2+2i with colors created with Mathematica 13.1]]

In [[mathematics]], a '''confluent [[hypergeometric function]]''' is a solution of a '''confluent hypergeometric equation''', which is a degenerate form of a [[hypergeometric differential equation]] where two of the three [[regular singular point|regular singularities]] merge into an [[irregular singularity]]. (The term "[[Confluence|''confluent]]"'' refers to the merging of singular points of families of differential equations; "''confluere"'' is Latin for "to flow together".) There are several common standard forms of confluent hypergeometric functions:

* '''Kummer's (confluent hypergeometric) function''' {{math|''M''(''a'', ''b'', ''z'')}}, introduced by {{harvs|txt|authorlink=Ernst Kummer| last=Kummer |year=1837}}, is a solution to '''Kummer's differential equation'''. This is also known as the confluent hypergeometric function of the first kind. There is a different and unrelated [[Kummer's function]] bearing the same name.

* '''Tricomi's (confluent hypergeometric) function''' {{math|''U''(''a'', ''b'', ''z'')}} introduced by {{harvs|txt|authorlink=Francesco Tricomi|first=Francesco|last=Tricomi|year=1947}}, sometimes denoted by {{math|Ψ(''a''; ''b''; ''z'')}}, is another solution to Kummer's equation. This is also known as the confluent hypergeometric function of the second kind.

* '''[[Whittaker function]]s''' (for [[Edmund Taylor Whittaker]]) are solutions to '''Whittaker's equation'''.

* '''[[Coulomb wave function]]s''' are solutions to the '''Coulomb wave equation'''.

The Kummer functions, Whittaker functions, and Coulomb wave functions are essentially the same, and differ from each other only by elementary functions and change of variables.

==Kummer's equation==

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:<math>z\frac{d^2w}{dz^2} + (b-z)\frac{dw}{dz} - aw = 0,</math>

with a regular singular point at <{{math>|''z'' {{=}} 0</math>}} and an irregular singular point at <{{math>|''z'' {{=\infty</math>}} ∞}}. It has two (usually) [[linearly independent]] solutions {{math|''M''(''a'', ''b'', ''z'')}} and {{math|''U''(''a'', ''b'', ''z'')}}.

Kummer's function (of the first kind) ''{{mvar|M''}} is a [[generalized hypergeometric series]] introduced in {{harv|Kummer|1837}}, given by:

:<math>M(a,b,z)=\sum_{n=0}^\infty \frac {a^{(n)} z^n} {b^{(n)} n!}={}_1F_1(a;b;z),</math>

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: <math>a^{(n)}=a(a+1)(a+2)\cdots(a+n-1)\, ,</math>

is the [[rising factorial]]. Another common notation for this solution is {{math|Φ(''a'', ''b'', ''z'')}}. Considered as a function of ''{{mvar|a''}}, ''{{mvar|b''}}, or {{mvar|z}} with the other two held constant, this defines an [[entire function]] of ''{{mvar|a''}} or ''{{mvar|z''}}, except when {{math|''b'' {{=}} 0, −1, −2, ...}} As a function of {{mvar|b}} it is [[analytic function|analytic]] except for poles at the non-positive integers.

Some values of ''{{mvar|a''}} and {{mvar|b}} yield solutions that can be expressed in terms of other known functions. See [[#Special cases]]. When ''{{mvar|a''}} is a non-positive integer, then Kummer's function (if it is defined) is a (generalized) [[Laguerre polynomial]].

Just as the confluent differential equation is a limit of the [[hypergeometric differential equation]] as the singular point at 1 is moved towards the singular point at ∞, the confluent hypergeometric function can be given as a limit of the [[hypergeometric function]]

:<math>M(a,c,z) = \lim_{b\to\infty}{}_2F_1(a,b;c;z/b)</math>

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and many of the properties of the confluent hypergeometric function are limiting cases of properties of the hypergeometric function.

Since Kummer's equation is second order there must be another, independent, solution. The [[indicial equation]] of the method of Frobenius tells us that the lowest power of a [[power series]] solution to the Kummer equation is either 0 or 1−{{math|1 − ''b''}}. If we let {{math|''w''(''z'')}} be

:<math>w(z)=z^{1-b}v(z)</math>

then the differential equation gives

:<math>z^{2-b}\frac{d^2v}{dz^2}+2(1-b)z^{1-b}\frac{dv}{dz}-b(1-b)z^{-b}v + (b-z)\left[z^{1-b}\frac{dv}{dz}+(1-b)z^{-b}v\right] - az^{1-b}v = 0</math>

which, upon dividing out <{{math>|''z^{1-''<sup>1−''b}''</mathsup>}} and simplifying, becomes

<!--:<math>z\frac{d^2v}{dz^2}+2(1-b)\frac{dv}{dz}-b(1-b)z^{-1}v + (b-z)\left[\frac{dv}{dz}+(1-b)z^{-1}v\right] - av = 0</math>-->

:<math>z\frac{d^2v}{dz^2}+(2-b-z)\frac{dv}{dz} - (a+1-b)v = 0.</math>

This means that <{{math>|''z^{1-''^1−''b}''''M''(''a-b'' + 1 − ''b'', 2- − ''b'', ''z'')</math>}} is a solution so long as ''{{mvar|b''}} is not an integer greater than 1, just as <{{math>|''M''(''a'', ''b'', ''z'')</math>}} is a solution so long as ''{{mvar|b''}} is not an integer less than 1. We can also use the Tricomi confluent hypergeometric function {{math|''U''(''a'', ''b'', ''z'')}} introduced by {{harvs|txt|authorlink=Francesco Tricomi| first=Francesco |last=Tricomi | year=1947}}, and sometimes denoted by {{math|Ψ(''a''; ''b''; ''z'')}}. It is a combination of the above two solutions, defined by

:<math>U(a,b,z)=\frac{\Gamma(1-b)}{\Gamma(a-b+1-b)}M(a,b,z)+\frac{\Gamma(b-1)}{\Gamma(a)}z^{1-b}M(a+1-b,2-b,z).</math>

Although this expression is undefined for integer {{mvar|b}}, it has the advantage that it can be extended to any integer {{mvar|b}} by continuity. Unlike Kummer's function which is an [[entire function]] of ''{{mvar|z''}}, {{math|''U''(''z'')}} usually has a [[singularity (mathematics)|singularity]] at zero. For example, if {{math|''b'' {{=}} 0}} and {{math|''a''≠0 ≠ 0}} then <{{math>\Gamma|Γ(''a''+1)''U''(''a'', ''b'',c ''z'')- − 1</math>}} is asymptotic to <{{math>|''az\'' ln z</math> as ''z''}} as {{mvar|z}} goes to zero. But see [[#Special cases]] for some examples where it is an entire function (polynomial).

Note that the solution {{math|''z''^1−''b''''U''(''a'' + 1 − ''b'', 2 − ''b'', ''z'')}} to Kummer's equation is the same as the solution {{math|''U''(''a'', ''b'', ''z'')}}, see [[#Kummer's transformation]].

For most combinations of ''a'' and ''b'', two of the three functions <math>M(a,b,z),\ U(a,b,z)</math> and <math>z^{1-b}M(a+1-b,2-b,z)</math> will be defined and independent. If <math>a=0</math>, then when <math>M(a,b,z)</math> is defined (that is, when ''b'' is not a non-positive integer) <math>U(a,b,z)=M(a,b,z)=1</math>, and when <math>a+1=b</math> and ''b'' is not an integer greater than 1 then <math>U(a,b,z)=z^{1-b}M(a+1-b,2-b,z).</math> When <math>a=0</math> one can usually use <math>z^{1-b}M(a+1-b,2-b,z)</math> as a second solution, but this is undefined if ''b'' is an integer greater than 1, and if ''b''=1 then <math>M(a,b,z),\ U(a,b,z)</math> and <math>z^{1-b}M(a+1-b,2-b,z)</math> are all the same solution, namely <math>w(z)=1</math>. But whenever <math>a=0</math> it is easy to solve the differential equation to find a non-constant solution, namely

:<math>w(z)=-\int_{-\infty}^z(-u)^{-b}e^u\mathrm{d}u.</math>

When <math>b=1</math> this is the [[exponential integral]] Ei(''z'').

For most combinations of real or complex {{mvar|a}} and {{mvar|b}}, the functions {{math|''M''(''a'', ''b'', ''z'')}} and {{math|''U''(''a'', ''b'', ''z'')}} are independent, and if {{mvar|b}} is a non-positive integer, so {{math|''M''(''a'', ''b'', ''z'')}} doesn't exist, then we may be able to use {{math|''z''^1−''b''''M''(''a''+1−''b'', 2−''b'', ''z'')}} as a second solution. But if {{mvar|a}} is a non-positive integer and {{mvar|b}} is not a non-positive integer, then {{math|''U''(''z'')}} is a multiple of {{math|''M''(''z'')}}. In that case as well, {{math|''z''^1−''b''''M''(''a''+1−''b'', 2−''b'', ''z'')}} can be used as a second solution if it exists and is different. But when {{mvar|b}} is an integer greater than 1, this solution doesn't exist, and if {{math|1=''b'' = 1}} then it exists but is a multiple of {{math|''U''(''a'', ''b'', ''z'')}} and of {{math|''M''(''a'', ''b'', ''z'')}} In those cases a second solution exists of the following form and is valid for any real or complex {{mvar|a}} and any positive integer {{mvar|b}} except when {{mvar|a}} is a positive integer less than {{mvar|b}}:

Similarly, when <math>a+1-b=0</math> we have, in addition to

:<math>UM(b-1a,b,z)=\ln z+z^{1-b},\sum_{k=0}^\infty C_kz^k</math>

When ''a'' = 0 we can alternatively use:

the solution

:<math>w(z)=z^{1-b}\int_{-\infty}^z(-u)^{b-2b}e^u\mathrm{d}u.</math>

When <{{math>|''b'' {{=}} 1</math>}} this is the [[exponential integral]] Ei{{math|''E''₁(''z−z'')}}.

Thus we have two independent solutions for any combination of ''a'' and ''b''.

A similar problem occurs when {{math|''a''−''b''}} is a negative integer and {{mvar|b}} is an integer less than 1. In this case {{math|''M''(''a'', ''b'', ''z'')}} doesn't exist, and {{math|''U''(''a'', ''b'', ''z'')}} is a multiple of {{math|''z''^1−''b''''M''(''a''+1−''b'', 2−''b'', ''z'').}} A second solution is then of the form:

:<math>z^{1-b}M(a+1-b,2-b,z)\ln z+\sum_{k=0}^\infty C_kz^k</math>

===Other equations===

Confluent Hypergeometric Functions can be used to solve the Extended Confluent Hypergeometric Equation whose general form is given as:

:<math>z\frac{d^2w}{dz^2} +(b-z)\frac{dw}{dz} -\left(\sum_{m=0}^M a_m z^m\right)w = 0</math> <ref>{{cite journal|last1=Campos|first1=LMBCL.M.B.C.|title=On Some Solutions of the Extended Confluent Hypergeometric Differential Equation|journal=Journal of Computational and Applied Mathematics|dateyear=2001|volume=Elsevier137|number=1|doi=10.1016/s0377-0427(00)00706-8|pages=177–200|bibcode=2001JCoAM.137..177C |mr=1865885}}</ref>

{NBNote that for {{math|''M'' {{=}} 0}} (or when the summation involves just one term), it reduces to the conventional Confluent Hypergeometric Equation}.

Thus Confluent Hypergeometric Functions can be used to solve "most" second-order ordinary differential equations whose variable coefficients are all linear functions of {{mvar|z}};, because they can be transformed to the Extended Confluent Hypergeometric Equation. Consider the equation:

:<math>(A+Bz)\frac{d^2w}{dz^2} + (C+Dz)\frac{dw}{dz} +(E+Fz)w = 0</math>

First we move the [[regular singular point]] to {{math|0}} by using the substitution of {{math|''A'' + ''Bz'' ↦ ''z''}}, which converts the equation to:

:<math>z\frac{d^2w}{dz^2} + (C+Dz)\frac{dw}{dz} +(E+Fz)w = 0</math>

Line 73 ⟶ 78:

:<math> z \mapsto \frac{1}{\sqrt{D^2-4F}} z</math>

and multiply the equation by the same factor, we getobtaining:

:<math>z\frac{d^2w}{dz^2}+\left(C+\frac{D}{\sqrt{D^2-4F}}z\right)\frac{dw}{dz}+\left(\frac{E}{\sqrt{D^2-4F}}+\frac{F}{D^2-4F}z\right)w=0</math>

Line 85 ⟶ 90:

:<math>a=\left (1+ \frac{D}{\sqrt{D^2-4F}} \right)\frac{C}{2}-\frac{E}{\sqrt{D^2-4F}}, \qquad b = C.</math>

Note that the square root may give an imaginary (or complex) number. If it is zero, another solution must be used, namely

:<math>\exp \left(-\tfrac{1}{2} Dz \right )w(z),</math>

Line 93 ⟶ 98:

:<math>zw''(z)+Cw'(z)+\left(E-\tfrac{1}{2}CD \right)w(z)=0.</math>

As noted lower downbelow, even the [[Bessel equation]] can be solved using confluent hypergeometric functions.

==Integral representations==

If {{math|Re ''b'' > Re ''a'' > 0}}, {{math|''M''(''a'', ''b'', ''z'')}} can be represented as an integral

:<math>M(a,b,z)= \frac{\Gamma(b)}{\Gamma(a)\Gamma(b-a)}\int_0^1 e^{zu}u^{a-1}(1-u)^{b-a-1}\,du.</math>

thus <{{math>|''M''(''a'', ''a''+''b'', ''it'')</math>}} is the [[characteristic function (probability)|characteristic function]] of the [[beta distribution]]. For ''{{mvar|a''}} with positive real part {{mvar|U}} can be obtained by the [[Laplace transform|Laplace integral]]

:<math>U(a,b,z) = \frac{1}{\Gamma(a)}\int_0^\infty e^{-zt}t^{a-1}(1+t)^{b-a-1}\,dt, \quad (\operatorname{Re}\ a>0) </math>

Line 117 ⟶ 122:

:<math>U(a,b,x)\sim x^{-a} \, _2F_0\left(a,a-b+1;\, ;-\frac 1 x\right),</math>

where <math>_2F_0(\cdot, \cdot; ;-1/x)</math> is a [[generalized hypergeometric series]] (with 1 as leading term), which generally converges nowhere, but exists as a [[formal power series]] in {{math|1/''x''}}. This [[asymptotic expansion]] is also valid for complex {{mvar|z}} instead of real ''{{mvar|x''}}, with <{{math>|\{{mabs|arg ''z|''}} <\tfrac 3 ''π''/2 \pi.</math>}}

The asymptotic behavior of Kummer's solution for large {{math|{{mabs|''z''|}}}} is:

:<math>M(a,b,z)\sim\Gamma(b)\left(\frac{e^zz^{a-b}}{\Gamma(a)}+\frac{(-z)^{-a}}{\Gamma(b-a)}\right)</math>

The powers of {{mvar|z}} are taken using <{{math>-\tfrac 3 |−3''π''/2\pi <\ arg ''z\le\tfrac'' 1≤ 2\pi<''π''/math>2}}.<ref>This is derived from Abramowitz and Stegun (see reference below), [http://people.math.sfu.ca/~cbm/aands/page_508.htm page 508]., They givewhere a full asymptotic series is given. They switch the sign of the exponent in {{math|exp(''iπa'')}} in the right half-plane but this is unimportantimmaterial, becauseas the term is negligible there or else ''{{mvar|a''}} is an integer and the sign doesn't matter.</ref> The first term is not needed when {{math|Γ(''b'' − ''a'')}} is finite, (that is, when {{math|''b'' − ''a''}} is not a non-positive integer) and the real part of {{mvar|z}} goes to negative infinity, whereas the second term is not needed when {{math|Γ(''a'')}} is finite, (that is, when ''{{mvar|a''}} is a not a non-positive integer) and the real part of {{mvar|z}} goes to positive infinity.

There is always some solution to Kummer's equation asymptotic to <{{math>|''e^zz^{^zz''<sup>''a-''−''b}''</mathsup>}} as {{math|''z'' → −∞}}. Usually this will be a combination of both {{math|''M''(''a'', ''b'', ''z'')}} and {{math|''U''(''a'', ''b'', ''z'')}} but can also be expressed as <{{math>|''e^^z'' (-1−1)^{^''a''-''b}'' ''U''(''b-'' − ''a'', ''b'',- −''z'')</math>}}.

==Relations==

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===Contiguous relations===

Given {{math|''M''(''a'', ''b'', ''z'')}}, the four functions {{math|''M''(''a'' ± 1, ''b'', ''z''), ''M''(''a'', ''b'' ± 1, ''z'')}} are called contiguous to {{math|''M''(''a'', ''b'', ''z'')}}. The function {{math|''M''(''a'', ''b'', ''z'')}} can be written as a linear combination of any two of its contiguous functions, with rational coefficients in terms of {{mvar|a, b}}, and {{mvar|z}}. This gives {{math|1= ({{su|lh=0.8em|p=4|b=2}}) = 6}} relations, given by identifying any two lines on the right hand side of

:<math>\begin{align}

Line 141 ⟶ 146:

\end{align}</math>

In the notation above, {{math|1=''M'' = ''M''(''a'', ''b'', ''z'')}}, {{math|1= ''M''(''a''+) = ''M''(''a'' + 1, ''b'', ''z'')}}, and so on.

Repeatedly applying these relations gives a linear relation between any three functions of the form {{math|''M''(''a'' + ''m'', ''b'' + ''n'', ''z'')}} (and their higher derivatives), where ''{{mvar|m''}}, ''{{mvar|n''}} are integers.

There are similar relations for ''{{mvar|U''}}.

===Kummer's transformation===

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In terms of [[Laguerre polynomials]], Kummer's functions have several expansions, for example

:<math>M\left(a,b,\frac{x y}{x-1}\right) = (1-x)^a \cdot \sum_n\frac{a^{(n)}}{b^{(n)}}L_n^{(b-1)}(y)x^n</math> {{harv|ErdelyiErdélyi|Magnus|Oberhettinger|Tricomi|1953|loc=6.12}}

or

:<math>M\left( a,\, b,\, z \right) = \frac{\Gamma\left( 1 - a \right) \cdot \Gamma\left( b \right)}{\Gamma\left( b - a \right)} \cdot L_{-a}^{(b - 1)}\left( z \right)</math>[https://functions.wolfram.com/HypergeometricFunctions/Hypergeometric1F1/27/01/0001/]

==Special cases==

Functions that can be expressed as special cases of the confluent hypergeometric function include:

*Some [[elementary function]]s (where the left-hand side is not defined when {{mvar|b}} is a non-positive integer, but the right-hand side is still a solution of the corresponding Kummer equation):

::<math>M(0,b,z)=1</math>

::<math>U(0,c,z)=1</math>

::<math>M(b,b,z)=e^z</math>

::<math>U(a,a,z)=e^z\int_z^\infty u^{-a}e^{-u}du</math> (a polynomial if ''{{mvar|a''}} is a non-positive integer)

::<math>\frac{U(1,b,z)}{\Gamma(b-1)}+\frac{M(1,b,z)}{\Gamma(b)}=z^{1-b}e^z</math>

::<math>M(n,b,z)</math> for non-positive integer ''{{mvar|n''}} is a [[generalized Laguerre polynomial]].

::<math>U(n,c,z)</math> for non-positive integer {{mvar|n}} is a multiple of a generalized Laguerre polynomial, equal to <math>\tfrac{\Gamma(1-c)}{\Gamma(n+1-c)}M(n,c,z)</math> when the latter exists.

::<math>U(c-n,c,z)</math> when {{mvar|n}} is a positive integer is a closed form with powers of {{mvar|z}}, equal to <math>\tfrac{\Gamma(c-1)}{\Gamma(c-n)}z^{1-c}M(1-n,2-c,z)</math> when the latter exists.

::<math>U(a,a+1,z)= z^{-a}</math>

::<math>U(-n,-2n,z)</math> for non-negative integer ''{{mvar|n''}} is a Bessel polynomial (see lower down).

::<math>M(1,2,z)=(e^z-1)/z,\ \ M(1,3,z)=2!(e^z-1-z)/z^2</math> etc.

::<math>M(n,b,z)</math> for non-positive integer ''n'' is a [[generalized Laguerre polynomial]].

::Using the contiguous relation <math>aM(a+)=(a+z)M+z(a-b)M(b+)/b</math> we get, for example, <math>M(2,1,z)=(1+z)e^z.</math>

*[[Bateman's function]]

*[[Bessel function]]s and many related functions such as [[Airy function]]s, [[Kelvin function]]s, [[Hankel function]]s. For example, in the special case {{math|''b'' {{=}} 2''a''}} the function reduces to a [[Bessel function]]:

::<math>{}_1F_1(a,2a,x)= e^{\frac x /2}\, {}_0F_1 \left(; a+\tfrac{1}{2}; \tfrac{x^2}{16} \right) = e^{\frac{x}{/2}} \left(\tfrac{x}{4}\right)^{\tfrac{1}{/2}-a}\Gamma\left(a+\tfrac{1}{2}\right)I_{a-\frac{1}{/2}}\left(\tfrac{x}{2}\right).</math>

:This identity is sometimes also referred to as [[Ernst Kummer|Kummer's]] second transformation. Similarly

::<math>U(a,2a,x)= \frac{e^\frac {x /2}}{\sqrt \pi} x^{\tfrac 1 /2 -a} K_{a-\tfrac 1 /2} \left(\tfrac x /2 \right),</math>

:When {{mvar|a}} is a non-positive integer, this equals <{{math>|2^{-^{−''a}\theta_{-''}''θ''_{−''a}\left''}(\tfrac ''x ''/2 \right)</math>}} where {{mvar|θ}} is a [[Bessel polynomial]].

* The [[error function]] can be expressed as

::<math>\mathrm{erf}(x)= \frac{2}{\sqrt{\pi}}\int_0^x e^{-t^2} dt= \frac{2x}{\sqrt{\pi}}\ {}_1F_1\left(\tfrac{1}{2},\tfrac{3}{2},-x^2\right).</math>

Line 194 ⟶ 205:

*[[Poisson–Charlier function]]

*[[Toronto function]]s

*[[Whittaker function]]s {{math|''M_κ,μ''(''z''), ''W_κ,μ''(''z'')}} are solutions of [[Whittaker's equation]] that can be expressed in terms of Kummer functions ''{{mvar|M''}} and ''{{mvar|U''}} by

::<math>M_{\kappa,\mu}(z) = e^{-\tfrac{z}{2}} z^{\mu+\tfrac{1}{2}}M\left(\mu-\kappa+\tfrac{1}{2}, 1+2\mu; z\right)</math>

::<math>W_{\kappa,\mu}(z) = e^{-\tfrac{z}{2}} z^{\mu+\tfrac{1}{2}}U\left(\mu-\kappa+\tfrac{1}{2}, 1+2\mu; z\right)</math>

* The general {{mvar|p}}-th raw moment ({{mvar|p}} not necessarily an integer) can be expressed as<ref>{{Cite web|title=Aspects of Multivariate Statistical Theory {{!}} Wiley|url=https://www.wiley.com/en-us/Aspects+of+Multivariate+Statistical+Theory-p-9780471769859|access-date=2021-01-23|website=Wiley.com|language=en-us}}</ref>

:: <math>\begin{align}

\operatorname{E} \left[\left|N\left(\mu, \sigma^2 \right)\right|^p \right] &= \frac{\left(2 \sigma^2\right)^{\frac{p}{/2}} \Gamma\left(\tfrac{1+p}{2}\right)}{\sqrt \pi} \ {}_1F_1\left(-\tfrac p 2, \tfrac 1 2, -\tfrac{\mu^2}{2 \sigma^2}\right)\\

\operatorname{E} \left[N \left(\mu, \sigma^2 \right)^p \right] &= \left (-2 \sigma^2\right)^\frac {p /2} U\left(-\tfrac p 2, \tfrac 1 2, -\tfrac{\mu^2}{2 \sigma^2} \right)

\end{align}</math>

:In the second formula the function's second [[branch cut]] can be chosen by multiplying with <{{math>|(-1−1)^<sup>''p''</mathsup>}}.

==Application to continued fractions==

By applying a limiting argument to [[Gauss's continued fraction]] it can be shown that<ref>{{cite journal|first1=Evelyn|last1=Frank | year=1956|title=A new class of continued fraction expansions for the ratios of hypergeometric functions| journal=Trans. Am. Math. Soc.|volume=81|number=2|pages=453–476|mr= 0076937|jstor=1992927|doi=10.1090/S0002-9947-1956-0076937-0}}</ref>

:<math>\frac{M(a+1,b+1,z)}{M(a,b,z)} = \cfrac{1}{1 - \cfrac{{\displaystyle\frac{b-a}{b(b+1)}z}}

Line 214 ⟶ 225:

</math>

and that this continued fraction converges uniformly to a [[meromorphic function]] of {{mvar|z}} in every bounded ___domain that does not include a pole.

==See also==

* [[Composite Bézier curve]]

==Notes==

{{Reflist}}

<references/>

==References==

* {{AS ref |13|504}}

* {{springereom|idtitle=c/c024700Confluent hypergeometric function|first=E.A. |last=Chistova}}

* {{dlmf|first=Adri B. Olde|last= Daalhuis|id=13}}

* {{cite book | last1= Erdélyi | first1= Arthur | author1-link= Arthur Erdélyi | last2= Magnus | first2= Wilhelm | author2-link= Wilhelm Magnus | last3= Oberhettinger | first3= Fritz | lastauthorampname-list-style= yesamp | last4= Tricomi | first4= Francesco G. | title= Higher transcendental functions. Vol. I | ___location= New York–Toronto–London | publisher= McGraw–Hill Book Company, Inc. | year= 1953 | mr= 0058756 | ref= harv}}

* {{cite journal | last= Kummer | first= Ernst Eduard | authorlinkauthor-link= Ernst Eduard Kummer | title= De integralibus quibusdam definitis et seriebus infinitis | language= Latinla | url= http://resolver.sub.uni-goettingen.de/purl?GDZPPN002141329 | format= | journal= [[Journal für die reine und angewandte Mathematik]] | year= 1837 | volume= 1837 | issue= 17 | pages= 228–242 | issn= 0075-4102 | ref= harv | doi=10.1515/crll.1837.17.228| s2cid= 121351583 }}

* {{cite book | last= Slater | first= Lucy Joan | authorlinkauthor-link= Lucy Joan Slater | title= Confluent hypergeometric functions | url= https://archive.org/details/confluenthyperge0000slat | url-access= registration | ___location= Cambridge, UK | publisher= Cambridge University Press | year= 1960 | mr= 0107026 | ref= harv}}

* {{cite journal | last= Tricomi | first= Francesco G. | authorlinkauthor-link= Francesco Giacomo Tricomi | title= Sulle funzioni ipergeometriche confluenti | language= Italianit | journal= Annali di Matematica Pura ed Applicata. Serie|series=Series Quarta4 | year= 1947 | volume= 26 | pages= 141–175 | issn= 0003-4622 | mr= 0029451 | ref= harv | doi=10.1007/bf02415375| s2cid= 119860549 | doi-access= free }}

* {{cite book | last= Tricomi | first= Francesco G. | title= Funzioni ipergeometriche confluenti | language= Italianit | ___location= Rome | publisher= Edizioni cremonese | year= 1954 | series= Consiglio Nazionale Delle Ricerche Monografie Matematiche | volume= 1 | isbn= 978-88-7083-449-9 | mr= 0076936 | ref=harv}}

* {{cite book | last1=Oldham | first1=K.B. | last2=Myland | first2=J. | last3=Spanier | first3=J. | title=An Atlas of Functions: with Equator, the Atlas Function Calculator | publisher=Springer New York | year=2010 | isbn=978-0-387-48807-3 | url=https://books.google.com/books?id=UrSnNeJW10YC&pg=PA75 | access-date=2017-08-23}}

==External links==

Line 233 ⟶ 247:

* [http://functions.wolfram.com/HypergeometricFunctions/Hypergeometric1F1/ Kummer hypergeometric function] on the Wolfram Functions site

* [http://functions.wolfram.com/HypergeometricFunctions/HypergeometricU/ Tricomi hypergeometric function] on the Wolfram Functions site

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[[Category:Hypergeometric functions]]

[[Category:Special hypergeometric functions]]

[[Category:Special functions]]