gamma

Gamma function

Use only in the MuPAD Notebook Interface.

This functionality does not run in MATLAB.

Syntax

gamma(x)
gamma(iv)

Description

gamma(x) represents the gamma function .

The gamma function is defined for all complex arguments apart from the singular points 0, - 1, - 2, ….

The gamma function is related to the factorial function: gamma(x)=fact(x-1)=(x-1)! for all positive integers x.

If x is a floating-point value, then gamma returns a floating-point value. If x is a floating-point interval, a floating-point interval is returned. If x is a positive integer not larger than the value given by Pref::autoExpansionLimit(), then an integer is returned. (Use expand(gamma(x)) to get an integer value for larger integers x.) If x is a rational number of domain type DOM_RAT not larger than the value given by Pref::autoExpansionLimit(), then the functional relation Γ(x + 1) = xΓ(x) is applied to "normalize" the result. (Again, use expand(gamma(x)) to enforce this normalization for larger rational numbers x.) The functional relation

is applied if is a rational number of domain type DOM_RAT that is an integer multiple of or . The call gamma(1/2) yields sqrt(PI); gamma(infinity) yields infinity.

For all other arguments, a symbolic function call is returned.

The float attribute of gamma is a kernel function, i.e., floating-point evaluation is fast.

The expand attribute uses the functional equation Γ(x + 1) = xΓ(x), the reflection formula

,

and the Gauß multiplication formula for Γ(kx) when k is a positive integer, to rewrite gamma(x). Cf. Example 3. For numerical x, the functional equation is used to shift the argument to the range 0 < x < 1.

The functional equations for gamma lead to various identities for lngamma which can be applied via expand. Cf. Example 3.

The logarithmic derivative of gamma is implemented by the digamma function psi.

Environment Interactions

When called with a floating-point argument, the functions are sensitive to the environment variable DIGITS which determines the numerical working precision.

Examples

Example 1

We demonstrate some calls with exact and symbolic input data:

gamma(15),
gamma(3/2),
gamma(-3/2),
gamma(sqrt(2)),
gamma(x + 1)

lngamma(15),
lngamma(3/2),
lngamma(-3/2),
lngamma(sqrt(2)),
lngamma(x + 1)

Floating point values are computed for floating-point arguments:

gamma(11.5),
gamma(2.0 + 10.0*I)

lngamma(11.5),
lngamma(2.0 + 10.0*I)

Example 2

gamma and lngamma are singular for nonpositive integers:

gamma(0)
Error: Singularity. [gamma]
lngamma(-2)
Error: Singularity. [lngamma]

Example 3

The functions diff, expand, float, limit, and series handle expressions involving gamma and lngamma:

diff(gamma(x^2 + 1), x),
diff(lngamma(x^2 + 1), x)

float(ln(3 + gamma(sqrt(PI)))),
float(ln(3 + lngamma(sqrt(PI))))

expand(gamma(x + 2)),
expand(lngamma(x + 2))

expand(gamma(2*x)),
expand(lngamma(2*x))

expand(gamma(2*x - 1)),
expand(lngamma(2*x - 1))

limit(1/gamma(x), x = infinity),
limit(1/lngamma(x), x = infinity)

limit(gamma(x - 4)/gamma(x - 10), x = 0),
limit(lngamma(x - 4) - lngamma(x - 10), x = 0)

series(gamma(x), x = 0, 3),
series(lngamma(x), x = 0, 3)

The Stirling formula is obtained as an asymptotic series:

series(gamma(x), x = infinity, 4),
series(lngamma(x), x = infinity, 4)

Example 4

The logarithm function ln has a branch cut along the negative real semi axis, where the values jump by 2 π i when crossing the cut. In the following plot of the imaginary part of the logarithm of the gamma function the lines in the complex z plane with and are clearly visible as discontinuities:

plotfunc3d(Im(ln(gamma(x + I*y))), x = -10 .. 10, y = -10 .. 10, 
           Submesh = [2, 2], CameraDirection = [0, -1, 1000]):

The function lngamma(z), however, adds suitable integer multiples of 2 π i to ln(gamma(z)) making the function analytic throughout the complex plane with a branch cut along the negative real semi axis:

plotfunc3d(Im(lngamma(x + I*y)), x = -10 .. 10, y = -10 .. 10, 
           Submesh = [2, 2], CameraDirection = [0, -1, 1000]):

Return Values

Arithmetical expression or a floating-point interval.

Overloaded By

x

See Also

MuPAD Functions

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