The bestknown properties and formulas for differentiated gamma functions
Real values for real arguments
For real values of the argument and nonnegative integer , the differentiated gamma functions , , , and are real (or infinity). The function is real (or infinity) for real values of argument and integer .
Simple values at zero
The differentiated gamma functions , , , and have simple values for zero arguments:
Values at fixed points
The differentiated gamma functions , , , and with rational arguments can sometimes be evaluated through classical constants and logarithms, for example:
Specific values for specialized variables
The previous relations are particular cases of the following general formulas:
The differentiated gamma functions and with integer parameters and have the following representations:
Analyticity
The digamma function and the harmonic number are defined for all complex values of the variable . The functions and are analytical functions of and over the whole complex ‐ and ‐planes. For fixed , the generalized harmonic number is an entire function of .
Poles and essential singularities
The differentiated gamma functions and have an infinite set of singular points , where for and for . These points are the simple poles with residues . The point is the accumulation point of poles for the functions , , and (with fixed nonnegative integer ), which means that is an essential singular point.
For fixed nonnegative integer , the function has an infinite set of singular points: are the simple poles with residues ; and are the poles of order with residues .
For fixed , the function does not have poles and the function has only one singular point at , which is an essential singular point.
Branch points and branch cuts
The functions and do not have branch points and branch cuts.
For integer , the function does not have branch points and branch cuts.
For fixed noninteger , the function has two singular branch points and , and it is a single‐valued function on the ‐plane cut along the interval , where it is continuous from above:
For fixed , the functions and do not have branch points and branch cuts.
Periodicity
The differentiated gamma functions , , , and do not have periodicity.
Parity and symmetry
The differentiated gamma functions , , , and have mirror symmetry:
Series representations
The differentiated gamma functions , , and have the following series expansions near regular points:
Near singular points, the differentiated gamma functions , , , and can be expanded through the following series:
Here is the Euler gamma constant .
Except for the generalized power series, there are other types of series through which differentiated gamma functions , , , and can be represented, for example:
Asymptotic series expansions
The asymptotic behavior of the differentiated gamma functions , , , and can be described by the following formulas (only the main terms of the asymptotic expansions are given):
Integral representations
The differentiated gamma functions , , , and can also be represented through the following equivalent integrals:
Transformations
The following formulas describe some of the transformations that change the differentiated gamma functions into themselves:
Transformations with arguments that are integer multiples take the following forms:
The following transformations represent summation theorems:
Identities
The differentiated gamma functions , , , and satisfy the following recurrence identities:
The previous formulas can be generalized to the following recurrence identities with a jump of length :
Representations of derivatives
The derivatives of the differentiated gamma functions , , , and have rather simple representations:
The corresponding symbolic order derivatives of all the differentiated gamma functions , , , and can be expressed by the following formulas:
