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Initialization: Set the display of special functions in output to typeset mathematical notation (textbook notation):
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The conditions for both the singular and the polynomial cases can also be seen from the AppellF2. For example, the fourteen polynomial cases of AppellF2 are
Likewise, the conditions for the singular cases of AppellF2 can be seen either using the FunctionAdvisor or entering AppellF2:-Singularities(), so with no arguments.
For particular values of its parameters, AppellF2 is related to the hypergeometric function. These hypergeometric cases are returned automatically. For example, for ,
To see all the hypergeometric cases, enter
Other special values of AppellF2 can be seen using FunctionAdvisor(special_values, AppellF2).
By requesting the sum form of AppellF2, besides its double power series definition, we also see the particular form the series takes when one of the summations is performed and the result expressed in terms of 2F1 hypergeometric functions:
As indicated in the formulas above, for AppellF2 (also for AppellF4), and unlike the case of AppellF1 and AppellF3, the domain of convergence with regards to the two variables and is entangled, i.e. it intrinsically depends on a combination of the two variables, so the hypergeometric coefficient in one variable in the single sum form does not extend the domain of convergence of the double sum but for particular cases, and from the formulas above one cannot conclude about the value of the function when one of or is equal to 1 unless the other one is exactly equal to 0.
AppellF2 admits identities analogous to Euler identities for the hypergeometric function. These Euler-type identities, as well as contiguity identities, are visible using the FunctionAdvisor with the option identities, or directly from the function. For example,
Among other situations, this identity is useful when the sum of the absolute values of and is larger than 1 but the same sum constructed with the arguments in the same position of AppellF2 on the right-hand side is smaller than 1. On the other hand, unlike the case of the other three Appell functions, none of the two Euler type transformations or hypergeometric special cases of AppellF2 are of help to analytically extend to the whole complex plane the AppellF2 series when either or .
A contiguity transformation for AppellF2
The contiguity transformations available in this way are
By using differential algebra techniques, the PDE system satisfied by AppellF2 can be transformed into an equivalent PDE system where one of the equations is a linear ODE in parametrized by . In the case of AppellF2 this linear ODE is of fourth order and can be computed as follows
This linear ODE has four regular singularities, one of which is depends on
You can also see a general presentation of AppellF2, organized into sections and including plots, using the FunctionAdvisor
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AppellF2
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describe
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definition
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classify function
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symmetries
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plot
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singularities
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branch points
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branch cuts
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special values
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identities
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sum form
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series
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integral form
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differentiation rule
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DE
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