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Convolution of a Gaussian with a simple pole: incomplete solution

Posted 3 years ago

Mathematica 13.0 does not output the full results of the convolution of a Gaussian with a simple pole.

Integrate[Exp[- x^2 ]/( c - x), {x, -Infinity, Infinity},   PrincipalValue -> True]

gives:

    2 Sqrt[Pi] DawsonF[c] if  Re[c] > 0 && Im[c] = 0

yet this is incomplete as with Im[z] > 0 one gets a compact expression using the well-known Faddeeva function (pervasive in plasma physics among numerous other fields). The expected result is:

-I Pi w[z]

with w[z] the Faddeeva (or Kramp) function. Mathematica 13.0 boasts having all Ambarowitz and Stegun functions on board (formula 7.1.3). Unfortunately I do not see this well-known function as a Mathematica primitive. It is implemented in MATLAB or Octave, though.
Or did I miss something?

POSTED BY: Fab Nicol
3 Replies
Posted 3 years ago

The above can be simplified as follows:

PowerExpand[Log[-(1/c)] + Log[c], Assumptions -> Arg[c] > 0]

which gives:

I Pi

As:

w[z] = Exp[-z^2] (1 + I Erfi[z])

the expected result follows straightforwardly for Im[z] > 0.
Edit:
For plotting and computation, I just found a Resource function contributed by Jan Mangaldan here:
https://resources.wolframcloud.com/FunctionRepository/resources/FaddeevaW/
Being a resource function, it will not be used by the core symbolic integration engine.

POSTED BY: Fab Nicol
Posted 3 years ago
POSTED BY: Fab Nicol
Posted 3 years ago
POSTED BY: Updating Name
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