[Octave-patch-tracker] [patch #8644] Add dirac and heaviside functions

[Colin Macdonald]

Follow-up Comment #18, patch #8644 (project octave):

@Carne, we may need to review the various special functions that have been
moved from this pkg into core...  I'm no expert on special functions but
Octave can be much better than this!

Here's a summary of where I'm at:

lambertw: good, based on the definitive paper.  I did some minor fixes for
inf/nan inputs.

laguerre: was numerically unstable, essentially new implementation in
Symbolic.

zeta: embarrassingly bad.  I found errors of at least 0.03 along the critical
line.  I can't possibly merge this!

sinint: also bad, see https://github.com/cbm755/octsympy/issues/416

cosint: shares implementation (and inaccuracies #47738) with expint

Why are we rolling our own for these things?  SciPy uses the "cephes" and
"specfun" Fortran libraries, maybe others...

- - - - - -

laplacian: I disagree: this does not belong in Symbolic: its a finite
difference discretization of the Laplacian: like 'del2' in core.  Looks
useful.  Also part of "BLOPEX".  (Its true that Symbolic does have a
'@sym/laplacian')

- - - - - -

multinom, multinom_coeff, multinom_exp: haven't looked into these yet.

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