Re: It is very difficult for Octave to calculate the inverse Laplace transform of some functions

[Colin Macdonald]

On 2020-01-06 12:28 a.m., billyandriam wrote:
> I had no intention to torture test Gnu Octave. I had this  Exercise from my
> Circuit Analysis Textbook
> <https://electronics.stackexchange.com/questions/474650/solving-a-linear-system-of-equations-from-a-mesh-analysis-in-the-s-domain/>
> which I was trying to solve. It is about circuit Analysis and one has to
> find Mesh currents from a planar circuit.
> In the last step, one has to find the inverse Laplace transform of the two
> functions.
> I only found out that Gnu Octave takes a huge amount of time to calculate
> the inverse Laplace transform of some functions. Yesterday night, I was
> going to bed and started the calculation only to find out in the morning
> that it was not yet done calculating.
>
> Here is a screenshot:
> <https://octave.1599824.n4.nabble.com/file/t373596/Annotation_2020-01-06_110927.png>
>
> I got the job done using Wolfram Alpha but, that leaves me a big concern. I
> don't know if I can rely on Octave's symbolic calculation abilities if I
> happen to encounter a comparable question in my exam and if Octave stalls.

Octave's Symbolic package relies on Sympy.  While it can do transforms, 
its perhaps not its strongest feature.

Some things you could try:

1.  Make sure you're running a recent version of Sympy (1.4 or 1.5)

2.  Try to isolate the simplest problem for which it fails, then you or 
I can report it upstream at https://github.com/sympy/sympy/issues
(For example, there are very large integers in the screenshot you 
provided: does it also fail for rational expressions of quadrics with 
smaller coefficients?)

best,
Colin