[petsc-users] VecPow clarification

[Matthew Knepley]

On Thu, Nov 14, 2024 at 9:39 AM Peder Jørgensgaard Olesen via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Given a vector containing roots of unity, v[i] = exp(i*k*x[i]) I wanted to
> compute the vector u[i]=exp(i*n*k*x[i]), for some real number n. From the
> face of it this should be easily achieved with VecPow, as u[i] = v[i]^n.
>
> That didn't work as expected, though I got around it using VecGetArray()
> and a loop with PetscPowComplex(). The source designated in the docs
> (src/vec/vec/utils/projection.c) reveals that VecPow() maps v[i] to
> PETSC_INFINITY when the PetscRealPart(v[i]) < 0, unless the power is any of
> 0, ±0.5, ±1 or ±2. Even in the simple case of a purely real vector (with
> negative entries) raised to any other integer power, the results would not
> be what one might  reasonably expect from the description of VecPow().
>
> While I do have a solution suiting my need, I'm left wondering what might
> be the rationale for VecPow working the way it does.
>

This is indeed wrong. It was coded only for real numbers. We will fix it.

  Thanks for reporting this,

     Matt


> Best,
> Peder
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!bNQOcaOJC5gkTat8nR3TNhd8LdtJY9sMS6rBMYVNwUdmQE2UkCPoXt7GmCWMleJs9EAJr_rfIaO2WqqNrQe-$  <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!bNQOcaOJC5gkTat8nR3TNhd8LdtJY9sMS6rBMYVNwUdmQE2UkCPoXt7GmCWMleJs9EAJr_rfIaO2WqfN0fHA$ >
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20241114/66846bbc/attachment-0001.html>