On Thu, Nov 14, 2024 at 9:39 AM Peder Jørgensgaard Olesen via petsc-users <
petsc-users@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.
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