[petsc-users] VecPow clarification

[Stefano Zampini]

That is a very old bug! Can you make an MR to just call PetscPowScalar in a
loop here
https://urldefense.us/v3/__https://gitlab.com/petsc/petsc/-/blob/main/src/vec/vec/utils/projection.c*L1022__;Iw!!G_uCfscf7eWS!bbSdSMnU5KpH03jHI7aV5j4WLGQ3yPvxWzR6Lwr14QLy_7EJ2MNT-qhL6J1x6z3vpF6M5GQk9lMQkMoLJ_JNSn1mqwqict4$ 
?

Thanks

Il giorno gio 14 nov 2024 alle ore 17:39 Peder Jørgensgaard Olesen via
petsc-users <petsc-users at mcs.anl.gov> ha scritto:

> 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.
>
> Best,
> Peder
>


-- 
Stefano
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