Good question. Looking at the commit logs from 2014 I see move tao vector operations over to Vec directory, fix a couple names and calling sequences
So previously, the function was problem-specific for Tao (which did not support complex numbers) used inside the semi-smooth methods where roots of negative numbers should be mapped to infinity (indicating a "bad" domain point). When we merged the Tao and PETSc source code base just blindly copied over the code without realizing it was not a general-purpose VecPow() and that it did not make sense for complex numbers. I should rework it for general use without breaking the use in Tao. Thanks for pointing out the problem. Barry > On 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. > > Best, > Peder
