[petsc-dev] Meaning of PETSc matrices with zero rows but nonzero columns?

[Stefano Zampini]

> On May 31, 2020, at 1:03 AM, Jed Brown <jed at jedbrown.org> wrote:
> 
> Stefano Zampini <stefano.zampini at gmail.com> writes:
> 
>> If A is 0x8 and B is 8x5 then C is correct to be of size 0x5. The rows and columns of the resulting matrix have to follow the rules.
> 
> Right, I think if you said C is 0x0 (which seems like Richard's proposal), you'd need to relax shape compatibility logic in many places, including in ways that might produce confusing errors.
> 

Richard

In the triple matrix product case, your code will break, because the operation will no longer be associative

A 3x0, B 0x8, C 8x7 -> (ABC) is a valid 3x7 matrix (empty) 

If I understand you right, (AB)  would be  a 0x0 matrix, and it can no longer be multiplied against C

> Richard, what is the hardship in preserving the shape relations?