[petsc-dev] TS Terminology

[Zhang, Hong]

> On Oct 20, 2017, at 3:15 PM, Jed Brown <jed at jedbrown.org> wrote:
> 
> "Zhang, Hong" <hongzhang at anl.gov> writes:
> 
>> Another confusion which is not related to this topic is the usage of the word "DAE".
>> I disagree with the statement "In general, this (the general form) is a differential algebraic equation (DAE)" on page 141 of the manual.
>> The word "DAE" has been abused in the comments of many TS functions (including TSComputeIFunction), where it actually should mean "DAE or ODE".
>> PETSc uses the same interfaces for both DAE and ODE, but it is wrong to consider ODE as a special case of DAE.
>> They are fundamentally different from each other and should be distinguished explicitly in the manual and the source code.
> 
> Is this not valid usage?
> 
>  "Thus, ODEs have index 0."

One can think of scalar as 0d matrix. But how often do people use 0d matrix for scalar?

When people say DAE, naturally it refers to DAE with index 1 or higher (ODE+algebraic constraints).
If PETSc TS developers are really DAE zealots, at least it should be specified that their "DAE" includes index 0 DAE.
Most parts of the manual distinguish well between ODE and DAE, shouldn't the comments in the source code be improved at least for consistency?

Hong (Mr.)

> 
>  http://www.scholarpedia.org/article/Differential-algebraic_equations