[petsc-dev] manual page for TSSetIJacobian()

[Jed Brown]

On Mon, Feb 7, 2011 at 23:35, Barry Smith <bsmith at mcs.anl.gov> wrote:

> Well what I want is why is this "the derivative we need"? There is no hint
> as to an explanation why the user providing this quantity has any meaning a
> all. You could just as easily say g_x where g(x) = f(x, a*x^3 + 94) and why
> would that be wrong, while what you write is correct?


That is the reason why I like introducing g(x) = f(x,xdot) where xdot is
approximated by the integrator as z+a*x for some a and z depending on the
integration method, step size, and past states. But perhaps the user (as
opposed to someone implementing a new TS) doesn't care at all and would be
happy to evaluate f_x + a*f_xdot without any knowledge of why this beast is
relevant.
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