On Jun 20, 11:09 am, Renato Coutinho <[email protected]>
wrote:
> On Mon, Jun 20, 2011 at 12:01 PM, Julien Rioux <[email protected]> wrote:
> > However, this is a new behavior compared to just a couple weeks ago (I
> > could not say when exactly was the last time I used git pull). The new
> > behavior assumes that the partial derivatives of the function f(x,y)
> > are continuous everywhere. See e.g.
> >http://en.wikipedia.org/wiki/Symmetry_of_second_derivatives#Non-symmetry
> > Is this change intended?
>
> Yes, it is. See commit 53fbd255 [0] and issue 1435 [1]. We assume by
> default continuity of derivatives to the order needed, if the
> Derivatives are evaluated. It's planned to implement an assumption to
> allow the user to assume discontinuity of derivatives instead - issue
> 2471 [2].
>
> Renato
>
> [0]https://github.com/sympy/sympy/commit/53fbd25510f0294d89398c5f9d0bee2...
> [1]http://code.google.com/p/sympy/issues/detail?id=1435
> [2]http://code.google.com/p/sympy/issues/detail?id=2471
Thanks for the pointers. That's fine, except for one nitpick.
Paraphrasing, you say: what is missing is a way to assume
discontinuity of derivatives. But, in fact, what is missing is a way
to remove the assumption on continuity of derivatives, i.e., don't
assume anything about derivatives.
I think of it like commutativity of Symbol. When you create a symbol
it is assumed commutative by default. That's fine, that's the decision
of the sympy devs. Internally, Symbol('x') carries the assumption
commutative=True. You can remove this assumption by
Symbol('x',commutative=False). A similar construct for Function would
be nice.
Cheers,
Julien
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