It has been a while since I looked at this, but I went ahead and implemented what Ronan suggested. The branch is on my git hub: http://github.com/hazelnusse/sympy/tree/expand_neg_int_powers
It passes all tests on my machine. Let me know what you think. ~Luke On Aug 9, 7:24 pm, "Aaron S. Meurer" <asmeu...@gmail.com> wrote: > On Aug 9, 2009, at 8:07 PM, Ronan Lamy wrote: > > > > > > > Le dimanche 09 août 2009 à 16:34 -0700, Luke a écrit : > > >>> * This line suggests that it would be helpful to create an 'inverse' > >>> property - less work is needed to create this inverse than for a > >>> generic > >>> Pow instance. > > >> Could you elaborate on what you mean here? Do you mean an inverse > >> property to Pow instances which have a integer exponent? Perhaps > >> 'reciprocal' instead of 'inverse'? > > Yes, I guess 'reciprocal' is better. I mean a class property defined > > for > > all instances that would look like: > > @property > > def reciprocal(self): > > if self.exp == -1: > > return self.base > > else: > > return Basic.__new__(self.__class__, self.base, -self.exp) > > >>> *I don't see the point of the third test: it's only testing that (x > >>> +y)**2*(x+y)**-4 == (x+y)**-2 which isn't related to expand. > > >> Yes, you are right, it isn't testing expand but I guess it probably > >> doesn't hurt to keep it in either. > > No, it doesn't hurt much, but each unneeded test makes running the > > full > > test suite a tiny bit more painful. > > Maybe we should replace it with something that doesn't have > automatically combining exponents, such as: > > (x + y)**(2 + x)*(x + y)**(-x - 4) > > Then it at least checks to see if multinomial is being called after > power_exp. Actually this isn't guarantied, because expand hints are > applied in arbitrary order. See the expand docstring. So I am not > sure if this will even work, and if so if it will on every machine > (with different hash values and thus different orders in the hints > dict). I forget how the expand functions are structured if > expand_power_exp will call expand_multinomial afterwords or not (with > the deep option set to True). > > Otherwise I agree. It is highly unlikely that we are ever going to > change the fundamental automatic simplification of (x + y)**2*(x + > y)**(-4) to (x + y)**(-2), which is the same sort of thing as x*x == > x**2. And if it is broken, there are plenty of other tests that will > break instead (such as the specifically structured > test_Mul_doesnt_expand_exp() and others in test_arit.py. > > Aaron Meurer > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "sympy-patches" group. To post to this group, send email to sympy-patches@googlegroups.com To unsubscribe from this group, send email to sympy-patches+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sympy-patches?hl=en -~----------~----~----~----~------~----~------~--~---
