No, the deduction of even/odd on Add isn't implemented yet. But the implication Implies(Q.odd(d + 1), Q.even(d)) is always true, so you can add it to the assumed facts unconditionally.
Aaron Meurer On Tue, May 30, 2017 at 3:43 PM, mike <[email protected]> wrote: > Thank you Aaron, your response is very helpful. > > Q.odd(d + 1) & Q.even(d) are deduced assumptions. It seems if I use the > current method, Sympy can not get the conclusion "d is even" from "if (d+1) > is odd" directly. > > > 在 2017年5月29日星期一 UTC-6下午9:59:11，Aaron Meurer写道： >> >> None in this case means that SymPy doesn't know how determine the >> fact. It looks like the sathandlers has an even/odd fact for Mul but >> not for Add >> https://github.com/sympy/sympy/blob/71eb404921a4596b9fe42a7a4a0ccfa7d63a62c0/sympy/assumptions/sathandlers.py#L339. >> If I remember correctly, it's because the corresponding fact for Add >> requires counting, and I wasn't sure how to do that efficiently >> (without adding an exponential number of clauses for large Adds). >> >> You can always tell SymPy the facts that it needs to know to deduce >> things, in this case >> >> >>> with assuming(Q.positive(c), Q.integer(c), Q.positive(d), >> >>> Q.integer(d), Q.odd(c*(d + 1)), Q.odd(d + 1) >> Q.even(d)): >> ... print(ask(Q.odd(d))) >> False >> >> here >> means "implies" (you could also use Implies(Q.odd(d + 1), >> Q.even(d))). >> >> Aaron Meurer >> >> >> On Mon, May 29, 2017 at 9:45 PM, mike <[email protected]> wrote: >> > Hi, >> > >> > I am new to learn sympy. This is a very simple question but I do not >> > know >> > how to deal with it. >> > >> > The question is: both c and d are positive integers, is c*d even if >> > c*(d+1) >> > is odd. >> > >> > my code: >> > >> > from sympy import Symbol >> > from sympy.assumptions import assuming, Q, ask >> > c = Symbol('c') >> > d = Symbol('d') >> > with assuming(Q.positive(c), Q.integer(c), Q.positive(d), Q.integer(d)): >> > with assuming(Q.odd(c*(d +1))): >> > print(ask(Q.odd(c))) >> > print(ask(Q.odd((d+1)))) >> > >> > This code works. c is odd and (d+1) is odd, but how to evaluate d is odd >> > or >> > even. >> > I run the code >> > ask(Q.odd(d)), >> > but the result is "NONE" >> > >> > Ask for help. >> > Thanks, >> > >> > -- >> > You received this message because you are subscribed to the Google >> > Groups >> > "sympy" group. >> > To unsubscribe from this group and stop receiving emails from it, send >> > an >> > email to [email protected]. >> > To post to this group, send email to [email protected]. >> > Visit this group at https://groups.google.com/group/sympy. >> > To view this discussion on the web visit >> > >> > https://groups.google.com/d/msgid/sympy/5206e651-feeb-4cd9-a4c5-ba85f17ec6f0%40googlegroups.com. >> > For more options, visit https://groups.google.com/d/optout. > > -- > You received this message because you are subscribed to the Google Groups > "sympy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > Visit this group at https://groups.google.com/group/sympy. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sympy/2d41dd13-587e-4fca-996f-e3892b9fe792%40googlegroups.com. > > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "sympy" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/sympy. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/CAKgW%3D6Ju6_044Loxh7JVi8mhV00tFZJXrdXG5AYC%2B-z5FWUiYA%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
