Hi Ondrej, Ok, thanks for letting me know. For the time being, would you agree that removing the root by exponentiation, as in *solve(y**2 >= 0*, is an acceptable workaround?
(I have started playing with sympy and I really like it, I might post more questions in the near future!) Dario On Monday, October 6, 2014 2:49:28 PM UTC+1, Ondřej Čertík wrote: > > Hi Dario, > > On Sun, Oct 5, 2014 at 6:05 AM, Dario Beraldi <[email protected] > <javascript:>> wrote: > > Hello, > > > > Apologies if I'm missing something trivial here... > > > > I have the function y= sqrt(4*x**2 - 9) for which I want to know the > real > > domain of definition. I.e. the answer should be: y is defined for x <= > -3/2 > > and x >= 3/2. > > > > If I try solve(y >= 0) I get a PolynomialError. I could do solve(y**2 >= > 0) > > to get the "right" answer but it feels like a hack and I'm wondering > whether > > there is a better strategy to solve this inequality. Any idea? > > > > I.m using sympy 0.7.5 and this below is the bit of code to reproduce my > > question above. > > > > Many thanks! > > Dario > > > > isympy > > > > x= symbols('x') > > y= sqrt(4*x**2 - 9) > > > > ## That's Ok: > > solve(y**2 >= 0) ## im(x) = 0 ∧ (re(x) ≥ 3/2 ∨ re(x) ≤ -3/2) > > > > ## Or even better: > > x= symbols('x', real= True) > > y= sqrt(4*x**2 - 9) > > solve(y**2 >= 0) ## x ≥ 3/2 ∨ x ≤ -3/2 > > > > ## This doesn't work: > > x= symbols('x') > > y= sqrt(4*x**2 - 9) > > solve(y >= 0) > > > --------------------------------------------------------------------------- > > PolynomialError Traceback (most recent call > last) > > <ipython-input-14-8865cc4d86ab> in <module>() > > ----> 1 solve(y >= 0) > > > > > /opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sympy/solvers/solvers.pyc > > > > in solve(f, *symbols, **flags) > > 686 elif isinstance(fi, (bool, C.BooleanAtom)) or > > fi.is_Relational: > > 687 return reduce_inequalities(f, > > assume=flags.get('assume'), > > --> 688 symbols=symbols) > > 689 > > 690 # Any embedded piecewise functions need to be brought > out to > > the > > > > > /opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sympy/solvers/inequalities.pyc > > > > in reduce_inequalities(inequalities, assume, symbols) > > 514 > > 515 for gen, exprs in poly_part.items(): > > --> 516 > poly_reduced.append(reduce_rational_inequalities([exprs], > > gen, assume)) > > 517 > > 518 for gen, exprs in abs_part.items(): > > > > > /opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sympy/solvers/inequalities.pyc > > > > in reduce_rational_inequalities(exprs, gen, assume, relational) > > 219 expr.together().as_numer_denom(), gen) > > 220 except PolynomialError: > > --> 221 raise PolynomialError("only polynomials and " > > 222 "rational functions are supported in this > > context") > > 223 > > > > PolynomialError: only polynomials and rational functions are supported > in > > this context > > I think you found a bug. Thanks for reporting this. I made this: > > https://github.com/sympy/sympy/issues/8206 > > (Feel free to report similar bugs directly to the bug trucker if you > want.) > > Ondrej > -- 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 http://groups.google.com/group/sympy. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/d430b751-95f4-4014-a210-2a464caa1237%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
