Hi Ondrej, Thanks for the help. I dug a little deeper and found that the problem was that both the LU and GaussElimination solver could not detect zeros in the pivot elements. The problems disappeared if the pivot elements were simplified first. For the GE solver I could just set the simplifed argument to True in the call to the "rref" method.
But there are probably cases where a simplify is not enough. I do not know if there is a more robust way of detecting if an expression is zero. For the circuit solver I will use the ADJ version from now on. I will keep you informed when it is ready for release. Best regards, Henrik 2008/10/17 Ondrej Certik <[EMAIL PROTECTED]> > Hi Henrik! > > On Fri, Oct 17, 2008 at 5:37 PM, henjo <[EMAIL PROTECTED]> wrote: > > > > Hi, > > > > I have been using Sympy for quite some time now in a symbolic electric > > circuit solver (which I will release soon). > > > > To the problem, I have a matrix that gives an incorrect inverse: > > > >>>> A = Matrix([[x+y, -x, 0], [-x-y, x, 1], [0,1,0]]) > >>>> Ainv = A.inv() > >>>> Ainv.simplify() > >>>> print Ainv > > ⎡0 0 0⎤ > > ⎢ ⎥ > > ⎢0 0 1⎥ > > ⎢ ⎥ > > ⎣0 1 0⎦ > >>>> print A*Ainv > > ⎡ 0 0 0⎤ > > ⎢ ⎥ > > ⎢ 0 1 0⎥ > > ⎢ ⎥ > > ⎣-x - y x 1⎦ > > Thanks for the bug report. I made it: > > http://code.google.com/p/sympy/issues/detail?id=1168 > > In the meantime, use a different method for calculating the inverse, e.g.: > > In [1]: A = Matrix([[x+y, -x, 0], [-x-y, x, 1], [0,1,0]]) > > In [2]: Ainv = A.inv("ADJ") > > In [3]: Ainv > Out[3]: > ⎡ -1 -x ⎤ > ⎢────── 0 ──────⎥ > ⎢-x - y -x - y⎥ > ⎢ ⎥ > ⎢ 0 0 1 ⎥ > ⎢ ⎥ > ⎣ 1 1 0 ⎦ > > In [4]: Ainv*A > Out[4]: > ⎡ x y ⎤ > ⎢- ────── - ────── 0 0⎥ > ⎢ -x - y -x - y ⎥ > ⎢ ⎥ > ⎢ 0 1 0⎥ > ⎢ ⎥ > ⎣ 0 0 1⎦ > > In [6]: (Ainv*A).applyfunc(simplify) > Out[6]: > ⎡1 0 0⎤ > ⎢ ⎥ > ⎢0 1 0⎥ > ⎢ ⎥ > ⎣0 0 1⎦ > > > If you have time, it'd be cool if you could figure out where the > problem is exactly, see the issue. Looking forward for your symbolic > electric > circuit solver. When you release it, please send also an email to this > list, I am sure people will like to play with it. Well, to speak for > myself, I will. :) > > Ondrej > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to sympy@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sympy?hl=en -~----------~----~----~----~------~----~------~--~---
