Hey Ondrej-- I'll keep this in mind, but I don't think I can post these particular calculations as they're proprietary...
One thing this experience suggests though is that it might speed up dsolve if the coefficients are replaced with simple symbols such as you had and then resubstituted after the solve. I don't know if that would be true in every case, or if it would prevent solutions in some cases... On Sunday, April 5, 2015 at 12:25:25 AM UTC-7, Ondřej Čertík wrote: > > On Sat, Apr 4, 2015 at 11:34 PM, G B <[email protected] <javascript:>> > wrote: > > Thanks, Ondrej. In my case the coefficients are the result of a series > of > > calculations earlier in the notebook. They are symbolic up until the > call > > to dsolve, but I am substituting values in right before the dsolve > because > > it takes several minutes to return a solution otherwise (and gives a > very > > complicated solution over a segmented domain). > > > > I let it run to completion symbolically and tried substituting into the > > result and I do get something that looks more reasonable. I can also > run it > > through lambdify and plot the result. So despite the slow computation, > at > > least it looks like I can continue what I'm doing. > > Would you mind posting your whole calculation, if it is possible? We > are developing a very fast C++ core (https://github.com/sympy/csympy) > and I am always looking for examples of computations that people found > to be slow in SymPy, like in your case. > > > > > Any idea why the huge integers though? I seem to remember somewhere > that > > Sympy will try to convert floats to rationals under certain conditions. > Is > > that what's happening? Can I turn that behavior off? > > > > Any insight into why it won't evalf or lamdify if I substitute before > dsolve > > rather than after? > > Aaron just answered these. > > Ondrej > > > > > Thanks, again! > > > > > > On Saturday, April 4, 2015 at 9:27:07 PM UTC-7, G B wrote: > >> > >> Still wrestling with dsolve... Below is a call with an arbitrary > >> differential equation. Any idea why dsolve is returning terms with > these > >> enormous integers? All of the coefficients are floats in this case. > The > >> expression is impervious to .n() (as mentioned in my earlier question). > >> Converting to a numpy function to evaluate the results works, but > throws an > >> exception when called. > >> > >> I can't seem to get past this point in the analysis. Any idea how I > can > >> get this into a form I can continue working with? > >> > >> A=symbols(r'A',cls=Function) > >> t=symbols(r't') > >> > Eq4=-123456.78*A(t)-9876.54*A(t).diff(t)-0.00032*A(t).diff(t,2)+1357908.64 > >> soln=dsolve(Eq4) > >> print(soln.n()) > >> > >> A(t) == C1*exp(125*t*(-33935533038108675 - > >> sqrt(1151618536954453541512853661417481))/274877906944) + > >> C2*exp(125*t*(-33935533038108675 + > >> sqrt(1151618536954453541512853661417481))/274877906944) + > 10.999060885923 > >> > >> > >> fn=lambdify(t,soln.rhs,'numpy') > >> > >> > >> fn(3.2) > >> > >> > >> > >> > --------------------------------------------------------------------------- > >> AttributeError Traceback (most recent call > >> last) > >> <ipython-input-42-bde0572cbbe1> in <module>() > >> ----> 1 fn(3.2) > >> > >> //anaconda/lib/python3.4/site-packages/numpy/__init__.py in > >> <lambda>(_Dummy_73) > >> > >> AttributeError: 'int' object has no attribute 'sqrt' > > > > -- > > 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] <javascript:>. > > To post to this group, send email to [email protected] > <javascript:>. > > 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/6f3572d3-07dd-40b3-a3a9-7673eddf1c87%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 http://groups.google.com/group/sympy. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/367442da-4a21-4ea7-a9f3-0688d261f1b3%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
