I guess that's related to the work at https://github.com/sympy/sympy/pull/2988, which is for https://github.com/sympy/sympy/issues/6247.
Aaron Meurer On Sun, Mar 9, 2014 at 12:26 AM, Harsh Gupta <[email protected]> wrote: > Our constant simplifier work for only one variable. Someone might go > try to implementing it for multiple variables. Our PDE module needs > that. > > There a paper on this by Neil Soiffer called "Collapsing Constants" > that deals with such stuff. > http://www.cs.berkeley.edu/~fateman/papers/CollapsingConstants-Soiffer.pdf > > On 9 March 2014 06:22, Aaron Meurer <[email protected]> wrote: >> This is in some sense a bug. The solution is correct mathematically. >> The biggest issue with it is actually that there are four constants, >> not two. This is because the solver currently outputs four terms and >> relies on the constant simplification to reduce them to two. But the >> terms don't simplify in this case because they are so complicated. >> >> If you want something simpler, you should assume that f is real, like >> >> f = symbols('f', real=True) >> >> If you do that, you will get >> >> In [8]: print(dsolve(d2udt2, u(t))) >> u(t) == C1*sin(t*Abs(f)) + C2*cos(t*Abs(f)) >> >> if you don't like the Abs you can assume f is positive instead. >> >> Ideally, dsolve would return a solution in terms of complex >> exponentials in these sorts of cases, which would be a lot less >> complicated. Any potential GSoC students out there, especially those >> interested in the ODE module, this is a nice little project for your >> patch requirement. >> >> Aaron Meurer >> >> On Sat, Mar 8, 2014 at 2:24 PM, Filipe Pires Alvarenga Fernandes >> <[email protected]> wrote: >>> Hi, >>> I'm new to sympy and I'm trying to understand how to use dsolve. (I'm >>> creating an ipython notebook for a class.) >>> >>> I'm creating my DE like this: >>> de = Eq(u(t).diff(t, t) + 4*u(t), 0) >>> print(de) >>> >>> 4*u(t) + Derivative(u(t), t, t) == 0 >>> >>> >>> soln = dsolve(de, u(t)) >>> print(soln) >>> >>> u(t) == C1*sin(2*t) + C2*cos(2*t) >>> >>> >>> So far now everything is perfect. But if I try to change the number 4 for a >>> "generic" symbol (f**2) I do not get >>> >>> u(t) == C1*sin(f*t) + C2*cos(f*t) as I expected, insted I get a more >>> "comprehensive" solution below. >>> >>> >>> What am I doing wrong? >>> >>> >>>>>> d2udt2 = Eq(u(t).diff(t, t) - f*(-f*u(t)), 0) >>>>>> print(d2udt2) >>> f**2*u(t) + Derivative(u(t), t, t) == 0 >>> >>> u(t) == (C1*sin(t*((-re(f)**2 + im(f)**2)**2 + >>> 4*re(f)**2*im(f)**2)**(1/4)*Abs(sin(atan2(-2*re(f)*im(f), -re(f)**2 + >>> im(f)**2)/2))) + C2*cos(t*((-re(f)**2 + im(f)**2)**2 + >>> 4*re(f)**2*im(f)**2)**(1/4)*sin(atan2(-2*re(f)*im(f), -re(f)**2 + >>> im(f)**2)/2)))*exp(-t*((-re(f)**2 + im(f)**2)**2 + >>> 4*re(f)**2*im(f)**2)**(1/4)*cos(atan2(-2*re(f)*im(f), -re(f)**2 + >>> im(f)**2)/2)) + (C3*sin(t*((-re(f)**2 + im(f)**2)**2 + >>> 4*re(f)**2*im(f)**2)**(1/4)*Abs(sin(atan2(-2*re(f)*im(f), -re(f)**2 + >>> im(f)**2)/2))) + C4*cos(t*((-re(f)**2 + im(f)**2)**2 + >>> 4*re(f)**2*im(f)**2)**(1/4)*sin(atan2(-2*re(f)*im(f), -re(f)**2 + >>> im(f)**2)/2)))*exp(t*((-re(f)**2 + im(f)**2)**2 + >>> 4*re(f)**2*im(f)**2)**(1/4)*cos(atan2(-2*re(f)*im(f), -re(f)**2 + >>> im(f)**2)/2)) >>> >>> >>> Thanks, >>> >>> -Filipe >>> >>> -- >>> 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/1395f3d9-f6ea-4167-bc0c-3a9b7e1e1e5d%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/CAKgW%3D6%2BKeqD5E8oghS0fw%2BxSzpG%3D94PSOv0LN%2BAfNYcSqZR9Xg%40mail.gmail.com. >> For more options, visit https://groups.google.com/d/optout. > > > > -- > Harsh > > -- > 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/CADN8iurs%3DCUsfRG-F%3Dw%2BBa0UGvFiWtK8vRAsohMxR%3DxSBYow4Q%40mail.gmail.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. 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