The reason this is happening is that both tanh(x) and coth(x) satisfy the same differential equation, namely f'(x) = 1 - f(x)**2, because d/dx tanh(x) = 1 - tanh(x)**2 and d/dx coth(x) = 1 - coth(x)**2. This is your ODE (up to a change in variable).
Aaron Meurer On Tue, Jun 28, 2016 at 12:54 PM, Aaron Meurer <[email protected]> wrote: > According to checkodesol(paraChute, solution), the solution is correct. > > Aaron Meurer > > On Tue, Jun 28, 2016 at 2:13 AM, Dan Lewis <[email protected]> wrote: >> Hi folks, >> >> Looks like sympy/dsolve produces an incorrect solution to an ODE: >> >> from sympy import * >> >> mass, g, b, t = symbols('mass g b t') >> >> v, x = symbols('v x', cls=Function) >> >> paraChute = mass*v(t).diff(t)-mass*g+b*(v(t))**2 >> >> solution = dsolve(paraChute,v(t),hint='lie_group') >> >> solution >> >> Out[1]: >> >> Eq(v(t), sqrt(g)*sqrt(mass)/(sqrt(b)*tanh(sqrt(b)*sqrt(g)*(C1*mass + >> t)/sqrt(mass)))) >> >> >> Sorry in advance if I'm doing something silly - the solution shouldn't be >> 1/tanh. Should be tanh. >> >> -- >> 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/92f8fe37-45df-4cc0-9ebc-7330f0bb20d9%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%3D6K1gZKPkR4ZXaCL_WAVYUZ_1cW-G_KWWeHNmfio8iurgg%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
