Thanks, Sergey. I've filed an issue on github. On Tuesday, July 29, 2014 4:15:52 AM UTC-4, Sergey Kirpichev wrote: > > Looks as a bug for me. > > Our bugtracker: > https://github.com/sympy/sympy/issues > > On Tuesday, July 29, 2014 1:29:59 AM UTC+4, J. S. Oishi wrote: >> >> Hi all, >> >> I'm using sympy 0.7.4.1, and I've come across an issue I don't quite >> understand. If I do >> >> from sympy import * >> x,z,t = symbols('x, z, t') >> >> psi = Function('psi')(x,z) >> >> j1 = Derivative(psi,x) - Derivative(psi,z) >> >> j1.subs(Derivative(psi,x), t) >> >> I get what I expect, t - d psi/dx. However, if I do the same thing with a >> second derivative with respect to z, I get the following: >> >> j2 = Derivative(psi,x) - Derivative(psi,z,2) >> >> j2.subs(Derivative(psi,x), t) >> >> it returns t - d^2 t/dz^2. I expect it to return t - d^2 psi/dz^2. Can >> anyone point me to anything I might be doing wrong? I've tried defining t >> as a Function, rather than as a symbol (t = Function('t')(x,z)), but that >> does the exact same thing. I've also tried writing the second derivative in >> j2 as Derivative(psi,z,z), but that also does the same thing (which I would >> expect). >> >> I've found this behavior both on my computer with Python 3.3.2 and on the >> sympy live shell on sympy.org, which is currently SymPy 0.7.4.1 (Python >> 2.7.5). >> >> Any advice you could provide would be very helpful. >> >> >> thanks, >> >> >> Jeff >> >
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