On Thu, Aug 30, 2012 at 1:28 AM, Ondřej Čertík <ondrej.cer...@gmail.com> wrote: > Hi, > > Let's say that I have just first derivative (but I would like second > derivatives too later): > > >>>> f(x).diff(x) > d > ──(f(x)) > dx > > and I would like to use the substitution y = 2*x**2 to transform the > derivative. Mathematically, here is how it works: > > df/dx = df/dy * dy/dx = df/dy * 4*x > > I can do the following: > > >>>> eq = f(x).diff(x) >>>> eq.subs(x, sqrt(y/2)) > ⎛d ⎞│ ___ ___ > ⎜──(f(x))⎟│ ╲╱ 2 ⋅╲╱ y > ⎝dx ⎠│x=─────────── > 2 > > it's a little ugly because I have to manually invert the substitution > and also the derivative is still expressed in terms of "x". I would > like to get: > > f'(y) * 4*x > > or > > f'(y) * 4*sqrt(y/2) > > I was wondering whether there is any support for changing a symbolic > ODE from one variable to another.
I don't think there is, but obviously it would be useful. Would this make more sense as a method on Expr or as a function in ode.py? > The confusing notion is that if you write f(x) and f(y), it's not > clear whether it's the same function, just relabeling the independent > variable, > or whether those are two different functions, one is f(x), and the > other is f(y) = f(2*x**2). So the real issue is that y should really be y(x), another function. Aaron Meurer > > So I think that when I change the independent variable like this, I > should also change the name of the function. What would be the best > way to do that? > > 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 > sympy+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/sympy?hl=en. > -- 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 sympy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sympy?hl=en.
