It seems you want to revert the Lorentz transformation: https://en.wikipedia.org/wiki/Lorentz_factor#Occurrence
In this case you have a system of equations relating the various quantities c, ga, t, tt, x, xx, v, where - ga is the gamma factor, c is the speed of light, - v is the relative speed of two inertial frames, - t, x are spacetime coordinates in one inertial frame, - tt, xx are spacetime coordinates in the other frame. In Sage form, the system looks like: tt == ga * (t - v*x/c^2) xx == ga * (x - v*t) It is a usual system of equations, not a system of differential equations, so you want to use `solve` and not `desolve` (note that "desolve is short for "differential equation solve"). You could do this: Define the variables (everything stays symbolic, we don't give specific values to any variable): sage: ga, t, tt, x, xx, v = SR.var("ga t tt x xx v") Define the two equations: sage: eq1 = tt == ga * (t - v*x/c^2) sage: eq2 = xx == ga * (x - v*t) Use `solve`, giving it a list of equations constituting the system to be solved, and the variables for which to solve: sage: solve([eq1, eq2], t, x) [[t == (c^2*tt + v*xx)/(c^2*ga - ga*v^2), x == (c^2*tt*v + c^2*xx)/(c^2*ga - ga*v^2)]] -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To post to this group, send email to sage-support@googlegroups.com. Visit this group at https://groups.google.com/group/sage-support. For more options, visit https://groups.google.com/d/optout.