Comment #9 on issue 2238 by [email protected]: Autodetection of dependent
variable in dsolve, checkodesol and other high level function in ODE module
http://code.google.com/p/sympy/issues/detail?id=2238
Pull https://github.com/sympy/sympy/pull/169 was replaced with
https://github.com/sympy/sympy/pull/568
But i'm not sure what advantage you imagine there being to leaving
`Derivative(f(x) + x, x, x, x))` unevaluated. Derivatives are easy to
compute compared to the rest of dsolve's work, so for now the preprocess
routine evaluates derivatives when there is something other than the
function of interest at their core. It wouldn't, however, evaluate the
following:
>>> preprocess(f(x).diff(x, 2))
(Derivative(f(x), x, x), f(x))
but it will reduce the following so an error does not occur in the solver:
>>> preprocess((f(x)+x).diff(x, 2))
(Derivative(f(x), x, x), f(x))
This preprocessing capability is also extended to classify_ode (and tests
were added).
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