Comment #3 on issue 2180 by smi...@gmail.com:
examples/advanced/relativity.py stopped working
http://code.google.com/p/sympy/issues/detail?id=2180
ode_order doesn't recognize it as 2nd order because the 2nd order term has
an expr of
-lambda(r)
and ode_order doesn't expect that. After doit, it would become
-D(lambda(r), r, r) and then be recognized. So the quick fix is to
append ".doit()" to line 213 of relativity. The bigger question is whether
dsolve should pre-process derivatives with doit:
eq = eq.subs(dict([(d, d.doit()) for d in eq.atoms(Derivative)])
There are lots of ways that derivatives could be written so ode_order will
fail, e.g.
D(D(f(x), x) + f(x)/2, x) is order 2 but will come back as order 1.
ode_order, too, recurses "args of args" without specifying what type of
expr is expected. I wonder if this should be
for a in Add.make_args(expr):
for m in Mul.make_args(a):
etc..
and then an error raised if anything other than a simple power is
encountered. Though such an expression as below is caught by dsolve, it
would pass through ode_order as order 1:
ode_order(D(f(x), x)**D(f(x), x), f(x)) -> 1
/c
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