I've done a lot of work on this in the last week, and have a couple
different ideas on how to make a fast way to do this. I've made a wiki page
here
<https://github.com/sympy/sympy/wiki/Design-Notes:-Evaluating-expr-that-results-in-%60nan%60-or-%60oo%60>.
It would be extremely helpful for people to take a look through and give me
their thoughts. Thanks!
On Saturday, June 28, 2014 1:24:31 PM UTC-5, Aaron Meurer wrote:
>
> You might also be able to get a custom trigsimp that only the
> simplifications you need from the fu module.
>
> Aaron Meurer
>
> On Fri, Jun 27, 2014 at 10:53 PM, Chris Smith <smi...@gmail.com
> <javascript:>> wrote:
> > Ahh...trisgsimp is like simplify in that it tries several things that
> are
> > known to generally work. If you know that it's trig-related because of
> these
> > sorts of ratios, you might just try rewriting
> >
> >>>> cancel((sin(x)/tan(x)).rewrite(exp)).subs(x,0)
> > 1
> >>>> w=Wild('w'); (sin(x)/tan(x)).replace(tan(w),sin(w)/cos(w)).subs(x,0)
> > 1
> >
> > You could also extract Muls (foo.atoms(Mul)) and build up a replacement
> > dictionary where the value of a given Mul is the value after
> substitution if
> > not nan or oo or else the value you get *after* simplifying that
> expression.
> > This way you are targeting only trouble expressions and not working with
> the
> > whole expression tree.
> >
> > On Friday, June 27, 2014 9:35:09 PM UTC-5, James Crist wrote:
> >>
> >> signsimp didn't seem to help. I'll try to get a gist of the expression
> up
> >> tomorrow, it's so huge I haven't succesfully ran simplify on it.
> However, I
> >> had the same thing happen with a readable expression. The issue was
> >> something like:
> >>
> >> >>> expr = sin(a)/tan(a)
> >> >>> expr.subs(a, 0)
> >> nan
> >> >>> expr = expr.trigsimp()
> >> >>> expr
> >> cos(a)
> >> >>> expr.subs(a, 0)
> >> 1
> >>
> >> Based on the form of equations of motion, I assume it's something along
> >> this line where a trig identity needs to be subbed out. For large
> >> expressions, trigsimp is too slow to actually use, but I can't see a
> way
> >> around it. I tried all methods of trigsimp (interestingly `fu` is
> faster for
> >> small expressions, but `matching` is faster for large). Looking through
> to
> >> see if I can find a way to generate more compact expressions to attempt
> to
> >> avoid this end result.
> >>
> >> On Friday, June 27, 2014 7:29:48 AM UTC-5, Chris Smith wrote:
> >>>
> >>> Try only signsimp before substitution. Can you post code to generate
> one
> >>> of these expressions?
> >
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