To simplify a huge expression efficiently, which involves a variable in a bunch of exponential functions going to infinity, I have tried to substitute
/.Exp[-x_]->0
where my intention was to set to zero any exponential function featuring a minus sign in front of an arbitrary function. Unfortunately this substitution had no effect.
How can I do what I intend? Please note, Simplify
and FullSimplify
functions involving any ComplexityFunction
are not an option, since they take forever to evaluate.
EDIT
On a similar note, I noticed that oftentimes expressions of the same power appear as multipliers in numerator and denominator, like in the following example
$$\frac{a f_{1}+af_2}{(a f_3+af_4)f_5}$$
still the $a$ does not cancel out automatically. Is there a command to explicitly look for such cancellations and carry them out without attempting any other simplifications? I tried PowerExpand
, Expand
and ExpandAll
. None of them work like this.
Exp[(-I | -1) x_]
$\endgroup$ – swish Apr 24 '13 at 22:52Exp[- a ] /. Exp[- x_] :> 0
gives0
butExp[-4 a ] /. Exp[- x_] :> 0
givesExp[-4 a ]
andExp[-4 a ] /. Exp[(-I | -1) x_] :> 0
givesExp[-4 a ]
$\endgroup$ – Kagaratsch Apr 24 '13 at 22:56/. x_/y_ :> Collect[x, a]/Collect[y, a]
$\endgroup$ – swish Apr 24 '13 at 22:56/. Exp[n_?Negative x_] :> 0
$\endgroup$ – swish Apr 24 '13 at 23:00