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Re: Inappropriate Domain Calculation Warnings
*To*: mathgroup at smc.vnet.net
*Subject*: [mg45704] Re: [mg45683] Inappropriate Domain Calculation Warnings
*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>
*Date*: Tue, 20 Jan 2004 05:08:15 -0500 (EST)
*References*: <200401191015.FAA16982@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
On 19 Jan 2004, at 10:15, Harold Noffke wrote:
> MathGroup:
>
> Here's an example of Solve with a user-specified Domain. It is
> accompanied by a Warning which is inappropriate for the specified
> Domain.
>
> In[1]:= Solve[(2^53 == 10^x), x, Element[x, Reals]] // N
>
> Solve::ifun: Inverse functions are being used by Solve, so some
> solutions may not be found; use Reduce for complete solution
> information.
>
> Out[1]= {{x -> 15.9546}}
>
> Given the Domain spec that x is a Real number, I would think this
> Solve::ifun: message would not propagate to printout -- i.e., that it
> would be suppressed.
>
> This is how I feel a new user would expect Mathematica 5.0 to work.
>
> Regards,
> Harold
>
The question is: where did you get the idea that Solve can use domain
specifications? Certainly not from the documentation. Mathematica is
intuitive but not as intuitive so that you can dispense entirely with
reading the documentation. If you are looking for a program that a new
user can use without any learning than you have chosen the wrong one.
As the message you got tells you, the function to use is Reduce. If you
do that you get
Reduce[2^53 == 10^x, x, Reals]
x == Log[9007199254740992]/Log[10]
The reason for this difference between Solve and Reduce has already
many times been explained on this list. They use essentially different
methods. Solve is more limited in the kind of equations it can deal
with (in particular you can't restrict it to only real solutions) but
in those cases which it can deal with it should be more efficient than
Reduce. One could quibble about the names of these functions but they
are part of Mathematica's legacy.
Andrzej Kozlowski
Chiba, Japan
http://www.mimuw.edu.pl/~akoz/
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