Re: Re: solve and Abs
- To: mathgroup at smc.vnet.net
- Subject: [mg69400] Re: [mg69398] Re: solve and Abs
- From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
- Date: Mon, 11 Sep 2006 05:38:31 -0400 (EDT)
On 10 Sep 2006, at 20:20, Peter Pein wrote:
> Richard J. Fateman schrieb:
>> Is there a reason (in 5.1) for Solve[x-Abs[x]==0,x] to return {{}}?
>> This supposedly means all variables can have all possible values.
>>
>> Reduce does better.
>>
> Oops, I should have read further...
> Help says
> "Solve gives {{}} if all variables can have all possible values."
> I would consider this a bug, not a feature.
>
> Sorry for overhasty posting,
> Peter
>
However, the documentation does not say "Solve gives {{}} if and only
if all variables can have all possible values." There are other cases
when Solve returns {{}}. For example, Solve also gives {{}} when, for
example, there is no "generic" solution. (This is stated in the Help
also). In addition, although this does not seem to be stated, Solve
never returns solutions that would have to be given in conditional
form (like the solution returned in this example by Reduce). This
does not appear to be stated in the help but has been pointed out
several times on this list.
Andrzej Kozlowski
Tokyo, Japan