Re: Questions about Abs[_]
- To: mathgroup at smc.vnet.net
- Subject: [mg55634] Re: Questions about Abs[_]
- From: Maxim <ab_def at prontomail.com>
- Date: Thu, 31 Mar 2005 01:25:49 -0500 (EST)
- References: <d2dpgh$lt5$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
On Wed, 30 Mar 2005 08:54:41 +0000 (UTC), Dan <dantopa at gmail.com> wrote:
> Hi Andrzej:
>
> Mathematica is considering the most general case. I am thinking that
> you are dealing with all positive numbers since you have stated the
> answer you want.
>
> Let's tell Mathematica we have all positive numbers.
>
> x = Abs[(2 z0 - 2 d m)^2]; (* define the function *)
>
> lst={z0,d,m}; (* assemble all variables in a list *)
>
> Simplify[x,lst\[Element]Reals && lst > 0] (* apply criteria *)
>
> output =
> (2 z0 - 2 d m)^2
>
> Hope this helps,
>
> Dan
>
Element[{x, y}, Reals] is just a conventional notation which means that
both x and y are in Reals; {x, y}>0 cannot be used in the same way.
Mathematica seems to interpret the assumption {x, y}>0 as equivalent to
Element[{x, y}, Reals] (most likely based on the rule that everything
appearing algebraically in inequalities is real):
In[1]:=
Refine[Element[x, Reals], {x, y} > 0]
Refine[x > 0, {x, y} > 0]
Out[1]=
True
Out[2]=
x > 0
Going slightly off topic, this is one of many cases where Mathematica's
parsing of vector input is shoddy:
In[3]:=
NSolve[{x, y} == {0}]
Out[3]=
{{False -> 0.}}
Of course we have an invalid input here, but Mathematica's output (and
absense of warning messages) leaves the user at a loss about what went
wrong.
Maxim Rytin
m.r at inbox.ru