       Re: Cross results?

• To: mathgroup at smc.vnet.net
• Subject: [mg63205] Re: [mg63201] Cross results?
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Mon, 19 Dec 2005 07:00:58 -0500 (EST)
• References: <200512181234.HAA24522@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```On 18 Dec 2005, at 21:34, Virgil Stokes wrote:

> I have the following
>
>   x=.
>   Cross[{10 Sin[x],-10 Cos[x],2.5},{0.12 Sin[x],-0.12 Cos[x], 0.0}]
>
> which gives,
>
>   {0.3 Cos[x], 0.3 Sin[x], 0.Cos[x]Sin[x]}
>
> why does it not set the last term to 0.0?

It would never make sense to set 0.0 * u to 0.0, where u is a
symbol since there is nothing known about the size of u. In general,
u could be arbitrarily large and in fact it can even be Infinity. Of
course for real x the value of 0. Cos[x]*Sin[x] will be approximately
0, but by taking x to be a purely imaginary number of huge modulus we
can make Cos[x]*Sin[x] also have arbitrarily large modulus. In fact,
taking x= I Infinity we get:

0. Sin[x]*Cos[x]/.x->I*Infinity

Indeterminate expression DirectedInfinity,  encountered.

Indeterminate

In any case, as a rule Mathematica never replaces 0.* non numeric
symbolic expression by 0. With numeric symbols the situation is of
course quite different (for the obvious reasons):

0.*Pi

0.

Andrzej Kozlowski

```

• Prev by Date: Re: Uninstall Mathematica v4
• Next by Date: Re: Uninstall Mathematica v4
• Previous by thread: Re: Cross results?
• Next by thread: Re: Re: Cross results?