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Re: Re: disable canceling

• To: mathgroup at smc.vnet.net
• Subject: [mg106523] Re: [mg106480] Re: [mg106465] disable canceling
• From: DrMajorBob <btreat1 at austin.rr.com>
• Date: Fri, 15 Jan 2010 03:19:39 -0500 (EST)
• References: <22549907.1263381381387.JavaMail.root@n11>
• Reply-to: drmajorbob at yahoo.com

MultiplyByOne seems especially useful.

It's the didactic principle I often use with math-illiterates (my mother,  
for instance) to explain changing feet to meters, and that kind of thing.

Bobby

On Thu, 14 Jan 2010 04:47:33 -0600, David Park <djmpark at comcast.net> wrote:

> It would be useful to know what your objective is here. What do you mean  
> by
> "after evaluation get some expression"? Is this for didactic reasons?
> Otherwise, what is gained by not canceling the factors?
>
> In any case, you could use HoldForm on selected factors to prevent
> canceling. But to do any real mathematics you will eventually have to use
> ReleaseHold and at that point any matching factors will cancel.
>
> g[s] = s HoldForm[s + 1]/((s + 1) (s + 2))
> % // ReleaseHold
>
> (s (s+1))/((1+s) (2+s))
> s/(2+s)
>
> g[s] = HoldForm[s (s + 1)]/(s + 2);
> c[s] = 1/(s (s + 1));
> l[s] = g[s] c[s]
> % // ReleaseHold
>
> s (s+1)/(s (1+s) (2+s))
> 1/(2+s)
>
> Sometimes there might be meaningful math that can be done while part of  
> an
> expression is held and them when the Hold is released it will no longer
> simplify as before. The Presentations package has a CreateSubexpression
> command that holds a sub-expression with a Tooltip and a tag so one can  
> more
> easily control the evaluations.
>
> Presentations also has a MultiplyByOne command that multiplies the  
> Numerator
> and Denominator of an expression by the same factor, but allows  
> operations
> on the two parts so the factor does not automatically cancel. For  
> example,
> we could construct g[s] by:
>
> Needs["Presentations`Master`"]
>
> s/(s + 2);
> % // MultiplyByOne[s + 1, Identity, Expand]
>
> (s + s^2)/((1 + s) (2 + s))
>
> Here the final expression has nothing held. You might have defined it  
> that
> way in the first place.
>
>
> David Park
> djmpark at comcast.net
> http://home.comcast.net/~djmpark/
>
>
>
> From: Petr Martinec [mailto:petr.martinec at gmail.com]
>
> Hi all,
> is here posibility to prevent canceling in Mathematica? I need it to
> work with transfer and contreller functions (dynamic system theory). I
> would like to input like this
>
> g[s] = s (s + 1)/((s + 1) (s + 2));
>
> and after evaluation get same expression. Another situation is:
>
> input:
>
> g[s] = s (s + 1)/(s + 2);
> c[s] =1/(s(s + 1));
> l[s] = g[s] c[s]
>
> output:
> l[s] = s(s + 1)/(s(s + 1)(s + 2))
>
> It is possible to get this result?
>
> thanks for your answers.
>
>
>


-- 
DrMajorBob at yahoo.com