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Re: "Assuming"

• To: mathgroup at smc.vnet.net
• Subject: [mg85599] Re: "Assuming"
• From: dh <dh at metrohm.ch>
• Date: Thu, 14 Feb 2008 06:30:20 -0500 (EST)
• References: <fp0m11$5u7$1@smc.vnet.net>

HI Markus,

I think you are fooling yourself.

Consider Simplify[(a^2 - 1)/(a - 1)]

As a^2 - 1)== (a+1)(a-1) this will give (a+1). This is true in general, 

for a==1 understood as limit. Therefore, this has nothing to do with 

assumptions.

On the other hand, (a^2 - 1)/(a - 1) /. a -> 1 can not be calculated as 

it gives 0/0.  Note that "/. and  ->" mean replacement, not limit. If 

you want a limit, you have to say so: Limit[(a^2 - 1)/(a - 1), a -> 1]

hope this helps, Daniel





markusbinder wrote:

> Hi,

> 

> I would appreciate some comments about the Assuming-function. Several examples using Mathematica 6 as follows:

> 

> In[1]:= Assuming[a > 0, Simplify[a == -1]]

> Out[1]= False

> In[2]:= Assuming[a \[Element] Integers, Simplify[a == -1]]

> Out[2]= a == -1

> In[3]:= Assuming[a \[Element] Integers && a > 0 && a < 2, Simplify[a^2< 1.1]]

> Out[3]= True

> In[4]:= Assuming[a \[Element] Reals && a > 0 && a < 2, Simplify[a^2 < 1.1]]

> Out[4]= a < 1.04881

> 

> So far, everything seems pretty reasonable, but

> 

> In[5]:= Assuming[a \[Element] Integers && a > 0 && a < 2, Simplify[(a^2 - 1)/(a - 1)]]

> Out[5]= 1 + a

> 

> whereas

> 

> In[6]:= (a^2 - 1)/(a - 1) /. a -> 1

> Out[6]= Indeterminate

> 

> I consider this as blunder (or bug?); I comprehend working with domains can be pretty tricky and of course I don't expect

> 

> In[7]:= Simplify[(a^2 - 1)/(a - 1)]

> 

> to yield

> 

> Out[7]= "Beware, a != 1 neccessary!"

> 

> instead of

> 

> Out[7]= 1 + a.

> 

> Am I tricked by some personal misconception of how Mathematica deals with Assuming plus fractions as shown above, or is this a mere flaw?

> Thanks in advance for your corrections & suggestions.

> 

> Best regards

> Markus Binder

>