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MathGroup Archive 1989

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Re: bug reports..

  • To: mathgroup
  • Subject: Re: bug reports..
  • From: stevec at ncsa.uiuc.edu
  • Date: Fri, 12 May 89 15:46:50 CDT
  • Reply-to: stevec at ncsa.uiuc.edu

For your bugs (from LFEAGIN at CALSTATE.BITNET), I get
(run in Sun verison 1.2):

In[1]:= Limit[ x/Sqrt[x^2], x->-Infinity ]

Out[1]= 1
 
In[2]:= Integrate[x b^(a x), x ]

         a x Log[b]        1            x
Out[2]= E           (-(----------) + --------)
                        2       2    a Log[b]
                       a  Log[b]

In[3]:= D[%,x]

         a x Log[b]
        E              a x Log[b]          1            x
Out[3]= ----------- + E           a (-(----------) + --------) Log[b]
         a Log[b]                       2       2    a Log[b]
                                       a  Log[b]

In[4]:= Expand[%]

         a x Log[b]
Out[4]= E             

When you ask Mathematica to Plot the function x/Sqrt[x] say from
-1 to 1, you get the step function with jump at 0, but the Limit
function does not give -1 as it probably should.       

In the second example, it is clear that the rules for E and Log's
are not automatic, but could easily be programmed in.

I note that Maple also gives 1 for In[1], But does simplify Out[4]
back to the x b^(a x).

Macsyma gives -1 for the limit, but does not simplify
the exponential automatically.

Steve

-------------------------------------------------------------------------

Steven M. Christensen
Senior Research Scientist (Theoretical Physics)
National Center for Supercomputing Applications
Beckman Institute for Advanced Science and Technology
University of Illinois at Urbana-Champaign
Urbana, Illinios 61801  USA

Phone: (217) 244-0544 (with answering machine)
Phone: (217) 244-0072 (main NCSA number for messages to a human being)
FAX:   (217) 244-2909
Email: stevec at ncsa.uiuc.edu (Internet)
Email: 14008 at ncsavmsa.bitnet (Bitnet)

-------------------------------------------------------------------------





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