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MathGroup Archive 1989

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Re: bug reports..

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

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.



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 (Internet)
Email: 14008 at ncsavmsa.bitnet (Bitnet)


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