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MathGroup Archive 1989

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Mathematica Problem

  • Subject: Mathematica Problem
  • From: jack at sun.acs.udel.edu
  • Date: Mon, 19 Jun 89 14:51:49 -0500
  • Apparently-to: mathgroup-out at yoda.ncsa.uiuc.edu

Perhaps someone can help solve the following Mathematica problem:

     Given y' = y, 

solve using a series solution and prove that the result is E[x].  

It is proving that the result is E[x] that I have not been able to do.
Here is a sample session:

In[1]:= Derivative[n_][y][x_]:=Derivative[n-1][y][x]

In[2]:= Series[y[x],{x,0,5}]

                              2         3         4         5
                        y[0] x    y[0] x    y[0] x    y[0] x        6
Out[2]= y[0] + y[0] x + ------- + ------- + ------- + ------- + O[x]
                           2         6        24        120

In[3]:= seriesbyy0[x_,n_]:=Normal[Expand[Cancel[Series[y[x],{x,0,n}]/y[0]]]]

In[4]:= expseries[x_,n_]:=Sum[x^i/i!,{i,0,n}]

In[5]:= seriesbyy0[x,6]

                 2    3    4    5     6
                x    x    x    x     x
Out[5]= 1 + x + -- + -- + -- + --- + ---
                2    6    24   120   720

In[6]:= expseries[x,6]

                 2    3    4    5     6
                x    x    x    x     x
Out[6]= 1 + x + -- + -- + -- + --- + ---
                2    6    24   120   720


In[7]:= seriesbyy0[x,100]-expseries[x,100]

Out[7]= 0


As shown, I can prove the results are identical for a constant value of
the argument "n", but not for general n.  I realize that the identity is 
proven by visual inspection, but I would like to prove it by showing the 
identity 

	    seriesbyy0[x,n]  ==  expseries[x,n]

evaluates to true.  I have tried LogicalExpand, but to no avail.


				Jack Seltzer
                                jack at sun.acs.udel.edu
 



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