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Re: Bug in Sum?

• To: mathgroup at smc.vnet.net
• Subject: [mg109146] Re: Bug in Sum?
• From: "Tony Harker" <a.harker at ucl.ac.uk>
• Date: Mon, 19 Apr 2010 04:11:26 -0400 (EDT)

What version are you using? 7.0 for Microsoft Windows (32-bit) (February 18,
2009) gives identical, correct, results for both.

  Tony Harker 

]-> -----Original Message-----
]-> From: gopher [mailto:gophergoon at gmail.com] 
]-> Sent: 13 April 2010 04:04
]-> To: mathgroup at smc.vnet.net
]-> Subject: [mg109109] Bug in Sum?
]-> 
]-> In the following, A and Expand[A] give different answers after when
]-> summed (a finite geometric series.) The result of summing A 
]-> is clearly
]-> wrong, since it is independent of the parameters r and s.
]-> 
]-> Abhishek
]-> 
]-> In[44]:= A = x^(i - n) (x^(i - n) (1 - r x^n) - s);
]-> A == Expand[A] // Simplify
]-> 
]-> Out[45]= True
]-> 
]-> In[46]:= Sum[A, {i, 0, n - 1}]
]-> 
]-> Out[46]= (x^(-2 n) (-1 + x^(2 n)))/(-1 + x^2)
]-> 
]-> In[47]:= Sum[Expand[A], {i, 0, n - 1}] // FullSimplify
]-> 
]-> Out[47]= -((
]->  x^(-2 n) (-1 + x^n) (-1 + x^n (-1 + r + s + s x + r 
]-> x^n)))/(-1 + x^2)
]->  )
]-> 
]->