Re: Bug in Sum?

*To*: mathgroup at smc.vnet.net*Subject*: [mg109142] Re: Bug in Sum?*From*: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>*Date*: Mon, 19 Apr 2010 04:10:43 -0400 (EDT)*References*: <hq0mqo$k1b$1@smc.vnet.net>

Abhishek, I don't seem to be able to reproduce your bug. For Sum A I get: -((x^(-2 n) (-1 + x^n) (-1 - x^n + r x^n + s x^n + r x^(2 n) + s x^(1 + n)))/(-1 + x^2)) which depends on r and s. Same result for Expand[A]. In[6]:= $Version Out[6]= "7.0 for Microsoft Windows (32-bit) (February 18, 2009)" Cheers -- Sjoerd On Apr 13, 5:01 am, gopher <gopherg... at gmail.com> wrote: > 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) > )