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bug ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg122317] bug ?
  • From: swiss <gregoire.nicollier at hevs.ch>
  • Date: Tue, 25 Oct 2011 06:16:06 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com

Following rational function has a double pole matched by a double root at (4+\sqrt{17})/4. The first result is correct, the other ones are false, but come without warning. Can somebody explain this to me?

In[50]:= Limit[-(((s (4 s - 5)^2 - (5 + 3 Sqrt[17])/
      8 (4 s + 1)) ((4 s - 5)^2 - 
     4 (5 + 3 Sqrt[17])/8 (4 s - 7)))/(16 s^2 - 32 s - 1)^2), 
 s -> 1/4 (4 + Sqrt[17])]

Out[50]= 1/544 (331 - 19 Sqrt[17])

In[51]:= 1/544 (331 - 19 Sqrt[17]) // N

Out[51]= 0.46445

In[52]:= -(((s (4 s - 5)^2 - (5 + 3 Sqrt[17])/
       8 (4 s + 1)) ((4 s - 5)^2 - 
      4 (5 + 3 Sqrt[17])/8 (4 s - 7)))/(16 s^2 - 32 s - 1)^2) /. 
  s -> 1/4 (4 + Sqrt[17]) // Simplify

Out[52]= 1/16 (1 + 2 Sqrt[17])

In[53]:= 1/16 (1 + 2 Sqrt[17]) // N

Out[53]= 0.577888

In[54]:= -(((s (4 s - 5)^2 - (5 + 3 Sqrt[17])/
      8 (4 s + 1)) ((4 s - 5)^2 - 
     4 (5 + 3 Sqrt[17])/8 (4 s - 7)))/(16 s^2 - 32 s - 1)^2) /. 
 s -> 1/4 (4 + Sqrt[17.])

Out[54]= 0.09375



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