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



• Follow-Ups:
• Re: bug ?
• From: Daniel Lichtblau <danl@wolfram.com>
• Re: bug ?
• From: DrMajorBob <btreat1@austin.rr.com>
• Prev by Date: Re: Integral points on elliptic curves
• Next by Date: Geometric series for matrices
• Previous by thread: Re: Another basic (?) question about RecurrenceTable and replacement
• Next by thread: Re: bug ?