Re: bug ?
- To: mathgroup at smc.vnet.net
- Subject: [mg122341] Re: bug ?
- From: DrMajorBob <btreat1 at austin.rr.com>
- Date: Wed, 26 Oct 2011 17:36:26 -0400 (EDT)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
- References: <201110251016.GAA05769@smc.vnet.net>
- Reply-to: drmajorbob at yahoo.com
That's odd! If f is continuous, all eight outputs should represent the same number, but N gives 4 distinct results: Clear[f, s] f[s_] = -(((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); rp = 1/4 (4 + Sqrt[17]); four = {Limit[f@s, s -> rp], f@rp, Simplify@f@rp, f@N@rp}; Grid@{four, N@four} (output suppressed) four//Column 1/544 (331-19 Sqrt[17]) -((((-1+Sqrt[17])^2-1/2 (-3+Sqrt[17]) (5+3 Sqrt[17])) (1/4 (-1+Sqrt[17])^2 (4+Sqrt[17])-1/8 (5+Sqrt[17]) (5+3 Sqrt[17])))/(-1-8 (4+Sqrt[17])+(4+Sqrt[17])^2)^2) 1/16 (1+2 Sqrt[17]) 0.09375 N@four//Column 0.46445 0. 0.577888 0.09375 Bobby On Tue, 25 Oct 2011 05:16:06 -0500, swiss <gregoire.nicollier at hevs.ch> wrote: > 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 > -- DrMajorBob at yahoo.com
- References:
- bug ?
- From: swiss <gregoire.nicollier@hevs.ch>
- bug ?