Re: Limits
- To: mathgroup at smc.vnet.net
- Subject: [mg7783] Re: [mg7741] Limits
- From: "Keith S. Mersman" <c621746 at everest.cclabs.missouri.edu>
- Date: Tue, 8 Jul 1997 22:41:01 -0400
- Sender: owner-wri-mathgroup at wolfram.com
Hans, The "problem" is that Mathematica does not know if your variables are positive. If you take the limit of the numerator, you get 3*(-sk^2*Us^2 + Sqrt[sk^4*Us^4]) which is 0 if sk and Us are both positive. However, for all applications, this might not be 0. You can alleviate this problem by using the ComplexExpand command on your expression. On Mon, 7 Jul 1997, Hans Steffani wrote: > Does Limit[] not know about L'Hospital? > > In[1]= (2 Mi OmegaSyn Rr-3 sk^2 Us^2+ > Sqrt[-4 Mi^2 OmegaSyn^2 Rr^2 sk^2+9 sk^4 Us^4])/(2 Mi Rr) > Out[1]= > 2 2 > (2 Mi OmegaSyn Rr - 3 sk Us + > 2 2 2 2 4 4 > Sqrt[-4 Mi OmegaSyn Rr sk + 9 sk Us ]) / (2 Mi Rr) > > In[2]= Limit[%, Mi->0] > Out[2]= > 2 2 4 4 > Infinity (-3 sk Us + 3 Sqrt[sk Us ]) > --------------------------------------- > Rr > > In[3]= Limit[ D[Numerator[%%], Mi]/ D[Denominator[%%], Mi] > ,Mi->0 ] (* Is L'Hospital possible? *) > Out[3]= OmegaSyn > > Hans Friedrich Steffani > -- > Hans Friedrich Steffani > Institut fuer Elektrische Maschinen und Antriebe, TU Chemnitz-Zwickau > mailto:hans.steffani at e-technik.tu-chemnitz.de > http://www.tu-chemnitz.de/~hfst/ > >