Re: Manipulating differential equations
- To: mathgroup at smc.vnet.net
- Subject: [mg18609] Re: [mg18552] Manipulating differential equations
- From: "Andrzej Kozlowski" <andrzej at tuins.ac.jp>
- Date: Tue, 13 Jul 1999 01:01:33 -0400
- Sender: owner-wri-mathgroup at wolfram.com
Here is one possible way. In[1]:= dg1 = c''[x] + g c'[x]; In[2]:= Block[{c = Function[x, Exp[-a x]]}, dg1] Out[2]= 2 a a g ---- - ---- a x a x E E In[3]:= Block[{c = Function[x, Sin[a x]]}, dg1] Out[3]= 2 a g Cos[a x] - a Sin[a x] In[4]:= dg1 Out[4]= g c'[x] + c''[x] -- Andrzej Kozlowski Toyama International University JAPAN http://sigma.tuins.ac.jp http://eri2.tuins.ac.jp ---------- >From: Martin Heimann <martin.heimann at bgc-jena.mpg.de> >To: mathgroup at smc.vnet.net >Subject: [mg18609] [mg18552] Manipulating differential equations >Date: Sat, Jul 10, 1999, 3:18 PM > > Dear colleagues, > > I would like to insert test functions into an expression that contains > differentials: > > dg1 = c''[x] + g c'[x] > > I would like e.g. to replace c[x] by Exp[-a x]: > > dg1/.c[x]->Exp[-a x] > > but this doesn't work; one has to put substitution rules for each of the > derivatives explicitly. Is there a more elegant way to achieve this? > > Cheers, > Martin Heimann > ------------------------------------------------------------------ > Max-Planck-Institut fuer Biogeochemie > Postfach 100164, D-07701 Jena, Germany > (Street Address: Tatzendpromenade 1a) > > Tel.: +49-3641-64-3773/3701, Fax.: +49-3641-64-3710 > Email: martin.heimann at bgc-jena.mpg.de