Re: Change of function in an ODE
- To: mathgroup at smc.vnet.net
- Subject: [mg109082] Re: Change of function in an ODE
- From: dh <dh at metrohm.com>
- Date: Mon, 12 Apr 2010 22:58:31 -0400 (EDT)
- References: <hps1fg$6l4$1@smc.vnet.net>
On 11.04.2010 10:32, Sam Takoy wrote: > Hi, > > I'm solving > DSolve[r^4 R''''[r] + 2 r^3 R'''[r] - r^2 R''[r] + r R'[r] - > L^4 r^4 R[r] == 0, R[r], r] // Simplify > > Now, I would like to make the substitution R[r]=r^2 Log[r] G[r], > formulate the equation for G[r] and solve it. What's the proper syntax > for that? > > Thanks, > > Sam > Hi Sam, the easiest way to write your equation in terms of G instead of R seems to replace the symbol "R" by an anonymous function. Here is a simple example: t = R'[r]; t /. R -> (#^2 Log[#] G[#] &) this will write R'' in terms o f G. For your problem we would have: t = r^4 R''''[r] + 2 r^3 R'''[r] - r^2 R''[r] + r R'[r] - L^4 r^4 R[r]; t /. R -> (#^2 Log[#] G[#] &) cheers, Daniel -- Daniel Huber Metrohm Ltd. Oberdorfstr. 68 CH-9100 Herisau Tel. +41 71 353 8585, Fax +41 71 353 8907 E-Mail:<mailto:dh at metrohm.com> Internet:<http://www.metrohm.com>