Re: Change of Variables

*To*: mathgroup at smc.vnet.net*Subject*: [mg32512] Re: Change of Variables*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Fri, 25 Jan 2002 02:57:40 -0500 (EST)*Organization*: Universitaet Leipzig*References*: <a2onns$5v9$1@smc.vnet.net>*Reply-to*: kuska at informatik.uni-leipzig.de*Sender*: owner-wri-mathgroup at wolfram.com

Hi, what is the difference between Replace[] and ReplaceAll[] ? ??ReplaceAll "expr /. rules applies a rule or list of rules in an attempt to transform \ each subpart of an expression expr" While ??Replace "Replace[expr, rules] applies a rule or list of rules in an attempt to \ transform the entire expression expr. Replace[expr, rules, levelspec] applies \ rules to parts of expr specified by levelspec." And test^2 /. v -> c*beta gives beta^2 and 2*Pi*c^2*h/lambda^5*(E^(h*c/(lambda*k*T)) - 1)^(-1) /. T -> h*c/(lambda*k x) gives (2*c^2*h*Pi)/((-1 + E^x)*lambda^5) Regards Jens John S wrote: > > Hello, > > I would greatly appreciate any help with the following problem. I am trying > to perform a change of variable in a function/definition so that I can > integrate it. In particular, I want to take Planck's Radiation Law: > > planck=2*Pi*c^2*h/lambda^5*(E^(h*c/(lambda*k*T))-1)^(-1) > > and substitute x=h*c/(lambda*k*T) and integrate wrt lambda from 0 to > infinity. I tried using replace, but that does not seem to try to > manipulate the function in terms of x, but simply seek out the replacement, > and if it exists, perform it. > > An even simpler example is the following: > > test=v/c > Replace[test^2,v/c -> beta] > > does not yield beta^2, but rather v^2/c^2. > > Again, any and all help would be greatly appreciated.