Re: Placeholders in matrix notation

*To*: mathgroup at smc.vnet.net*Subject*: [mg29907] Re: [mg29892] Placeholders in matrix notation*From*: "Hugh Philipp" <hph at com.dtu.dk>*Date*: Wed, 18 Jul 2001 02:08:49 -0400 (EDT)*References*: <200107170500.BAA15919@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

I've had this problem before and I have used at least a dozen clumsy work arounds for it. Here is one: ff ={D[#[[1]],x],0}&; ff@{f[x],g[x]} Of course, I don't really like this method because you have to do the matrix operations explicitly in the function - designating the elements. The other way of doing it goes something like this: Unprotect[Times] d1 f_:=D[f,x] Protect[Times] {d1,0,d1}.{f[x],g[x],h[x]} I don't know if the gurus here think this is O.K. - I'm sure there will be comments. Notice to that: d1 d1 f[x] evaluates to f''[x] ...a nice 'feature'. - but BE CAREFUL becaue d1 d1 evaluates to 0. So - there might be some other hidden 'features' with this approach. For example, the vector: {d1 d1,d1} evaluates to {0,d1} Hope this helps, Hugh. ----- Original Message ----- From: "Douglas F." <dooglefish at aol.communicate> To: mathgroup at smc.vnet.net Subject: [mg29907] [mg29892] Placeholders in matrix notation > I want to use the partial differential operator in matrix algebra. The function > it operates on would be in another matrix. > > Here, I am using "d" to represent the partial operator > {esc}pd{esc}{ctrl-_}x{ctrl-space} > > [d 0] . Transpose[ f[x]| g[x] ] > > But Mathematica won't let me do the matrix multiplication. How do you do that? > > -D

**References**:**Placeholders in matrix notation***From:*dooglefish@aol.communicate (Douglas F.)