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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
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