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

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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