       Re: A simple question?

• To: mathgroup at smc.vnet.net
• Subject: [mg3644] Re: A simple question?
• From: espen.haslund at fys.uio.no (Espen Haslund)
• Date: Sun, 31 Mar 1996 00:51:45 -0500
• Organization: Universitet i Oslo
• Sender: owner-wri-mathgroup at wolfram.com

```In article <4jd5ej\$pmr at dragonfly.wolfram.com>, nader at math.chalmers.se
says...
>
>
;>        Hi,
;>
;>        I guess this must be a simple question, but I can't seem to
;>find a good solution to it. For a function f[x,y] I want to create a
;>2x2x2x2 matrix called fourthDs. The elements of fourthDs will be the
;>fourth derivatives of f with respect to x and y in the following form:
;>
;>fourthDs[[1,1,1,1]]=D[f[x,y],{x,4}]
;>fourthDs[[2,2,2,2]]=D[f[x,y],{y,4}]
;>fourthDs[[1,2,1,1]]=D[f[x,y],{x,3},{y,1}]
;>
;>        This simply means that the number of 1's in the index will
;>give the number of derivations with respect to x and the number of 2's
;>in the index will give the number of derivations with respect to y. In
;>this case there will be 5 different elements in fourthDs. For example,
;>the last element in the above list will be the same for
;>fourthDs[[2,1,1,1]], fourthDs[[1,1,2,1]] and so on.
:>
;>        I wonder how I can do this in Mma. Thanks in advance for your
;>time.
>
>
>
>

This is not so elegant:

fourthDs[f_, {x_, y_}] :=
Module[{res},
res = Table[0, {2},{2},{2},{2}];
Do[
res[[i,j,k,l]] =
D[f, {x, Length[Select[{i,j,k,l}, # == 1 &] ] },
{y, Length[Select[{i,j,k,l}, # == 2 &] ] }],
{i,1,2},{j,1,2},{k,1,2},{l,1,2}];
res
]

but I hope it will do what you want.
- Espen

==== [MESSAGE SEPARATOR] ====

```

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