A simple question?

• To: mathgroup at smc.vnet.net
• Subject: [mg3627] A simple question?
• Date: Sat, 30 Mar 1996 02:46:33 -0500
• Organization: Dept. of Mathematics, Chalmers, Sweden
• Sender: owner-wri-mathgroup at wolfram.com

```	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.