       • To: mathgroup at yoda.physics.unc.edu (Mathgroup)
• Subject: Question about Cases command.
• From: Scott Herod <sherod at gauss.colorado.edu>
• Date: Mon, 27 Jun 1994 14:58:29 -0600 (MDT)

```I am trying to grab a list of the derivatives of functions that
appears in a large list of differential equations.  I have been using
the Cases command to do this and have a question about some curious
behavior.

Consider the list of two derivatives:

In:= {Derivative[1,2][a][x,y], Derivative[1,1][b][x,y]}

3       2
d a     d b
Out= {------, ------}
1  2    1  1
dx dy   dx dy

I would like to select the derivatives of "a" .  First let's
check the Head of the terms:

In:= myterms = %;

(1,2)   (1,1)
Out= {a     , b     }

Now try to uses Cases:

In:= Cases[myterms,_Derivative[__][a]]

Out= {}

Why didn't it select the "a" term out?  Try:

In:= Cases[myterms, _Derivative[__][_]]

Out= {}

In:= Cases[myterms, _Derivative[__][__]]

3       2
d a     d b
Out= {------, ------}
1  2    1  1
dx dy   dx dy

Ok, now why must I use two underscores in this form?  ( In )
The "a" and the "b" are single elements.

BTW. Here is something which works.

In:= Select[myterms, (derivdepfn[#1] === a) &]

3
d a
Out= {------}
1  2
dx dy

Scott A. Herod
Program in Applied Mathematics