D and Derivative - help please

• To: mathgroup at yoda.ncsa.uiuc.edu
• Subject: D and Derivative - help please
• From: uunet!uthvm1.bitnet!AN123651
• Date: Thu, 22 Mar 90 12:41:28 CST

```I would appreciate some help with this from anyone who understands
Mathematica better than I. Suppose I define a function:

f[x_,y_] := x + y + Gamma[x,y]

When I take its derivative using D, everything is fine.

D[f[x,y],x]

(1,0)
1 + Gamma     [x, y]

Derivative, on the other hand, can't get as far.

Derivative[1,0][f]

(1,0)
f

Now, suppose I define my own value for the derivative of the
Gamma function.

Derivative[1,0][Gamma] = Plus

D understands this perfectly, but Derivative not at all.

D[f[x,y],x]

1 + x + y

Derivative[1,0][f]

(1,0)
f

Derivative[1,0][f][x,y]

(1,0)
f     [x, y]

What's the difference here? How can I get Derivative to use my
definition?

Thanks for any help.

E. Neely Atkinson
AN123651 at UTHVM1.BITNET
neely at mdaali.cancer.utexas.edu

```

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