[SciPy-user] Derivative() usage?

Robert Kern rkern at ucsd.edu
Wed Nov 3 12:53:59 CST 2004

Giovanni Samaey wrote:
>> 'n' as in d^n/dx^n .
>> Somewhat confusingly, 'N' is being used in the docstring as a synonym 
>> for 'order' and is the number of discrete points used to evaluate the 
>> numerical derivative. I'm going to fix that.
>> For example, when n=2, and order=3, one is computing the second 
>> central derivative using 3 points [x0-dx, x0, x0+dx].
> Normally, one would expect "order" to be an indication of the accuracy 
> of the derivative.  So, order=p should mean
> d^n/dx^n (exact answer) - (what's computed) = O(dx^p) where dx is the 
> mesh spacing.
> Clearly, with three points for the second derivative, the order is only 
> two, so this is clearly not a mathematically correct use of the word 
> "order"...

Not really. "Order" is an incredibly overloaded word in this area. Its 
usage here is one of several acceptable ones. "numpoints" may be 
clearer, though.

Robert Kern
rkern at ucsd.edu

"In the fields of hell where the grass grows high
  Are the graves of dreams allowed to die."
   -- Richard Harter

More information about the SciPy-user mailing list