global name 'sqrt' is not defined

[Rob]

> Any ideas?  If I do something like "import math" in the subfunction, 
> then the error changes to "global name 'math' is not defined".

Presumably ipython does an import so you need to import it yourself
something like:

from math import sqrt

at the top of your source file.  I don't know why math would not be
defined, it's built in -- probably a scoping logic problem in your code. 
Put the form/import statement at the top of your module file (as is
usually done), not inside a function or other block.

Rob