In response to my remark about the need of feedback in higher
order filters,
Magnus Danielson wrote:
> But, this feedback hardly renders a Butterworth filter.

I stand corrected.  I wrote a little bit in haste, and it has been a 
long time ago since I dealt with filters (I still owe the Csound
community an introduction of myself).  I was also wrong about
cascading filters.  I was thinking of cascading a series of
filters that are exactly the same, but that is of course not how
it is done.  

In the meantime I found yet another useful book (which I
actually have in my collection):

   "Elements of Computer Music" by F. Richard Moore (Prentice Hall,
   1990)

It includes a section (Sec 2.4) about digital filters with a 
C program to calculate poles and zeros.

I must say I rather enjoy these discussions, and I don't think this
list is sleepy at all, David!