Re: Generation of permutations

[dewar@gnat.com (Robert Dewar)]

> Actually I am interested where you picked up the phrase 
> "recursively undecidable".  A better phrase would be "not 
> recursive" since we are referring to recursive languages.  

It's a very standard phrase in my vocabulary. And I did not
invent it. To pick up one quick reference from many, the
following is the statement of Richardson's Theorem from
Eric Weisstein's world of mathematics:

Let R be the class of expressions generated by 

1. The rational numbers and the two real
   numbers pi and ln 2, 
2. The variable x, 
3. The operations of addition, multiplication,
   and composition, and 
4. The sine, exponential, and absolute
   value functions. 

Then if E in R, the predicate "E = 0"
is recursively undecidable. 

(I must say this thread has gone splendidly off topic,
even for CLA :-)