Look a little closer, it's not Z, but _Z (the underscore is part of the name). And, this expression appears in the argument to the RootOf command. RootOf gives many users some trouble, until they understand that RootOf is a way to refer to solutions (specific or general) of a univariate equation and _Z is the generic name of the variable. For example:

z := RootOf(x^2+1=0);
                                     /  2    \
                               RootOf\_Z  + 1/
z^4;
                                             4
                                    /  2    \ 
                              RootOf\_Z  + 1/ 
simplify( % );
                                      1

a := RootOf(x^2+1=0,index=1);
b := RootOf(x^2+1=0,index=2);
                               /  2               \
                         RootOf\_Z  + 1, index = 1/
                               /  2               \
                         RootOf\_Z  + 1, index = 2/
evalc([a,b]);
                                   [I, -I]

In your case, the argument to RootOf is much more complicated. But, it's the same idea being used.

Doug

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Douglas B. Meade  <><
Math, USC, Columbia, SC 29208  E-mail: mailto:meade@math.sc.edu       
Phone:  (803) 777-6183         URL:    http://www.math.sc.ed