The computations of zeros graphed in our figures were performed
in double precision (approx. 18 decimal places) on a Silicon
Graphics workstation.
Some of the zeros were checked for accuracy by recomputing them
in double precision (approx. 28 decimal places) on a Cray X-MP.
The zero-finding program used the Jenkins-Traub algorithm and
was taken from a standard subroutine library.
Checks showed that the values that were obtained were accurate
on average to at least 10 decimal places, which was sufficient
for our graphs.
The program that was used appeared to produce accurate values on
the Cray for the zeros
for polynomials of degrees up to about 150.
(Computation of zeros of polynomials of much higher degree would
have required better algorithms, cf. [9].)