Dear GAP-forum,

Sebastian Egner asked about a function to test whether a given cyclotomic
field element is a root of unity.

The following article by my colleagues at Bath
is highly relevant:

"Effective Tests for Cyclotomic Polynomials," in Proceedings of
ISSAC 1988, Springer LNCS no.~368, P.~Gianni (Ed).
(R..J.~Bradford and J.H.~Davenport) MR 90m:11201 (11Y16; 11Y40). (1989)

Geoff Smith