A DYNAMICAL SYSTEM PROOF OF NIVEN'S THEOREM AND ITS EXTENSIONS.

Niven's theorem asserts that $\{\cos (r\pi) \mid r\in \mathbb {Q}\}\cap \mathbb {Q}=\{0,\pm 1,\pm 1/2\}.$ In this paper, we use elementary techniques and results from arithmetic dynamics to obtain an algorithm for classifying all values in the set $\{\cos (r\pi) \mid r\in \mathbb {Q}\}\cap K$ , where K is an arbitrary number field. [ABSTRACT FROM AUTHOR]