Editing Constructible number (section)

==Trigonometric numbers==
{{main|Trigonometric number}}
Trigonometric numbers are the cosines or sines of angles that are rational multiples of <math>\pi</math>. These numbers are always algebraic, but they may not be constructible. The cosine or sine of the angle <math>2\pi/n</math> is constructible only for certain special numbers <math>n</math>:{{sfnp|Martin|1998|p=46}}
*The [[power of two|powers of two]]
*The [[Fermat prime]]s, prime numbers that are one plus a power of two
*The products of powers of two and any number of distinct Fermat primes.
Thus, for example, <math>\cos(\pi/15)</math> is constructible because 15 is the product of the Fermat primes 3 and 5; but <math>\cos(\pi/9)</math> is not constructible (not being the product of {{em|distinct}} Fermat primes) and neither is <math>\cos(\pi/7)</math> (being a non-Fermat prime).