are quadratic integer s. Gaussian integer s: those complex numbers a+bi where both a and b are integers are also quadratic integers. Trigonometric functions of rational multiples of \pi (except when undefined). For example, each of cos(\pi/7), cos(3\pi/7), cos(5\pi/7) satisfies 8x^3 - 4x^2 - 4x + 1. This polynomial is irreducible