In the mathematical theory of dynamical systems, an irrational rotation is a map where θ is an irrational number. Under the identification of a circle with R/Z, or with the interval [0, 1] with the boundary points glued together, this map becomes a rotation of a circle by a proportion θ of a full revolution (i.e., an angle of 2πθ radians). Since θ is irrational, the rotation has infinite order in the circle group and the map Tθ has no periodic orbits. Alternatively, we can use multiplicative notation for an irrational rotation by introducing the map . It can be shown that φ is an isometry.