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Irrational rotation

Rotation of a circle by an angle of π times an irrational number

In the mathematical theory of dynamical systems, an irrational rotation is a map

${\displaystyle T_{\theta }:[0,1]\rightarrow [0,1],\quad T_{\theta }($x)\triangleq x+\theta \mod 1,}

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 T_θ has no periodic orbits.

Alternatively, we can use multiplicative notation for an irrational rotation by introducing the map

${\displaystyle T_{\theta }:S^{1}\to S^{1},\quad \quad \quad T_{\theta }($x)=xe^{2\pi i\theta }}

The relationship between the additive and multiplicative notations is the group isomorphism

${\displaystyle \varphi :([0,1],+)\to (S^{1},\cdot )\quad \varphi ($x)=xe^{2\pi i\theta }} .

It can be shown that φ is an isometry.

There is a strong distinction in circle rotations that depends on whether θ is rational or irrational. Rational rotations are less interesting examples of dynamical systems because if ${\displaystyle \theta ={\frac {a}{b}}}$ and ${\displaystyle \gcd(a,b)=1}$, then ${\displaystyle T_{\theta }^{b}($x)=x} when ${\displaystyle x\in [0,1]}$. It can also be shown that ${\displaystyle T_{\theta }^{i}($x)\neq x} when ${\displaystyle 1\leq i<b}$.

Significance[edit]

Irrational rotations form a fundamental example in the theory of dynamical systems. According to the Denjoy theorem, every orientation-preserving C²-diffeomorphism of the circle with an irrational rotation number θistopologically conjugatetoT_θ. An irrational rotation is a measure-preserving ergodic transformation, but it is not mixing. The Poincaré map for the dynamical system associated with the Kronecker foliation on a torus with angle θ> is the irrational rotation by θ. C*-algebras associated with irrational rotations, known as irrational rotation algebras, have been extensively studied.

Properties[edit]

• Ifθ is irrational, then the orbit of any element of [0, 1] under the rotation T_θisdensein[0, 1]. Therefore, irrational rotations are topologically transitive.
• Irrational (and rational) rotations are not topologically mixing.
• Irrational rotations are uniquely ergodic, with the Lebesgue measure serving as the unique invariant probability measure.
• Suppose [a, b] ⊂ [0, 1]. Since T_θ is ergodic,
  ${\displaystyle {\text{lim}}_{N\to \infty }{\frac {1}{N}}\sum _{n=0}^{N-1}\chi _{[a,b)}(T_{\theta }^{n}($t))=b-a} .

Generalizations[edit]

• Circle rotations are examples of group translations.
• For a general orientation preserving homomorphism fofS¹ to itself we call a homeomorphism ${\displaystyle F:\mathbb {R} \to \mathbb {R} }$aliftoffif${\displaystyle \pi \circ F=f\circ \pi }$ where ${\displaystyle \pi ($t)=t{\bmod {1}}} .^[1]
• The circle rotation can be thought of as a subdivision of a circle into two parts, which are then exchanged with each other. A subdivision into more than two parts, which are then permuted with one-another, is called an interval exchange transformation.
• Rigid rotations of compact groups effectively behave like circle rotations; the invariant measure is the Haar measure.

Applications[edit]

• Skew Products over Rotations of the Circle: In 1969^[2] William A. Veech constructed examples of minimal and not uniquely ergodic dynamical systems as follows: "Take two copies of the unit circle and mark off segment J of length 2πα in the counterclockwise direction on each one with endpoint at 0. Now take θ irrational and consider the following dynamical system. Start with a point p, say in the first circle. Rotate counterclockwise by 2πθ until the first time the orbit lands in J; then switch to the corresponding point in the second circle, rotate by 2πθ until the first time the point lands in J; switch back to the first circle and so forth. Veech showed that if θ is irrational, then there exists irrational α for which this system is minimal and the Lebesgue measure is not uniquely ergodic."^[3]

See also[edit]

• Bernoulli map
• Modular arithmetic
• Siegel disc
• Toeplitz algebra
• Phase locking (circle map)
• Weyl sequence

References[edit]

Further reading[edit]

• C. E. Silva, Invitation to ergodic theory, Student Mathematical Library, vol 42, American Mathematical Society, 2008 ISBN 978-0-8218-4420-5