# Circle

Manifolds.Circle โ Type
Circle{๐ฝ} <: AbstractManifold{๐ฝ}

The circle $๐^1$ is a manifold here represented by real-valued points in $[-ฯ,ฯ)$ or complex-valued points $z โ โ$ of absolute value $\lvert z\rvert = 1$.

Constructor

Circle(๐ฝ=โ)

Generate the โ-valued Circle represented by angles, which alternatively can be set to use the AbstractNumbers ๐ฝ=โ to obtain the circle represented by โ-valued circle of unit numbers.

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Base.exp โ Method
exp(M::Circle, p, X)

Compute the exponential map on the Circle.

$$$\exp_p X = (p+X)_{2ฯ},$$$

where $(โ )_{2ฯ}$ is the (symmetric) remainder with respect to division by $2ฯ$, i.e. in $[-ฯ,ฯ)$.

For the complex-valued case, the same formula as for the Sphere $๐^1$ is applied to values in the complex plane.

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Base.log โ Method
log(M::Circle, p, q)

Compute the logarithmic map on the Circle M.

$$$\log_p q = (q-p)_{2ฯ},$$$

where $(โ )_{2ฯ}$ is the (symmetric) remainder with respect to division by $2ฯ$, i.e. in $[-ฯ,ฯ)$.

For the complex-valued case, the same formula as for the Sphere $๐^1$ is applied to values in the complex plane.

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Base.rand โ Method
Random.rand(M::Circle{โ}; vector_at = nothing, ฯ::Real=1.0)

If vector_at is nothing, return a random point on the Circle $\mathbb S^1$ by picking a random element from $[-\pi,\pi)$ uniformly.

If vector_at is not nothing, return a random tangent vector from the tangent space of the point vector_at on the Circle by using a normal distribution with mean 0 and standard deviation ฯ.

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Manifolds.sym_rem โ Method
sym_rem(x,[T=ฯ])

Compute symmetric remainder of x with respect to the interall 2*T, i.e. (x+T)%2T, where the default for T is $ฯ$

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ManifoldsBase.check_point โ Method
check_point(M::Circle, p)

Check whether p is a point on the Circle M. For the real-valued case, p is an angle and hence it checks that $p โ [-ฯ,ฯ)$. for the complex-valued case, it is a unit number, $p โ โ$ with $\lvert p \rvert = 1$.

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ManifoldsBase.check_vector โ Method
check_vector(M::Circle, p, X; kwargs...)

Check whether X is a tangent vector in the tangent space of p on the Circle M. For the real-valued case represented by angles, all X are valid, since the tangent space is the whole real line. For the complex-valued case X has to lie on the line parallel to the tangent line at p in the complex plane, i.e. their inner product has to be zero.

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ManifoldsBase.distance โ Method
distance(M::Circle, p, q)

Compute the distance on the Circle M, which is the absolute value of the symmetric remainder of p and q for the real-valued case and the angle between both complex numbers in the Gaussian plane for the complex-valued case.

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ManifoldsBase.inner โ Method
inner(M::Circle, p, X, Y)

Compute the inner product of the two tangent vectors X,Y from the tangent plane at p on the Circle M using the restriction of the metric from the embedding, i.e.

$$$g_p(X,Y) = X*Y$$$

for the real case and

$$$g_p(X,Y) = Y^\mathrm{T}X$$$

for the complex case interpreting complex numbers in the Gaussian plane.

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ManifoldsBase.parallel_transport_to โ Method
 parallel_transport_to(M::Circle, p, X, q)

Compute the parallel transport of X from the tangent space at p to the tangent space at q on the Circle M. For the real-valued case this results in the identity. For the complex-valued case, the formula is the same as for the Sphere(1) in the complex plane.

$$$\mathcal P_{qโp} X = X - \frac{โจ\log_p q,Xโฉ_p}{d^2_{โ}(p,q)} \bigl(\log_p q + \log_q p \bigr),$$$

where log denotes the logarithmic map on M.

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ManifoldsBase.project โ Method
project(M::Circle, p, X)

Project a value X onto the tangent space of the point p on the Circle M.

For the real-valued case this is just the identity. For the complex valued case X is projected onto the line in the complex plane that is parallel to the tangent to p on the unit circle and contains 0.

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ManifoldsBase.project โ Method
project(M::Circle, p)

Project a point p onto the Circle M. For the real-valued case this is the remainder with respect to modulus $2ฯ$. For the complex-valued case the result is the projection of p onto the unit circle in the complex plane.

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Statistics.mean โ Method
mean(M::Circle{โ}, x::AbstractVector[, w::AbstractWeights])

Compute the Riemannian mean of x of points on the Circle $๐^1$, represented by complex numbers, i.e. embedded in the complex plane. Comuting the sum

$$$s = \sum_{i=1}^n x_i$$$

the mean is the angle of the complex number $s$, so represented in the complex plane as $\frac{s}{\lvert s \rvert}$, whenever $s \neq 0$.

If the sum $s=0$, the mean is not unique. For example for opposite points or equally spaced angles.

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Statistics.mean โ Method
mean(M::Circle{โ}, x::AbstractVector[, w::AbstractWeights])

Compute the Riemannian mean of x of points on the Circle $๐^1$, represented by real numbers, i.e. the angular mean

$$$\operatorname{atan}\Bigl( \sum_{i=1}^n w_i\sin(x_i), \sum_{i=1}^n w_i\sin(x_i) \Bigr).$$$
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