Hyperplane

Hyperplane{N, VN<:AbstractVector{N}} <: AbstractPolyhedron{N}

Type that represents a hyperplane of the form $a⋅x = b$.

Fields

• a – normal direction (non-zero)
• b – constraint

Examples

The line $y = 0$ in 2D:

julia> Hyperplane([0, 1.], 0.)
Hyperplane{Float64, Vector{Float64}}([0.0, 1.0], 0.0)

convert(::Type{Hyperplane{N}}, expr::Expr; vars=Vector{Basic}=Basic[]) where {N}

Return a LazySet.Hyperplane given a symbolic expression that represents a hyperplane.

Input

• expr – a symbolic expression
• vars – (optional, default: Basic[]): set of variables with respect to which the gradient is taken; if empty, we take the free symbols in the given expression

Output

A Hyperplane, in the form ax = b.

Examples

julia> convert(Hyperplane, :(x1 = -0.03))
Hyperplane{Float64, Vector{Float64}}([1.0], -0.03)

julia> convert(Hyperplane, :(x1 + 0.03 = 0))
Hyperplane{Float64, Vector{Float64}}([1.0], -0.03)

julia> convert(Hyperplane, :(x1 + x2 = 2*x4 + 6))
Hyperplane{Float64, Vector{Float64}}([1.0, 1.0, -2.0], 6.0)

You can also specify the set of "ambient" variables in the hyperplane, even if not all of them appear:

julia> using SymEngine: Basic

julia> convert(Hyperplane, :(x1 + x2 = 2*x4 + 6), vars=Basic[:x1, :x2, :x3, :x4])
Hyperplane{Float64, Vector{Float64}}([1.0, 1.0, 0.0, -2.0], 6.0)

an_element(X::LazySet)

Return some element of a nonempty set.

Input

• X – set

Output

An element of X unless X is empty.

Extended help

an_element(H::Hyperplane)

Algorithm

We compute a point on the hyperplane $a⋅x = b$ as follows:

• We first find a nonzero entry of $a$ in dimension, say, $i$.
• We set $x[i] = b / a[i]$.
• We set $x[j] = 0$ for all $j ≠ i$.

constrained_dimensions(H::Hyperplane)

Return the dimensions in which a hyperplane is constrained.

Input

• H – hyperplane

Output

A vector of ascending indices i such that the hyperplane is constrained in dimension i.

Examples

A 2D hyperplane with constraint $x_1 = 0$ is constrained in dimension 1 only.

isbounded(X::LazySet)

Check whether a set is bounded.

Input

• X – set

Output

true iff the set is bounded.

Notes

See also isboundedtype(::Type{<:LazySet}).

Extended help

isbounded(H::Hyperplane)

Algorithm

The result is true iff H is one-dimensional.

isuniversal(X::LazySet, witness::Bool=false)

Check whether a set is universal.

Input

• X – set
• witness – (optional, default: false) compute a witness if activated

Output

• If the witness option is deactivated: true iff $X = ℝ^n$
• If the witness option is activated:
  • (true, []) iff $X = ℝ^n$
  • (false, v) iff $X ≠ ℝ^n$ for some $v ∉ X$

Extended help

isuniversal(H::Hyperplane, [witness]::Bool=false)

Algorithm

A witness is produced by adding the normal vector to an element on the hyperplane.

normalize(H::Hyperplane{N}, p::Real=N(2)) where {N}

Normalize a hyperplane.

Input

• H – hyperplane
• p – (optional, default: 2) norm

Output

A new hyperplane whose normal direction $a$ is normalized, i.e., such that $‖a‖_p = 1$ holds.

rand(T::Type{<:LazySet}; [N]::Type{<:Real}=Float64, [dim]::Int=2,
     [rng]::AbstractRNG=GLOBAL_RNG, [seed]::Union{Int, Nothing}=nothing
    )

Create a random set of the given set type.

Input

• T – set type
• N – (optional, default: Float64) numeric type
• dim – (optional, default: 2) dimension
• rng – (optional, default: GLOBAL_RNG) random number generator
• seed – (optional, default: nothing) seed for reseeding

Output

A random set of the given set type.

Extended help

rand(::Type{Hyperplane}; [N]::Type{<:Real}=Float64, [dim]::Int=2,
     [rng]::AbstractRNG=GLOBAL_RNG, [seed]::Union{Int, Nothing}=nothing)

Algorithm

All numbers are normally distributed with mean 0 and standard deviation 1. Additionally, the constraint a is nonzero.

∈(x::AbstractVector, X::LazySet)

Check whether a point lies in a set.

Input

• x – point/vector
• X – set

Output

true iff $x ∈ X$.

Extended help

∈(x::AbstractVector, H::Hyperplane)

Algorithm

We just check whether $x$ satisfies $a⋅x = b$.

project(x::AbstractVector, H::Hyperplane)

Project a point onto a hyperplane.

Input

• x – point
• H – hyperplane

Output

The projection of x onto H.

Algorithm

The projection of $x$ onto the hyperplane of the form $a⋅x = b$ is

\[ x - \dfrac{a (a⋅x - b)}{‖a‖²}\]

reflect(x::AbstractVector, H::Hyperplane)

Reflect (mirror) a vector in a hyperplane.

Input

• x – point/vector
• H – hyperplane

Output

The reflection of x in H.

Algorithm

The reflection of a point $x$ in the hyperplane $a ⋅ x = b$ is

\[ x − 2 \frac{x ⋅ a − b}{a ⋅ a} a\]

where $u · v$ denotes the dot product.

ρ(d::AbstractVector, X::LazySet)

Evaluate the support function of a set in a given direction.

Input

• d – direction
• X – set

Output

The evaluation of the support function of X in direction d.

Notes

The convenience alias support_function is also available.

We have the following identity based on the support vector $σ$:

\[ ρ(d, X) = d ⋅ σ(d, X)\]

Extended help

ρ(d::AbstractVector, H::Hyperplane)

Output

If the set is unbounded in the given direction, the result is Inf.

σ(d::AbstractVector, X::LazySet)

Compute a support vector of a set in a given direction.

Input

• d – direction
• X – set

Output

A support vector of X in direction d.

Notes

The convenience alias support_vector is also available.

Extended help

σ(d::AbstractVector, H::Hyperplane)

Output

A support vector in the given direction, which is only defined in the following two cases:

1. The direction has norm zero.
2. The direction is the hyperplane's normal direction or its opposite direction.

In all cases, any point on the hyperplane is a solution. Otherwise this function throws an error.

translate(X::LazySet, v::AbstractVector)

Compute the translation of a set with a vector.

Input

• X – set
• v – vector

Output

A set representing $X + \{v\}$.

Extended help

translate(H::Hyperplane, v::AbstractVector; share::Bool=false)

Notes

The normal vector of the hyperplane (vector $a$ in $a⋅x = b$) is shared with the original hyperplane if share == true.

Algorithm

A hyperplane $a⋅x = b$ is transformed to the hyperplane $a⋅x = b + a⋅v$. In other words, we add the dot product $a⋅v$ to $b$.

_ishyperplanar(expr::Expr)

Determine whether the given expression corresponds to a hyperplane.

Input

• expr – a symbolic expression

Output

true if expr corresponds to a half-space or false otherwise.

Examples

julia> using LazySets: _ishyperplanar

julia> _ishyperplanar(:(x1 = 0))
true

julia> _ishyperplanar(:(x1 <= 0))
false

julia> _ishyperplanar(:(2*x1 = 4))
true

julia> _ishyperplanar(:(6.1 = 5.3*f - 0.1*g))
true

julia> _ishyperplanar(:(2*x1^2 = 4))
false

julia> _ishyperplanar(:(x1^2 = 4*x2 - x3))
false

julia> _ishyperplanar(:(x1 = 4*x2 - x3))
true