A Julia package for describing domains in Euclidean space



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DomainSets.jl is a package designed to represent simple infinite sets, that can be used to encode domains of functions. For example, the domain of the function log(x::Float64) is the infinite open interval, which is represented by the type HalfLine{Float64}().



DomainSets.jl uses IntervalSets.jl for closed and open intervals. In addition, it defines a few standard intervals.

julia> using DomainSets, StaticArrays

julia> UnitInterval()
0.0..1.0 (Unit)

julia> ChebyshevInterval()
-1.0..1.0 (Chebyshev)

julia> HalfLine()
0.0..Inf (closed–open) (HalfLine)


Rectangles can be constructed as a product of intervals, where the elements of the domain are SVector{2}:

julia> using DomainSets: ×

julia> (-1..1) × (0..3) × (4.0..5.0)
(-1.0..1.0) × (0.0..3.0) × (4.0..5.0)

julia> SVector(1,2) in (-1..1) × (0..3)

julia> UnitInterval()^3

Circles and Spheres

A UnitSphere contains x if norm(x) == 1. The unit sphere is N-dimensional, and its dimension is specified with the constructor. The element types are SVector{N,T} when the dimension is specified as Val(3), and they are Vector{T} when the dimension is specified by an integer value instead:

julia> SVector(0,0,1.0) in UnitSphere(Val(3))

julia> [0.0,1.0,0.0,0.0] in UnitSphere(4)

UnitSphere itself is an abstract type, hence the examples above return concrete types <:UnitSphere. The intended element type can also be explicitly specified with the UnitSphere{T} constructor:

julia> typeof(UnitSphere{SVector{3,BigFloat}}())
EuclideanUnitSphere{3, BigFloat} (alias for StaticUnitSphere{SArray{Tuple{3}, BigFloat, 1, 3}})

julia> typeof(UnitSphere{Vector{Float32}}(6))
VectorUnitSphere{Float32} (alias for DynamicUnitSphere{Array{Float32, 1}})

Without arguments, UnitSphere() defaults to a 3D domain with SVector{3,Float64} elements. Similarly, there is a special case UnitCircle in 2D:

julia> SVector(1,0) in UnitCircle()

Disks and Balls

A UnitBall contains x if norm(x) ≤ 1. As with UnitSphere, the dimension is specified via the constructor by type or by value:

julia> SVector(0.1,0.2,0.3) in UnitBall(Val(3))

julia> [0.1,0.2,0.3,-0.1] in UnitBall(4)

By default N=3, but UnitDisk is a special case in 2D, and so are ComplexUnitDisk and ComplexUnitCircle in the complex plane:

julia> SVector(0.1,0.2) in UnitDisk()

julia> 0.5+0.2im ∈ ComplexUnitDisk()

UnitBall itself is an abstract type, hence the examples above return concrete types <:UnitBall. The types are similar to those associated with UnitSphere. Like intervals, balls can also be open or closed:

julia> EuclideanUnitBall{3,Float64,:open}()
the 3-dimensional open unit ball

Product domains

The cartesian product of domains is constructed with the ProductDomain or ProductDomain{T} constructor. This abstract constructor returns concrete types best adapted to the arguments given.

If T is not given, ProductDomain makes a suitable choice based on the arguments. If all arguments are Euclidean, i.e., their element types are numbers or static vectors, then the product is a Euclidean domain as well:

julia> ProductDomain(0..2, UnitCircle())
0.0..2.0 x the unit circle

julia> eltype(ans)
SVector{3, Float64} (alias for SArray{Tuple{3}, Float64, 1, 3})

The elements of the interval and the unit circle are flattened into a single vector, much like the vcat function. The result is a VcatDomain.

If a Vector of domains is given, the element type is a Vector as well:

julia> 1:5 in ProductDomain([0..i for i in 1:5])

In other cases, the points are concatenated into a tuple and membership is evaluated element-wise:

julia> ("a", 0.4) ∈ ProductDomain(["a","b"], 0..1)

Some arguments are recognized and return a more specialized product domain. Examples are the unit box and more general hyperrectangles:

julia> ProductDomain(UnitInterval(), UnitInterval())
0.0..1.0 (Unit) x 0.0..1.0 (Unit)

julia> ProductDomain(0..2, 4..5, 6..7.0)
0.0..2.0 x 4.0..5.0 x 6.0..7.0

julia> typeof(ans)
Rectangle{SVector{3, Float64}}

Union, intersection, and setdiff of domains

Domains can be unioned and intersected together:

julia> d = UnitCircle() ∪ 2UnitCircle();

julia> in.([SVector(1,0),SVector(0,2), SVector(1.5,1.5)], d)
3-element BitArray{1}:

julia> d = UnitCircle() ∩ (2UnitCircle() .+ SVector(1.0,0.0))
the intersection of 2 domains:
    1.  : the unit circle
    2.  : A mapped domain based on the unit circle

julia> SVector(1,0) in d

julia> SVector(-1,0) in d

Level sets

A domain can be defined by the level sets of a function. The domains of all points [x,y] for which x*y = 1 or x*y >= 1 are represented as follows:

julia> d = LevelSet{SVector{2,Float64}}(prod, 1.0)
level set f(x) = 1.0 with f = prod

julia> [0.5,2] ∈ d

julia> SuperlevelSet{SVector{2,Float64}}(prod, 1.0)
superlevel set f(x) >= 1.0 with f = prod

There is also SublevelSet, and there are the special cases ZeroSet, SubzeroSet and SuperzeroSet.

Indicator functions

A domain can be defined by an indicator function or a characteristic function. This is a function f(x) which evaluates to true or false, depending on whether or not the point x belongs to the domain.

julia> d = IndicatorFunction{Float64}( t ->  cos(t) > 0)
indicator domain defined by function f = #5

julia> 0.5 ∈ d, 3.1 ∈ d
(true, false)

This enables generator syntax to define domains:

julia> d = Domain(x>0 for x in -1..1)
indicator function bounded by: -1..1

julia> 0.5 ∈ d, -0.5 ∈ d
(true, false)

julia> d = Domain( x*y > 0 for (x,y) in UnitDisk())
indicator function bounded by: the 2-dimensional closed unit ball

julia> [0.2, 0.3] ∈ d, [0.2, -0.3] ∈ d
(true, false)

julia> d = Domain( x+y+z > 0 for (x,y,z) in ProductDomain(UnitDisk(), 0..1))
indicator function bounded by: the 2-dimensional closed unit ball x 0..1

julia> [0.3,0.2,0.5] ∈ d

The domain interface

A domain is any type that implements the functions eltype and in. If d is an instance of a type that implements the domain interface, then the domain consists of all x that is an eltype(d) such that x in d returns true.

Domains often represent continuous mathematical domains, for example, a domain d representing the interval [0,1] would have eltype(d) == Int but still have 0.2 in d return true.

The Domain type

DomainSets.jl contains an abstract type Domain{T}. All subtypes of Domain{T} must implement the domain interface, and in addition support convert(Domain{T}, d).

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