Spaces

# Spaces

A Space is an abstract type whose subtypes indicate which space a function lives in. This typically corresponds to the span of a (possibly infinite) basis.

## Classical orthogonal polynomial spaces

Chebyshev, Ultraspherical, Jacobi, Hermite, and Laguerre are spaces corresponding to expansion in classical orthogonal polynomials.

Note that we always use the classical normalization: the basis are not orthonormal. This is because this normalization leads to rational recurrence relationships, which are more efficient than their normalized counterparts. See the Digital Library of Mathematical Functions for more information.

### Chebyshev space

The default space in ApproxFun is Chebyshev, which represents expansions in Chebyshev polynomials:

$f(x) = \sum_{k=0}^\infty f_k T_k(x)$

where $T_k(x) = \cos k \,{\rm acos} x$, which are orthogonal polynomials with respect to the weight ${1 \over \sqrt{1-x^2}} \qquad\hbox{for}\qquad -1 \leq x \leq 1.$ Note that there is an intrinsic link between Chebyshev and CosSpace:

$g(\theta) = f(\cos \theta) = \sum_{k=0}^\infty f_k \cos k \theta$

In other words:

julia> f=Fun(exp,Chebyshev());

julia> g=Fun(CosSpace(),f.coefficients); # specify the coefficients directly

julia> f(cos(0.1))
2.70473560723178

julia> g(0.1)
2.7047356072317794

### Ultraspherical spaces

A key tool for solving differential equations are the ultraspherical spaces, encoded as Ultraspherical(λ) for λ ≠ 0, which can be defined by the span of derivatives of Chebyshev polynomials, or alternatively as polynomials orthogonal with respect to the weight $(1-x^2)^{\lambda - 1/2} \qquad\hbox{for}\qquad -1 \leq x \leq 1.$

Note that Ultraspherical(1) corresponds to the Chebyshev basis of the second kind: $U_k(x) = {\sin (k+1) {\rm acos} x \over \sin {\rm acos} x}$. The relationship with Chebyshev polynomials follows from trigonemetric identities: $T_k'(x) = k U_{k-1}(x)$.

Converting between ultraspherical polynomials (with integer orders) is extremely efficient: it requires $O(n)$ operations, where $n$ is the number of coefficients.

### Jacobi spaces

Jacobi(b,a) represents the space spanned by the Jacobi polynomials, which are orthogonal polynomials with respect to the weight $(1+x)^b(1-x)^a$ using the standard normalization.

## Fourier and Laurent spaces

There are several different spaces to represent functions on periodic domains, which are typically a PeriodicSegment, Circle or PeriodicLine.

CosSpace represents expansion in cosine series:

$f(\theta) = \sum_{k=0}^\infty f_k \cos k \theta$

SinSpace represents expansion in sine series:

$f(\theta) = \sum_{k=0}^\infty f_k \sin (k+1) \theta$

Taylor represents expansion with only non-negative complex exponential terms:

$f(\theta) = \sum_{k=0}^\infty f_k {\rm e}^{{\rm i} k \theta}$

Hardy{false} represents expansion with only negative complex exponential terms:

$f(\theta) = \sum_{k=0}^\infty f_k {\rm e}^{-{\rm i} (k+1) \theta}$

Fourier represents functions that are sums of sines and cosines. Note that if a function has the form

$f(\theta) = f_0 + \sum_{k=1}^\infty f_k^{\rm c} \cos k \theta + f_k^{\rm s} \sin k\theta$

then the coefficients of the resulting Fun are order as $[f_0,f_1^{\rm s},f_1^{\rm c},…]$. For example:

julia> f = Fun(Fourier(),[1,2,3,4]);

julia> f(0.1)
4.979356652307978

julia> 1 + 2sin(0.1) + 3cos(0.1) + 4sin(2*0.1)
4.979356652307979

Laurent represents functions that are sums of complex exponentials. Note that if a function has the form

$f(\theta) = \sum_{k=-\infty}^\infty f_k {\rm e}^{{\rm i} k \theta}$

then the coefficients of the resulting Fun are order as $[f_0,f_{-1},f_1,…]$. For example:

julia> f = Fun(Laurent(),[1,2,3,4]);

julia> f(0.1)
9.895287137755096 - 0.694843906533417im

julia> 1 + 2exp(-im*0.1) + 3exp(im*0.1) + 4exp(-2im*0.1)
9.895287137755094 - 0.6948439065334167im

## Modifier spaces

Some spaces are built out of other spaces:

### JacobiWeight

JacobiWeight(β,α,space) weights space, which is typically Chebyshev() or Jacobi(b,a), by a Jacobi weight (1+x)^α*(1-x)^β: in other words, if the basis for space is $\psi_k(x)$ and the domain is the unit interval -1 .. 1, then the basis for JacobiWeight(β,α,space) is $(1+x)^α(1-x)^β \psi_k(x)$. If the domain is not the unit interval, then the basis is determined by mapping back to the unit interval: that is, if $M(x)$ is the map dictated by tocanonical(space, x), where the canonical domain is the unit interval, then the basis is $(1+M(x))^α(1-M(x))^β \psi_k(x)$. For example, if the domain is another interval a .. b, then $M(x) = {2x-b-a \over b-a}$ and the basis becomes $\left({2 \over (b-a)}\right)^{\alpha+\eta} (x-a)^α(b-x)^β \psi_k(x)$

### SumSpace

SumSpace((space_1,space_2,…,space_n)) represents the direct sum of the spaces, where evaluation is defined by adding up each component. A simple example is the following, showing that the coefficients are stored by interlacing:

julia> x = Fun(identity,-1..1);

julia> f = cos(x-0.1)*sqrt(1-x^2) + exp(x);

julia> space(f) # isa SumSpace
(1-x^2)^0.5[Chebyshev(【-1.0,1.0】)]⊕Chebyshev(【-1.0,1.0】)

julia> a, b = components(f);

julia> a(0.2) # cos(0.2-0.1)*sqrt(1-0.2^2)
0.9749009987500246

julia> b(0.2) # exp(0.2)
1.2214027581601699

julia> f(0.2) # a(0.2) + b(0.2)
2.1963037569101944

julia> norm(f.coefficients[1:2:end] - a.coefficients)
0.0

julia> norm(f.coefficients[2:2:end] - b.coefficients)
0.0

More complicated examples may interlace the coefficients using a different strategy. Note that it is difficult to represent the first component of function $f$ by a Chebyshev series because the derivatives of $f$ at its boundaries blow up, whereas the derivative of a polynomial is a polynomial.

Note that Fourier and Laurent are currently implemented as SumSpace, but this may change in the future.

### PiecewiseSpace

PiecewiseSpace((space_1,space_2,…,space_n)) represents the direct sum of the spaces, where evaluation is defined in a piecewise way. A simple example is the following:

julia> x = Fun(Domain(-1 .. 0) ∪ Domain( 1 .. 2));

julia> f = exp(x);

julia> a, b = components(f);

julia> f(-0.5) - a(-0.5)
0.0

julia> f(1.5) - b(1.5)
0.0

julia> f(0.5) # outside domain components
0.0

julia> norm(f.coefficients[2:2:end] - b.coefficients)
0.0

julia> norm(f.coefficients[1:2:end] - a.coefficients)
0.0

More complicated examples may interlace the coefficients using a different strategy.

### ArraySpace

ArraySpace(::Array{<:Space}) represents the direct sum of the spaces, where evaluation is defined in an array-wise manner. A simple example is the following:

julia> x = Fun(identity,-1..1);

julia> f = [exp(x); sqrt(1-x^2)*cos(x-0.1)];

julia> space(f)
2-element ArraySpace:
Chebyshev(【-1.0,1.0】)
(1-x^2)^0.5[Chebyshev(【-1.0,1.0】)]

julia> a, b = components(f);

julia> norm(f(0.5) - [a(0.5); b(0.5)])
0.0

julia> norm(f.coefficients[1:2:end] - a.coefficients)
0.0

julia> norm(f.coefficients[2:2:end] - b.coefficients)
0.0

More complicated examples may interlace the coefficients using a different strategy.

### TensorSpace

TensorSpace((space_1,space_2)) represents the tensor product of two spaces. See documentation of TensorSpace for more details on how the coefficients are interlaced. Note that more than two spaces is only partially supported.

## Unset space

UnsetSpace is a special space that is used as a stand in when a space has not yet been determined, particularly by operators.