`Copulas.AMHGenerator`

— Type`AMHGenerator{T}`

Fields:

- θ::Real - parameter

Constructor

```
AMHGenerator(θ)
AMHCopula(d,θ)
```

The AMH copula in dimension $d$ is parameterized by $\theta \in [-1,1)$. It is an Archimedean copula with generator :

\[\phi(t) = 1 - \frac{1-\theta}{e^{-t}-\theta}\]

It has a few special cases:

- When θ = 0, it is the IndependentCopula

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.ArchimedeanCopula`

— Type`ArchimedeanCopula{d, TG}`

Fields: - G::TG : the generator <: Generator.

Constructor:

`ArchimedeanCopula(d::Int,G::Generator)`

For some Archimedean `Generator`

`G::Generator`

and some dimenson `d`

, this class models the archimedean copula which has this generator. The constructor checks for validity by ensuring that `max_monotony(G) ≥ d`

. The $d$-variate archimedean copula with generator $\phi$ writes:

\[C(\mathbf u) = \phi^{-1}\left(\sum_{i=1}^d \phi(u_i)\right)\]

The default sampling method is the Radial-simplex decomposition using the Williamson transformation of $\phi$.

There exists several known parametric generators that are implement in the package. For every `NamedGenerator <: Generator`

implemented in the package, we provide a type alias ``NamedCopula{d,...} = ArchimedeanCopula{d,NamedGenerator{...}}`

to be able to manipulate the classic archimedean copulas without too much hassle for known and usefull special cases.

A generic archimdean copula can be constructed as follows:

```
struct MyGenerator <: Generator end
ϕ(G::MyGenerator,t) = exp(-t) # your archimedean generator, can be any d-monotonous function.
max_monotony(G::MyGenerator) = Inf # could depend on generators parameters.
C = ArchimedeanCopula(d,MyGenerator())
```

The obtained model can be used as follows:

```
spl = rand(C,1000) # sampling
cdf(C,spl) # cdf
pdf(C,spl) # pdf
loglikelihood(C,spl) # llh
```

Bonus: If you know the Williamson d-transform of your generator and not your generator itself, you may take a look at `WilliamsonGenerator`

that implements them. If you rather know the frailty distribution, take a look at `WilliamsonFromFrailty`

.

References:

- williamson1955multiply Williamson, R. E. (1956). Multiply monotone functions and their Laplace transforms. Duke Math. J. 23 189–207. MR0077581
- mcneil2009 McNeil, A. J., & Nešlehová, J. (2009). Multivariate Archimedean copulas, d-monotone functions and ℓ 1-norm symmetric distributions.

`Copulas.ClaytonGenerator`

— Type`ClaytonGenerator{T}`

Fields:

- θ::Real - parameter

Constructor

```
ClaytonGenerator(θ)
ClaytonCopula(d,θ)
```

The Clayton copula in dimension $d$ is parameterized by $\theta \in [-1/(d-1),\infty)$. It is an Archimedean copula with generator :

\[\phi(t) = \left(1+\mathrm{sign}(\theta)*t\right)^{-1\frac{1}{\theta}}\]

It has a few special cases:

- When θ = -1/(d-1), it is the WCopula (Lower Frechet-Hoeffding bound)
- When θ = 0, it is the IndependentCopula
- When θ = ∞, is is the MCopula (Upper Frechet-Hoeffding bound)

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.EllipticalCopula`

— Type`EllipticalCopula{d,MT}`

This is an abstract type. It implements an interface for all Elliptical copulas. We construct internally elliptical copulas using the sklar's theorem, by considering the copula $C$ to be defined as :

\[C = F \circ (F_1^{-1},...,F_d^{-1}),\]

where $F$ and $F_1,...,F_d$ are respectively the multivariate distribution function of some elliptical random vector and the univariate distribution function of its marginals. For a type `MyCop <: EllipitcalCopula`

, it is necessary to implement the following methods:

`N(::Type{MyCOp})`

, returning the constructor of the elliptical random vector from its correlation matrix. For example,`N(GaussianCopula)`

simply returns`MvNormal`

from`Distributions.jl`

.`U(::Type{MyCOp})`

, returning the constructor for the univariate marginal, usually in standardized form. For example,`U(GaussianCopula)`

returns`Normal`

from`Distributions.jl`

.

From these two functions, the abstract type provides a fully functional copula.

**Details**

Recall the definition of spherical random vectors:

Definition (Spherical and elliptical random vectors):A random vector $\bm X$ is said to be spherical if for all orthogonal matrix $\bm A \in O_d(\mathbb R)$, $\bm A\bm X \sim \bm X$.For every matrix $\bm B$ and vector $\bm c$, the random vector $\bm B \bm X + \bm c$ is then said to be elliptical.

Recall that spherical random vectors are random vectors which characteristic functions (c.f.) only depend on the norm of their arguments. Indeed, for any $\bm A \in O_d(\mathbb R)$,

\[\phi(\bm t) = \mathbb E\left(e^{\langle \bm t, \bm X \rangle}\right)= \mathbb E\left(e^{\langle \bm t, \bm A\bm X \rangle}\right) = \mathbb E\left(e^{\langle \bm A\bm t, \bm X \rangle}\right) = \phi(\bm A\bm t).\]

We can therefore express this characteristic function as $\phi(\bm t) = \psi(\lVert \bm t \rVert_2^2)$, where $\psi$ is a function that characterizes the spherical family, called the *generator* of the family. Any characteristic function that can be expressed as a function of the norm of its argument is the characteristic function of a spherical random vector, since $\lVert \bm A \bm t \rVert_2 = \lVert \bm t \rVert_2$ for any orthogonal matrix $\bm A$.

However, note that this is not how the underlying code is working, we do not check for validity of the proposed generator (we dont even use it). You can construct such an elliptical family using simply Sklar:

```
struct MyElliptical{d,T} <: EllipticalCopula{d,T}
θ:T
end
U(::Type{MyElliptical{d,T}}) where {d,T} # Distribution of the univaraite marginals, Normal() for the Gaussian case.
N(::Type{MyElliptical{d,T}}) where {d,T} # Distribution of the mutlivariate random vector, MvNormal(C.Σ) for the Gaussian case.
```

These two functions are enough to implement the rest of the interface.

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.EmpiricalCopula`

— Type`EmpiricalCopula{d,MT}`

Fields:

- u::MT - the matrix of observations.

Constructor

`EmpiricalCopula(u;pseudos=true)`

The EmpiricalCopula in dimension $d$ is parameterized by a pseudo-data matrix which should have shape (d,N). Its expression is given as :

\[C(\mathbf x) = \frac{1}{N}\sum_{i=1}^n \mathbf 1_{\mathbf u_i \le \mathbf x}\]

This function is very practical, be be aware that this is not a true copula (since $\mathbf u$ are only pseudo-observations). The constructor allows you to pass dirctly pseudo-observations (the default) or will compute them for you. You can then compute the `cdf`

of the copula, and sample it through the standard interface.

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.FGMCopula`

— Type`FGMCopula{d,T}`

Fields:

- θ::Real - parameter

Constructor

`FGMCopula(d, θ)`

The Multivariate Farlie-Gumbel-Morgenstern (FGM) copula of dimension d has $2^d-d-1$ parameters $\theta$ and function

\[C(\boldsymbol{u})=\prod_{i=1}^{d}u_i \left[1+ \sum_{k=2}^{d}\sum_{1 \leq j_1 < \cdots < j_k \leq d} \theta_{j_1 \cdots j_k} \bar{u}_{j_1}\cdots \bar{u}_{j_k} \right],\]

where $\bar{u}=1-u$.

More details about Farlie-Gumbel-Morgenstern (FGM) copula are found in :

`Nelsen, Roger B. An introduction to copulas. Springer, 2006. Exercise 3.38.`

We use the stochastic representation of the copula to obtain random samples.

`Blier-Wong, C., Cossette, H., & Marceau, E. (2022). Stochastic representation of FGM copulas using multivariate Bernoulli random variables. Computational Statistics & Data Analysis, 173, 107506.`

It has a few special cases:

- When d=2 and θ = 0, it is the IndependentCopula.

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.FrankGenerator`

— Type`FrankGenerator{T}`

Fields:

- θ::Real - parameter

Constructor

```
FrankGenerator(θ)
FrankCopula(d,θ)
```

The Frank copula in dimension $d$ is parameterized by $\theta \in [-\infty,\infty)$. It is an Archimedean copula with generator :

\[\phi(t) = -\frac{\log\left(1+e^{-t}(e^{-\theta-1})\right)}{ heta}\]

It has a few special cases:

- When θ = -∞, it is the WCopula (Lower Frechet-Hoeffding bound)
- When θ = 1, it is the IndependentCopula
- When θ = ∞, is is the MCopula (Upper Frechet-Hoeffding bound)

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.GaussianCopula`

— Type`GaussianCopula{d,MT}`

Fields:

- Σ::MT - covariance matrix

Constructor

`GaussianCopula(Σ)`

The Gaussian Copula is the copula of a Multivariate normal distribution. It is constructed as:

\[C(\mathbf{x}; \boldsymbol{\Sigma}) = F_{\Sigma}(F_{\Sigma,i}^{-1}(x_i),i\in 1,...d)\]

where $F_{\Sigma}$ is a cdf of a gaussian random vector and $F_{\Sigma,i}$ is the ith marginal cdf, while $\Sigma$ is the covariance matrix.

It can be constructed in Julia via:

`C = GaussianCopula(Σ)`

You can sample it, compute pdf and cdf, or even fit the distribution via:

```
u = rand(C,1000)
Random.rand!(C,u) # other calling syntax for rng.
pdf(C,u) # to get the density
cdf(C,u) # to get the distribution function
Ĉ = fit(GaussianCopula,u) # to fit on the sampled data.
```

GaussianCopulas have a special case:

- When
`isdiag(Σ)`

, the constructor returns an`IndependentCopula(d)`

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.Generator`

— Type`Generator`

Abstract type. Implements the API for archimedean generators.

An Archimedean generator is simply a function $\phi :\mathbb R_+ \to [0,1]$ such that $\phi(0) = 1$ and $\phi(+\infty) = 0$.

To generate an archimedean copula in dimension $d$, the function also needs to be $d$-monotone, that is :

- $\phi$ is $d-2$ times derivable.
- $(-1)^k \phi^{(k)} \ge 0 \;\forall k \in \{1,..,d-2\},$ and if $(-1)^{d-2}\phi^{(d-2)}$ is a non-increasing and convex function.

The access to the function $\phi$ itself is done through the interface:

`ϕ(G::Generator, t)`

We do not check algorithmically that the proposed generators are d-monotonous. Instead, it is up to the person implementing the generator to tell the interface how big can $d$ be through the function

`max_monotony(G::MyGenerator) = # some integer, the maximum d so that the generator is d-monotonous.`

More methods can be implemented for performance, althouhg there are implement defaults in the package :

`ϕ⁻¹( G::Generator, x)`

gives the inverse function of the generator.`ϕ⁽¹⁾(G::Generator, t)`

gives the first derivative.`ϕ⁽ᵏ⁾(G::Generator, k, t)`

gives the kth derivative.`williamson_dist(G::Generator, d)`

gives the Wiliamson d-transform of the generator, see WilliamsonTransforms.jl.

References:

- mcneil2009 McNeil, A. J., & Nešlehová, J. (2009). Multivariate Archimedean copulas, d-monotone functions and ℓ 1-norm symmetric distributions.

`Copulas.GumbelBarnettGenerator`

— Type`GumbelBarnettGenerator{T}`

Fields:

- θ::Real - parameter

Constructor

```
GumbelBarnettGenerator(θ)
GumbelBarnettCopula(d,θ)
```

The Gumbel-Barnett copula is an archimdean copula with generator:

\[\phi(t) = \exp{θ^{-1}(1-e^{t})}, 0 \leq \theta \leq 1.\]

It has a few special cases:

- When θ = 0, it is the IndependentCopula

References:

- joe2014 Joe, H. (2014). Dependence modeling with copulas. CRC press, Page.437
- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.GumbelGenerator`

— Type`GumbelGenerator{T}`

Fields:

- θ::Real - parameter

Constructor

```
GumbelGenerator(θ)
GumbelCopula(d,θ)
```

The Gumbel copula in dimension $d$ is parameterized by $\theta \in [1,\infty)$. It is an Archimedean copula with generator :

\[\phi(t) = \exp{-t^{\frac{1}{θ}}}\]

It has a few special cases:

- When θ = 1, it is the IndependentCopula
- When θ = ∞, is is the MCopula (Upper Frechet-Hoeffding bound)

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.IndependentGenerator`

— Type`IndependentGenerator`

Constructor

```
IndependentGenerator()
IndependentCopula(d)
```

The Independent Copula in dimension $d$ is the simplest copula, that has the form :

\[C(\mathbf{x}) = \prod_{i=1}^d x_i.\]

It happends to be an Archimedean Copula, with generator :

\[\phi(t) = \exp{-t}\]

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.InvGaussianGenerator`

— Type`InvGaussianGenerator{T}`

Fields:

- θ::Real - parameter

Constructor

```
InvGaussianGenerator(θ)
InvGaussianCopula(d,θ)
```

The Inverse Gaussian copula in dimension $d$ is parameterized by $\theta \in [0,\infty)$. It is an Archimedean copula with generator :

\[\phi(t) = \exp{\frac{1-\sqrt{1+2θ^{2}t}}{θ}}.\]

More details about Inverse Gaussian Archimedean copula are found in :

`Mai, Jan-Frederik, and Matthias Scherer. Simulating copulas: stochastic models, sampling algorithms, and applications. Vol. 6. # N/A, 2017. Page 74.`

It has a few special cases:

- When θ = 0, it is the IndependentCopula

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.JoeGenerator`

— Type`JoeGenerator{T}`

Fields:

- θ::Real - parameter

Constructor

```
JoeGenerator(θ)
JoeCopula(d,θ)
```

The Joe copula in dimension $d$ is parameterized by $\theta \in [1,\infty)$. It is an Archimedean copula with generator :

\[\phi(t) = 1 - \left(1 - e^{-t}\right)^{\frac{1}{\theta}}\]

It has a few special cases:

- When θ = 1, it is the IndependentCopula
- When θ = ∞, is is the MCopula (Upper Frechet-Hoeffding bound)

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.MGenerator`

— Type`MGenerator`

Constructor

```
MGenerator()
MCopula(d)
```

The Upper Frechet-Hoeffding bound is the copula with the greatest value among all copulas. It correspond to comonotone random vectors.

For any copula $C$, if $W$ and $M$ are (respectively) the lower and uppder Frechet-Hoeffding bounds, we have that for all $\mathbf{u} \in [0,1]^d$,

\[W(\mathbf{u}) \le C(\mathbf{u}) \le M(\mathbf{u})\]

The two Frechet-Hoeffding bounds are also Archimedean copulas.

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.PlackettCopula`

— Type`PlackettCopula{P}`

Fields: - θ::Real - parameter

Constructor

`PlackettCopula(θ)`

Parameterized by $\theta > 0$ The Plackett copula is

\[C_{\theta}(u,v) = \frac{\left [1+(\theta-1)(u+v)\right]- \sqrt{[1+(\theta-1)(u+v)]^2-4uv\theta(\theta-1)}}{2(\theta-1)}\]

and for $\theta = 1$

\[C_{1}(u,v) = uv \]

It has a few special cases:

- When θ = 0, is is the MCopula (Upper Frechet-Hoeffding bound)
- When θ = 1, it is the IndependentCopula
- When θ = ∞, is is the WCopula (Lower Frechet-Hoeffding bound)

References:

- joe2014 Joe, H. (2014). Dependence modeling with copulas. CRC press, Page.164
- johnson1987multivariate Johnson, Mark E. Multivariate statistical simulation: A guide to selecting and generating continuous multivariate distributions. Vol. 192. John Wiley & Sons, 1987. Page 193.
- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006. Exercise 3.38.

`Copulas.RafteryCopula`

— Type`RafteryCopula{d, P}`

Fields: - θ::Real - parameter

Constructor

`RafteryCopula(d, θ)`

The Multivariate Raftery Copula of dimension d is parameterized by $\theta \in [0,1]$

\[C_{\theta}(\mathbf{u}) = u_{(1)} + \frac{(1 - \theta)(1 - d)}{1 - \theta - d} \left(\prod_{j=1}^{d} u_j\right)^{\frac{1}{1-\theta}} - \sum_{i=2}^{d} \frac{\theta(1-\theta)}{(1-\theta-i)(2-\theta-i)} \left(\prod_{j=1}^{i-1}u_{(j)}\right)^{\frac{1}{1-\theta}}u_{(i)}^{\frac{2-\theta-i}{1-\theta}}\]

where $u_{(1)}, \ldots , u_{(d)}$ denote the order statistics of $u_1, \ldots ,u_d$. More details about Multivariate Raftery Copula are found in the references below.

It has a few special cases:

- When θ = 0, it is the IndependentCopula.
- When θ = 1, it is the the Fréchet upper bound

References:

- Raftery2023 Saali, T., M. Mesfioui, and A. Shabri, 2023: Multivariate Extension of Raftery Copula. Mathematics, 11, 414, https://doi.org/10.3390/math11020414.
- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006. Exercise 3.6.

`Copulas.SklarDist`

— Type`SklarDist{CT,TplMargins}`

Fields:

`C::CT`

- The copula`m::TplMargins`

- a Tuple representing the marginal distributions

Constructor

`SklarDist(C,m)`

This function allows to construct a random vector specified, through the Sklar Theorem, by its marginals and its copula separately. See Sklar's theorem:

Theorem (Sklar 1959):For every random vector $\bm X$, there exists a copula $C$ such that$\forall \bm x\in \mathbb R^d, F(\bm x) = C(F_{1}(x_{1}),...,F_{d}(x_{d})).$ The copula $C$ is uniquely determined on $\mathrm{Ran}(F_{1}) \times ... \times \mathrm{Ran}(F_{d})$, where $\mathrm{Ran}(F_i)$ denotes the range of the function $F_i$. In particular, if all marginals are absolutely continuous, $C$ is unique.

The obtain random vector follows `Distributions.jl`

's API and can be sampled, pdf and cdf can be evaluated, etc... We even provide a fit function. See the folowing exemple code :

```
using Copulas, Distributions, Random
X₁ = Gamma(2,3)
X₂ = Pareto()
X₃ = LogNormal(0,1)
C = ClaytonCopula(3,0.7) # A 3-variate Clayton Copula with θ = 0.7
D = SklarDist(C,(X₁,X₂,X₃)) # The final distribution
simu = rand(D,1000) # Generate a dataset
# You may estimate a copula using the `fit` function:
D̂ = fit(SklarDist{ClaytonCopula,Tuple{Gamma,Normal,LogNormal}}, simu)
```

References:

- sklar1959 Sklar, M. (1959). Fonctions de répartition à n dimensions et leurs marges. In Annales de l'ISUP (Vol. 8, No. 3, pp. 229-231).
- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.SubsetCopula`

— Type`SubsetCopula{d,CT}`

Fields:

`C::CT`

- The copula`dims::Tuple{Int64}`

- a Tuple representing which dimensions are used.

Constructor

`SubsetCopula(C::Copula,dims)`

This class allows to construct a random vector corresponding to a few dimensions of the starting copula. If $(X_1,...,X_n)$ is the random vector corresponding to the copula `C`

, this returns the copula of `(`

$X_i$ `for i in dims)`

. The dependence structure is preserved. There are specialized methods for some copulas.

`Copulas.SurvivalCopula`

— Type`SurvivalCopula(C,indices)`

Computes the survival version of any copula on given indices. From a copula $C$ in dimension $d$, and some indices $i_1,...i_k$ in ${1,...,d}$, the survival copula associated simply reverses its arguments on chosen indices. For exemple, for $d=4$ and indices $(2,3)$, we have:

\[S(u_1,...u_4) = C(u_1,1-u_2,1-u3,u_4)\]

This constructor allows to derive new "survival" families. For exemple, in bivariate cases, this allows to do "rotations". The obtained models can be treated as the starting one, i.e. as a random vector in [0,1]^d with uniforms marginals.

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.TCopula`

— Type`TCopula{d,MT}`

Fields:

- df::Int - number of degree of freedom
- Σ::MT - covariance matrix

Constructor

`TCopula(df,Σ)`

The Student's T Copula is the copula of a Multivariate Student distribution. It is constructed as :

\[C(\mathbf{x}; \boldsymbol{n,\Sigma}) = F_{n,\Sigma}(F_{n,\Sigma,i}^{-1}(x_i),i\in 1,...d)\]

where $F_{n,\Sigma}$ is a cdf of a multivariate student random vector with covariance matrix $\Sigma$ and $n$ degrees of freedom. and `F_{n,\Sigma,i}`

is the ith marignal cdf.

It can be constructed in Julia via:

`C = TCopula(2,Σ)`

You can sample it, compute pdf and cdf, or even fit the distribution via:

```
u = rand(C,1000)
Random.rand!(C,u) # other calling syntax for rng.
pdf(C,u) # to get the density
cdf(C,u) # to get the distribution function
Ĉ = fit(TCopula,u) # to fit on the sampled data.
```

Except that currently it does not work since `fit(Distributions.MvTDist,data)`

does not dispatch.

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.WGenerator`

— Type`WGenerator`

Constructor

```
WGenerator()
WCopula(d)
```

The Lower Frechet-Hoeffding bound is the copula with the lowest value among all copulas. Note that $W$ is only a proper copula when $d=2$, in greater dimensions it is still the (pointwise) lower bound, but not a copula anymore. For any copula $C$, if $W$ and $M$ are (respectively) the lower and uppder Frechet-Hoeffding bounds, we have that for all $\mathbf{u} \in [0,1]^d$,

\[W(\mathbf{u}) \le C(\mathbf{u}) \le M(\mathbf{u})\]

The two Frechet-Hoeffding bounds are also Archimedean copulas.

References:

- nelsen2006 Nelsen, Roger B. An introduction to copulas. Springer, 2006.

`Copulas.WilliamsonGenerator`

— Type```
WilliamsonGenerator{TX}
i𝒲{TX}
```

Fields:

`X::TX`

– a random variable that represents its Williamson d-transform`d::Int`

– the dimension of the transformation.

Constructor

```
WilliamsonGenerator(X::Distributions.UnivariateDistribution, d)
i𝒲(X::Distributions.UnivariateDistribution,d)
```

The `WilliamsonGenerator`

(alias `i𝒲`

) allows to construct a d-monotonous archimedean generator from a positive random variable `X::Distributions.UnivariateDistribution`

. The transformation, which is called the inverse Williamson transformation, is implemented in WilliamsonTransforms.jl.

For a univariate non-negative random variable $X$, with cumulative distribution function $F$ and an integer $d\ge 2$, the Williamson-d-transform of $X$ is the real function supported on $[0,\infty[$ given by:

\[\phi(t) = 𝒲_{d}(X)(t) = \int_{t}^{\infty} \left(1 - \frac{t}{x}\right)^{d-1} dF(x) = \mathbb E\left( (1 - \frac{t}{X})^{d-1}_+\right) \mathbb 1_{t > 0} + \left(1 - F(0)\right)\mathbb 1_{t <0}\]

This function has several properties:

- We have that $\phi(0) = 1$ and $\phi(Inf) = 0$
- $\phi$ is $d-2$ times derivable, and the signs of its derivatives alternates : $\forall k \in 0,...,d-2, (-1)^k \phi^{(k)} \ge 0$.
- $\phi^{(d-2)}$ is convex.

These properties makes this function what is called a *d-monotone archimedean generator*, able to generate *archimedean copulas* in dimensions up to $d$. Our implementation provides this through the `Generator`

interface: the function $\phi$ can be accessed by

```
G = WilliamsonGenerator(X, d)
ϕ(G,t)
```

Note that you'll always have:

`max_monotony(WilliamsonGenerator(X,d)) === d`

References:

- williamson1955multiply Williamson, R. E. (1956). Multiply monotone functions and their Laplace transforms. Duke Math. J. 23 189–207. MR0077581
- mcneil2009 McNeil, Alexander J., and Johanna Nešlehová. "Multivariate Archimedean copulas, d-monotone functions and ℓ 1-norm symmetric distributions." (2009): 3059-3097.

`Copulas.pseudos`

— Method`pseudos(sample)`

Compute the pseudo-observations of a multivariate sample. Note that the sample has to be given in wide format (d,n), where d is the dimension and n the number of observations.

Warning: the order used is ordinal ranking like https://en.wikipedia.org/wiki/Ranking#Ordinal*ranking*.28.221234.22_ranking.29, see `StatsBase.ordinalrank`

for the ordering we use. If you want more flexibility, checkout `NormalizeQuantiles.sampleranks`

.

`Copulas.subsetdims`

— Method```
subsetdims(C::Copula,dims)
subsetdims(D::SklarDist, dims)
```

If $(X_1,...,X_n)$ is the random vector corresponding to the model `C`

or `D`

, this returns the distribution on `(`

$X_i$ `for i in dims)`

, preserving the dependence structure between the dimensions in `dims`

. There are specialized methods for some copulas.