rInvCholWishart {CholWishart} R Documentation

## Cholesky Factor of Random Inverse Wishart Distributed Matrices

### Description

Generate n random matrices, distributed according to the Cholesky factor of an inverse Wishart distribution with parameters `Sigma` and `df`, W_p(Sigma, df).

Note there are different ways of parameterizing the Inverse Wishart distribution, so check which one you need. Here, if X ~ IW_p(Sigma, df) then X^{-1} ~ W_p(Sigma^{-1}, df). Dawid (1981) has a different definition: if X ~ W_p(Sigma^{-1}, df) and df > p - 1, then X^{-1} = Y ~ IW(Sigma, delta), where delta = df - p + 1.

### Usage

```rInvCholWishart(n, df, Sigma)
```

### Arguments

 `n` integer sample size. `df` numeric parameter, "degrees of freedom". `Sigma` positive definite (p * p) "scale" matrix, the matrix parameter of the distribution.

### Value

a numeric array, say `R`, of dimension p * p * n, where each `R[,,i]` is a Cholesky decomposition of a realization of the Wishart distribution W_p(Sigma, df). Based on a modification of the existing code for the `rWishart` function

### References

Anderson, T. W. (2003). An Introduction to Multivariate Statistical Analysis (3rd ed.). Hoboken, N. J.: Wiley Interscience.

Dawid, A. (1981). Some Matrix-Variate Distribution Theory: Notational Considerations and a Bayesian Application. Biometrika, 68(1), 265-274. doi: 10.2307/2335827

Gupta, A. K. and D. K. Nagar (1999). Matrix variate distributions. Chapman and Hall.

Mardia, K. V., J. T. Kent, and J. M. Bibby (1979) Multivariate Analysis, London: Academic Press.

`rWishart` and `rCholWishart`

### Examples

```# How it is parameterized:
set.seed(20180211)
A <- rCholWishart(1L, 10, 3*diag(5L))[,,1]
A
set.seed(20180211)
B <- rInvCholWishart(1L, 10, 1/3*diag(5L))[,,1]
B
crossprod(A) %*% crossprod(B)

set.seed(20180211)
C <- chol(stats::rWishart(1L, 10, 3*diag(5L))[,,1])
C
```

[Package CholWishart version 1.1.0 Index]