NAG Library Function Document
nag_rand_copula_normal (g05rdc)
1 Purpose
nag_rand_copula_normal (g05rdc) sets up a reference vector and generates an array of pseudorandom numbers from a Normal (Gaussian) copula with covariance matrix .
2 Specification
#include <nag.h> |
#include <nagg05.h> |
void |
nag_rand_copula_normal (Nag_OrderType order,
Nag_ModeRNG mode,
Integer n,
Integer m,
const double c[],
Integer pdc,
double r[],
Integer lr,
Integer state[],
double x[],
Integer pdx,
NagError *fail) |
|
3 Description
The Gaussian copula,
, is defined by
where
is the number of dimensions,
is the multivariate Normal density function with mean zero and covariance matrix
and
is the inverse of the univariate Normal density function with mean zero and variance
.
nag_rand_matrix_multi_normal (g05rzc) is used to generate a vector from a multivariate Normal distribution and
nag_prob_normal (g01eac) is used to convert each element of that vector into a uniformly distributed value between zero and one.
One of the initialization functions
nag_rand_init_repeatable (g05kfc) (for a repeatable sequence if computed sequentially) or
nag_rand_init_nonrepeatable (g05kgc) (for a non-repeatable sequence) must be called prior to the first call to nag_rand_copula_normal (g05rdc).
4 References
Nelsen R B (1998) An Introduction to Copulas. Lecture Notes in Statistics 139 Springer
Sklar A (1973) Random variables: joint distribution functions and copulas Kybernetika 9 499–460
5 Arguments
- 1:
order – Nag_OrderTypeInput
-
On entry: the
order argument specifies the two-dimensional storage scheme being used, i.e., row-major ordering or column-major ordering. C language defined storage is specified by
. See
Section 3.2.1.3 in the Essential Introduction for a more detailed explanation of the use of this argument.
Constraint:
or .
- 2:
mode – Nag_ModeRNGInput
On entry: a code for selecting the operation to be performed by the function.
- Set up reference vector only.
- Generate variates using reference vector set up in a prior call to nag_rand_copula_normal (g05rdc).
- Set up reference vector and generate variates.
Constraint:
, or .
- 3:
n – IntegerInput
On entry: , the number of random variates required.
Constraint:
.
- 4:
m – IntegerInput
On entry: , the number of dimensions of the distribution.
Constraint:
.
- 5:
c[] – const doubleInput
-
Note: the dimension,
dim, of the array
c
must be at least
.
The
th element of the matrix
is stored in
- when ;
- when .
On entry: the covariance matrix of the distribution. Only the upper triangle need be set.
Constraint:
must be positive semidefinite to machine precision.
- 6:
pdc – IntegerInput
-
On entry: the stride separating row or column elements (depending on the value of
order) in the array
c.
Constraint:
.
- 7:
r[lr] – doubleCommunication Array
On entry: if , the reference vector as set up by nag_rand_copula_normal (g05rdc) in a previous call with or .
On exit: if or , the reference vector that can be used in subsequent calls to nag_rand_copula_normal (g05rdc) with .
- 8:
lr – IntegerInput
On entry: the dimension of the array
r. If
, it must be the same as the value of
lr specified in the prior call to nag_rand_copula_normal (g05rdc) with
or
.
Constraint:
.
- 9:
state[] – IntegerCommunication Array
-
Note: the dimension,
, of this array is dictated by the requirements of associated functions that must have been previously called. This array MUST be the same array passed as argument
state in the previous call to
nag_rand_init_repeatable (g05kfc) or
nag_rand_init_nonrepeatable (g05kgc).
On entry: contains information on the selected base generator and its current state.
On exit: contains updated information on the state of the generator.
- 10:
x[] – doubleOutput
-
Note: the dimension,
dim, of the array
x
must be at least
- when ;
- when .
Where
appears in this document, it refers to the array element
- when ;
- when .
On exit: the array of values from a multivariate Gaussian copula, with holding the th dimension for the th variate.
- 11:
pdx – IntegerInput
-
On entry: the stride used in the array
x.
Constraints:
- if ,
;
- if , .
- 12:
fail – NagError *Input/Output
-
The NAG error argument (see
Section 3.6 in the Essential Introduction).
6 Error Indicators and Warnings
- NE_BAD_PARAM
-
On entry, argument had an illegal value.
- NE_INT
-
On entry,
lr is not large enough,
: minimum length required
.
On entry, .
Constraint: .
On entry, .
Constraint: .
- NE_INT_2
-
On entry, and .
Constraint: .
On entry, and .
Constraint: .
On entry, and .
Constraint: .
- NE_INTERNAL_ERROR
-
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact
NAG for assistance.
- NE_INVALID_STATE
-
On entry,
state vector has been corrupted or not initialized.
- NE_POS_DEF
-
On entry, the covariance matrix is not positive semidefinite to machine precision.
- NE_PREV_CALL
-
m is not the same as when
r was set up in a previous call.
Previous value of
and
.
7 Accuracy
See
Section 7 in nag_rand_matrix_multi_normal (g05rzc) for an indication of the accuracy of the underlying multivariate Normal distribution.
8 Parallelism and Performance
nag_rand_copula_normal (g05rdc) is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
nag_rand_copula_normal (g05rdc) makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the
Users' Note for your implementation for any additional implementation-specific information.
The time taken by nag_rand_copula_normal (g05rdc) is of order .
It is recommended that the diagonal elements of should not differ too widely in order of magnitude. This may be achieved by scaling the variables if necessary. The actual matrix decomposed is , where is a diagonal matrix with small positive diagonal elements. This ensures that, even when is singular, or nearly singular, the Cholesky factor corresponds to a positive definite covariance matrix that agrees with within machine precision.
10 Example
This example prints ten pseudorandom observations from a Normal copula with covariance matrix
generated by nag_rand_copula_normal (g05rdc). All ten observations are generated by a single call to nag_rand_copula_normal (g05rdc) with
. The random number generator is initialized by
nag_rand_init_repeatable (g05kfc).
10.1 Program Text
Program Text (g05rdce.c)
10.2 Program Data
None.
10.3 Program Results
Program Results (g05rdce.r)