NAG CL Interface
g13dlc (multi_​diff)

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1 Purpose

g13dlc differences and/or transforms a multivariate time series.

2 Specification

#include <nag.h>
void  g13dlc (Integer k, Integer n, const double z[], const Integer tr[], const Integer id[], const double delta[], double w[], Integer *nd, NagError *fail)
The function may be called by the names: g13dlc or nag_tsa_multi_diff.

3 Description

For certain time series it may first be necessary to difference the original data to obtain a stationary series before calculating autocorrelations, etc. This function also allows you to apply either a square root or a log transformation to the original time series to stabilize the variance if required.
If the order of differencing required for the ith series is di, then the differencing operator is defined by δi(B)=1-δi1B-δi2B2--δidiBdi, where B is the backward shift operator; that is, BZt=Zt-1. Let d denote the maximum of the orders of differencing, di, over the k series. The function computes values of the differenced/transformed series Wt = (w1t,w2t,,wkt) T , for t=d+1,,n, as follows:
wit=δi(B)zit*,  i=1,2,,k  
where zit* are the transformed values of the original k-dimensional time series Zt = (z1t,z2t,,zkt) T .
The differencing parameters δij, for i=1,2,,k and j=1,2,,di, must be supplied by you. If the ith series does not require differencing, then di=0.

4 References

Box G E P and Jenkins G M (1976) Time Series Analysis: Forecasting and Control (Revised Edition) Holden–Day
Wei W W S (1990) Time Series Analysis: Univariate and Multivariate Methods Addison–Wesley

5 Arguments

1: k Integer Input
On entry: k, the dimension of the multivariate time series.
Constraint: k1.
2: n Integer Input
On entry: n, the number of observations in the series, prior to differencing.
Constraint: n1.
3: z[k×n] const double Input
On entry: z[(t-1)k+i-1] must contain the ith series at time t, for t=1,2,,n and i=1,2,,k.
4: tr[k] const Integer Input
On entry: tr[i-1] indicates whether the ith series is to be transformed, for i=1,2,,k.
A square root transformation is used.
No transformation is used.
A log transformation is used.
Constraint: tr[i-1]=−1, 0 or 1, for i=1,2,,k.
5: id[k] const Integer Input
On entry: the order of differencing for each series, d1,d2,,dk.
Constraint: 0id[i]<n, for i=0,1,,k-1.
6: delta[dim] const double Input
Note: the dimension, dim, of the array delta must be at least k×max(1,d), where d=max(id[i-1]).
On entry: if id[i-1]>0 then delta[(j-1)k+i-1] must be set to δij, for j=1,2,,dl and i=1,2,,k.
7: w[dim] double Output
Note: the dimension, dim, of the array w must be at least k×(n-d), where d=max(id[i-1]).
On exit: w[(t-1)k+i-1] contains the value of wi,t+d, for i=1,2,,k and t=1,2,,n-d.
8: nd Integer * Output
On exit: the number of differenced values, n-d, in the series, where d=max(id[i-1]).
9: fail NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
On entry, argument value had an illegal value.
On entry, k=value.
Constraint: k1.
On entry, n=value.
Constraint: n1.
On entry, i=value and id[i-1]=value.
Constraint: id[i-1]0.
On entry, i=value, id[i-1]=value and n=value.
Constraint: id<n.
On entry, tr[value]=value.
Constraint: tr[i]=−1, 0 or 1.
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.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.
On entry, one (or more) of the transformations requested is invalid. Check that you are not trying to log or square-root a series, some of whose values are negative.

7 Accuracy

The computations are believed to be stable.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
g13dlc 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 X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

The same differencing operator does not have to be applied to all the series. For example, suppose we have k=2, and wish to apply the second-order differencing operator 2 to the first series and the first-order differencing operator to the second series:
w1t =2z1t= (1-B) 2z1t=(1-2B+B2)z1t,   and w2t =z2t=(1-B)z2t.  
Then d1=2,d2=1, d=max(d1,d2)=2, and
delta = [ δ11 δ12 δ21 ] = [ 2 −1 1 ] .  

10 Example

A program to difference (non-seasonally) each of two time series of length 48. No transformation is to be applied to either of the series.

10.1 Program Text

Program Text (g13dlce.c)

10.2 Program Data

Program Data (g13dlce.d)

10.3 Program Results

Program Results (g13dlce.r)