Integer, Intent (In)	::	n
Integer, Intent (Inout)	::	ifail
Real (Kind=nag_wp), Intent (In)	::	t(0:n)
Real (Kind=nag_wp), Intent (Inout)	::	x(*), v
Real (Kind=nag_wp), Intent (Out)	::	work(n-1)

C Header Interface

#include nagmk26.h

void	f04mef_ ( const Integer n, const double t[], double x[], double v, double work[], Integer *ifail)

3

Description

f04mef solves the equations

T_{n} x_{n} = - t_{n},

where

T_{n}

is the

n

n

symmetric positive definite Toeplitz matrix

T_{n} = (\begin{array}{l} τ_{0} & τ_{1} & τ_{2} & \dots & τ_{n - 1} \\ τ_{1} & τ_{0} & τ_{1} & \dots & τ_{n - 2} \\ τ_{2} & τ_{1} & τ_{0} & \dots & τ_{n - 3} \\ . & . & . & . \\ τ_{n - 1} & τ_{n - 2} & τ_{n - 3} & \dots & τ_{0} \end{array})

and

t_{n}

is the vector

t_{n}^{T} = (τ_{1} τ_{2} \dots τ_{n}),

given the solution of the equations

T_{n - 1} x_{n - 1} = - t_{n - 1} .

The routine will normally be used to successively solve the equations

T_{k} x_{k} = - t_{k}, k = 1, 2, \dots, n .

If it is desired to solve the equations for a single value of

n

, then routine f04fef may be called. This routine uses the method of Durbin (see Durbin (1960) and Golub and Van Loan (1996)).

4

References

Bunch J R (1985) Stability of methods for solving Toeplitz systems of equations SIAM J. Sci. Statist. Comput. 6 349–364

Bunch J R (1987) The weak and strong stability of algorithms in numerical linear algebra Linear Algebra Appl. 88/89 49–66

Cybenko G (1980) The numerical stability of the Levinson–Durbin algorithm for Toeplitz systems of equations SIAM J. Sci. Statist. Comput. 1 303–319

Durbin J (1960) The fitting of time series models Rev. Inst. Internat. Stat. 28 233

Golub G H and Van Loan C F (1996) Matrix Computations (3rd Edition) Johns Hopkins University Press, Baltimore

5

Arguments

1: $n$ – IntegerInput: On entry: the order of the Toeplitz matrix $T$ .

Constraint: $n \geq 0$ . When $n = 0$ , an immediate return is effected.
2: $t (0 : n)$ – Real (Kind=nag_wp) arrayInput: On entry: $t (0)$ must contain the value $τ_{0}$ of the diagonal elements of $T$ , and the remaining n elements of t must contain the elements of the vector $t_{n}$ .

Constraint: $t (0) > 0.0$ . Note that if this is not true, the Toeplitz matrix cannot be positive definite.
3: $x (*)$ – Real (Kind=nag_wp) arrayInput/Output: Note: the dimension of the array x must be at least $\max (1, n)$ .

On entry: with $n > 1$ the ( $n - 1$ ) elements of the solution vector $x_{n - 1}$ as returned by a previous call to f04mef. The element $x (n)$ need not be specified.

Constraint: $|x (n - 1)| < 1.0$ . Note that this is the partial (auto)correlation coefficient, or reflection coefficient, for the $(n - 1)$ th step. If the constraint does not hold, $T_{n}$ cannot be positive definite.

On exit: the solution vector $x_{n}$ . The element $x (n)$ returns the partial (auto)correlation coefficient, or reflection coefficient, for the $n$ th step. If $|x (n)| \geq 1.0$ , the matrix $T_{n + 1}$ will not be positive definite to working accuracy.
4: $v$ – Real (Kind=nag_wp)Input/Output: On entry: with $n > 1$ the mean square prediction error for the ( $n - 1$ )th step, as returned by a previous call to f04mef.

On exit: the mean square prediction error, or predictor error variance ratio, $ν_{n}$ , for the $n$ th step. (See Section 9 and the Introduction to Chapter G13.)
5: $work (n - 1)$ – Real (Kind=nag_wp) arrayWorkspace
6: $ifail$ – IntegerInput/Output: On entry: ifail must be set to $0$ , $- 1 or 1$ . If you are unfamiliar with this argument you should refer to Section 3.4 in How to Use the NAG Library and its Documentation for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value $- 1 or 1$ is recommended. If the output of error messages is undesirable, then the value $1$ is recommended. Otherwise, if you are not familiar with this argument, the recommended value is $0$ . When the value $- 1 or 1$ is used it is essential to test the value of ifail on exit.

On exit: $ifail = 0$ unless the routine detects an error or a warning has been flagged (see Section 6).

6

Error Indicators and Warnings

If on entry

ifail = 0

- 1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = - 1$

On entry,	$n < 0$ ,
or	$t (0) \leq 0.0$ ,
or	$n > 1$ and $\|x (n - 1)\| \geq 1.0$ .

$ifail = 1$

The Toeplitz matrix

T_{n + 1}

is not positive definite to working accuracy. If, on exit,

x (n)

is close to unity, then the principal minor was probably close to being singular, and the sequence

τ_{0}, τ_{1}, \dots, τ_{n}

may be a valid sequence nevertheless. x returns the solution of the equations

T_{n} x_{n} = - t_{n},

and v returns

v_{n}

, but it may not be positive.

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 3.9 in How to Use the NAG Library and its Documentation for further information.

$ifail = - 399$: Your licence key may have expired or may not have been installed correctly.
See Section 3.8 in How to Use the NAG Library and its Documentation for further information.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.

7

Accuracy

The computed solution of the equations certainly satisfies

r = T_{n} x_{n} + t_{n},

where

{‖r‖}_{1}

is approximately bounded by

{‖r‖}_{1} \leq c ε (\prod_{i = 1}^{n} (1 + |p_{i}|) - 1),

c

being a modest function of

n

ε

being the machine precision and

p_{k}

being the

k

th element of

x_{k}

. This bound is almost certainly pessimistic, but it has not yet been established whether or not the method of Durbin is backward stable. For further information on stability issues see Bunch (1985), Bunch (1987), Cybenko (1980) and Golub and Van Loan (1996). The following bounds on

{‖T_{n}^{- 1}‖}_{1}

hold:

\max (\frac{1}{v_{n - 1}}, \frac{1}{\prod_{i = 1}^{n - 1} (1 - p_{i})}) \leq {‖T_{n}^{- 1}‖}_{1} \leq \prod_{i = 1}^{n - 1} (\frac{1 + |p_{i}|}{1 - |p_{i}|}),

where

v_{n}

is the mean square prediction error for the

n

th step. (See Cybenko (1980).) Note that

v_{n} < v_{n - 1}

. The norm of

T_{n}^{- 1}

may also be estimated using routine f04ydf.

8

Parallelism and Performance

f04mef 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 routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9

Further Comments

The number of floating-point operations used by this routine is approximately

4 n

The mean square prediction errors,

v_{i}

, is defined as

v_{i} = (τ_{0} + t_{i - 1}^{T} x_{i - 1}) / τ_{0} .

Note that

v_{i} = (1 - p_{i}^{2}) v_{i - 1}

10

Example

This example finds the solution of the Yule–Walker equations

T_{k} x_{k} = - t_{k}

k = 1, 2, 3, 4

where

T_{4} = (\begin{array}{l} 4 & 3 & 2 & 1 \\ 3 & 4 & 3 & 2 \\ 2 & 3 & 4 & 3 \\ 1 & 2 & 3 & 4 \end{array}) and t_{4} = (\begin{array}{l} 3 \\ 2 \\ 1 \\ 0 \end{array}) .

NAG Library Routine Document

f04mef (real_toeplitz_yule_update)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

1

Purpose

2

Specification

3

Description

4

References

5

Arguments

6

Error Indicators and Warnings

7

Accuracy

8

Parallelism and Performance

9

Further Comments

10

Example

10.1

Program Text

10.2

Program Data

10.3

Program Results