g05 Chapter Contents
g05 Chapter Introduction
NAG Library Manual

# NAG Library Function Documentnag_rand_field_2d_predef_setup (g05zrc)

## 1  Purpose

nag_rand_field_2d_predef_setup (g05zrc) performs the setup required in order to simulate stationary Gaussian random fields in two dimensions, for a preset variogram, using the circulant embedding method. Specifically, the eigenvalues of the extended covariance matrix (or embedding matrix) are calculated, and their square roots output, for use by nag_rand_field_2d_generate (g05zsc), which simulates the random field.

## 2  Specification

 #include #include
 void nag_rand_field_2d_predef_setup (const Integer ns[], double xmin, double xmax, double ymin, double ymax, const Integer maxm[], double var, Nag_Variogram cov, Nag_NormType norm, Integer np, const double params[], Nag_EmbedPad pad, Nag_EmbedScale corr, double lam[], double xx[], double yy[], Integer m[], Integer *approx, double *rho, Integer *icount, double eig[], NagError *fail)

## 3  Description

A two-dimensional random field $Z\left(\mathbf{x}\right)$ in ${ℝ}^{2}$ is a function which is random at every point $\mathbf{x}\in {ℝ}^{2}$, so $Z\left(\mathbf{x}\right)$ is a random variable for each $\mathbf{x}$. The random field has a mean function $\mu \left(\mathbf{x}\right)=𝔼\left[Z\left(\mathbf{x}\right)\right]$ and a symmetric positive semidefinite covariance function $C\left(\mathbf{x},\mathbf{y}\right)=𝔼\left[\left(Z\left(\mathbf{x}\right)-\mu \left(\mathbf{x}\right)\right)\left(Z\left(\mathbf{y}\right)-\mu \left(\mathbf{y}\right)\right)\right]$. $Z\left(\mathbf{x}\right)$ is a Gaussian random field if for any choice of $n\in ℕ$ and ${\mathbf{x}}_{1},\dots ,{\mathbf{x}}_{n}\in {ℝ}^{2}$, the random vector ${\left[Z\left({\mathbf{x}}_{1}\right),\dots ,Z\left({\mathbf{x}}_{n}\right)\right]}^{\mathrm{T}}$ follows a multivariate Normal distribution, which would have a mean vector $\stackrel{~}{\mathbf{\mu }}$ with entries ${\stackrel{~}{\mu }}_{i}=\mu \left({\mathbf{x}}_{i}\right)$ and a covariance matrix $\stackrel{~}{C}$ with entries ${\stackrel{~}{C}}_{ij}=C\left({\mathbf{x}}_{i},{\mathbf{x}}_{j}\right)$. A Gaussian random field $Z\left(\mathbf{x}\right)$ is stationary if $\mu \left(\mathbf{x}\right)$ is constant for all $\mathbf{x}\in {ℝ}^{2}$ and $C\left(\mathbf{x},\mathbf{y}\right)=C\left(\mathbf{x}+\mathbf{a},\mathbf{y}+\mathbf{a}\right)$ for all $\mathbf{x},\mathbf{y},\mathbf{a}\in {ℝ}^{2}$ and hence we can express the covariance function $C\left(\mathbf{x},\mathbf{y}\right)$ as a function $\gamma$ of one variable: $C\left(\mathbf{x},\mathbf{y}\right)=\gamma \left(\mathbf{x}-\mathbf{y}\right)$. $\gamma$ is known as a variogram (or more correctly, a semivariogram) and includes the multiplicative factor ${\sigma }^{2}$ representing the variance such that $\gamma \left(0\right)={\sigma }^{2}$.
The functions nag_rand_field_2d_predef_setup (g05zrc) and nag_rand_field_2d_generate (g05zsc) are used to simulate a two-dimensional stationary Gaussian random field, with mean function zero and variogram $\gamma \left(\mathbf{x}\right)$, over a domain $\left[{x}_{\mathrm{min}},{x}_{\mathrm{max}}\right]×\left[{y}_{\mathrm{min}},{y}_{\mathrm{max}}\right]$, using an equally spaced set of ${N}_{1}×{N}_{2}$ points; ${N}_{1}$ points in the $x$-direction and ${N}_{2}$ points in the $y$-direction. The problem reduces to sampling a Gaussian random vector $\mathbf{X}$ of size ${N}_{1}×{N}_{2}$, with mean vector zero and a symmetric covariance matrix $A$, which is an ${N}_{2}$ by ${N}_{2}$ block Toeplitz matrix with Toeplitz blocks of size ${N}_{1}$ by ${N}_{1}$. Since $A$ is in general expensive to factorize, a technique known as the circulant embedding method is used. $A$ is embedded into a larger, symmetric matrix $B$, which is an ${M}_{2}$ by ${M}_{2}$ block circulant matrix with circulant blocks of size ${M}_{1}$ by ${M}_{1}$, where ${M}_{1}\ge 2\left({N}_{1}-1\right)$ and ${M}_{2}\ge 2\left({N}_{2}-1\right)$. $B$ can now be factorized as $B=W\Lambda {W}^{*}={R}^{*}R$, where $W$ is the two-dimensional Fourier matrix (${W}^{*}$ is the complex conjugate of $W$), $\Lambda$ is the diagonal matrix containing the eigenvalues of $B$ and $R={\Lambda }^{\frac{1}{2}}{W}^{*}$. $B$ is known as the embedding matrix. The eigenvalues can be calculated by performing a discrete Fourier transform of the first row (or column) of $B$ and multiplying by ${M}_{1}×{M}_{2}$, and so only the first row (or column) of $B$ is needed – the whole matrix does not need to be formed.
As long as all of the values of $\Lambda$ are non-negative (i.e., $B$ is positive semidefinite), $B$ is a covariance matrix for a random vector $\mathbf{Y}$ which has ${M}_{2}$ blocks of size ${M}_{1}$. Two samples of $\mathbf{Y}$ can now be simulated from the real and imaginary parts of ${R}^{*}\left(\mathbf{U}+i\mathbf{V}\right)$, where $\mathbf{U}$ and $\mathbf{V}$ have elements from the standard Normal distribution. Since ${R}^{*}\left(\mathbf{U}+i\mathbf{V}\right)=W{\Lambda }^{\frac{1}{2}}\left(\mathbf{U}+i\mathbf{V}\right)$, this calculation can be done using a discrete Fourier transform of the vector ${\Lambda }^{\frac{1}{2}}\left(\mathbf{U}+i\mathbf{V}\right)$. Two samples of the random vector $\mathbf{X}$ can now be recovered by taking the first ${N}_{1}$ elements of the first ${N}_{2}$ blocks of each sample of $\mathbf{Y}$ – because the original covariance matrix $A$ is embedded in $B$, $\mathbf{X}$ will have the correct distribution.
If $B$ is not positive semidefinite, larger embedding matrices $B$ can be tried; however if the size of the matrix would have to be larger than maxm, an approximation procedure is used. We write $\Lambda ={\Lambda }_{+}+{\Lambda }_{-}$, where ${\Lambda }_{+}$ and ${\Lambda }_{-}$ contain the non-negative and negative eigenvalues of $B$ respectively. Then $B$ is replaced by $\rho {B}_{+}$ where ${B}_{+}=W{\Lambda }_{+}{W}^{*}$ and $\rho \in \left(0,1\right]$ is a scaling factor. The error $\epsilon$ in approximating the distribution of the random field is given by
 $ε= 1-ρ 2 trace⁡Λ + ρ2 trace⁡Λ- M .$
Three choices for $\rho$ are available, and are determined by the input argument corr:
• setting ${\mathbf{corr}}=\mathrm{Nag_EmbedScaleTraces}$ sets
 $ρ= trace⁡Λ trace⁡Λ+ ,$
• setting ${\mathbf{corr}}=\mathrm{Nag_EmbedScaleSqrtTraces}$ sets
 $ρ= trace⁡Λ trace⁡Λ+ ,$
• setting ${\mathbf{corr}}=\mathrm{Nag_EmbedScaleOne}$ sets $\rho =1$.
nag_rand_field_2d_predef_setup (g05zrc) finds a suitable positive semidefinite embedding matrix $B$ and outputs its sizes in the vector m and the square roots of its eigenvalues in lam. If approximation is used, information regarding the accuracy of the approximation is output. Note that only the first row (or column) of $B$ is actually formed and stored.

## 4  References

Dietrich C R and Newsam G N (1997) Fast and exact simulation of stationary Gaussian processes through circulant embedding of the covariance matrix SIAM J. Sci. Comput. 18 1088–1107
Schlather M (1999) Introduction to positive definite functions and to unconditional simulation of random fields Technical Report ST 99–10 Lancaster University
Wood A T A and Chan G (1997) Algorithm AS 312: An Algorithm for Simulating Stationary Gaussian Random Fields Journal of the Royal Statistical Society, Series C (Applied Statistics) (Volume 46) 1 171–181

## 5  Arguments

1:    $\mathbf{ns}\left[2\right]$const IntegerInput
On entry: the number of sample points to use in each direction, with ${\mathbf{ns}}\left[0\right]$ sample points in the $x$-direction, ${N}_{1}$ and ${\mathbf{ns}}\left[1\right]$ sample points in the $y$-direction, ${N}_{2}$. The total number of sample points on the grid is therefore ${\mathbf{ns}}\left[0\right]×{\mathbf{ns}}\left[1\right]$.
Constraints:
• ${\mathbf{ns}}\left[0\right]\ge 1$;
• ${\mathbf{ns}}\left[1\right]\ge 1$.
2:    $\mathbf{xmin}$doubleInput
On entry: the lower bound for the $x$-coordinate, for the region in which the random field is to be simulated.
Constraint: ${\mathbf{xmin}}<{\mathbf{xmax}}$.
3:    $\mathbf{xmax}$doubleInput
On entry: the upper bound for the $x$-coordinate, for the region in which the random field is to be simulated.
Constraint: ${\mathbf{xmin}}<{\mathbf{xmax}}$.
4:    $\mathbf{ymin}$doubleInput
On entry: the lower bound for the $y$-coordinate, for the region in which the random field is to be simulated.
Constraint: ${\mathbf{ymin}}<{\mathbf{ymax}}$.
5:    $\mathbf{ymax}$doubleInput
On entry: the upper bound for the $y$-coordinate, for the region in which the random field is to be simulated.
Constraint: ${\mathbf{ymin}}<{\mathbf{ymax}}$.
6:    $\mathbf{maxm}\left[2\right]$const IntegerInput
On entry: determines the maximum size of the circulant matrix to use – a maximum of ${\mathbf{maxm}}\left[0\right]$ elements in the $x$-direction, and a maximum of ${\mathbf{maxm}}\left[1\right]$ elements in the $y$-direction. The maximum size of the circulant matrix is thus ${\mathbf{maxm}}\left[0\right]$$×$${\mathbf{maxm}}\left[1\right]$.
Constraint: ${\mathbf{maxm}}\left[\mathit{i}\right]\ge {2}^{k}$, where $k$ is the smallest integer satisfying ${2}^{k}\ge 2\left({\mathbf{ns}}\left[\mathit{i}\right]-1\right)$, for $\mathit{i}=0,1$.
7:    $\mathbf{var}$doubleInput
On entry: the multiplicative factor ${\sigma }^{2}$ of the variogram $\gamma \left(\mathbf{x}\right)$.
Constraint: ${\mathbf{var}}\ge 0.0$.
8:    $\mathbf{cov}$Nag_VariogramInput
On entry: determines which of the preset variograms to use. The choices are given below. Note that ${x}^{\prime }=‖\frac{x}{{\ell }_{1}},\frac{y}{{\ell }_{2}}‖$, where ${\ell }_{1}$ and ${\ell }_{2}$ are correlation lengths in the $x$ and $y$ directions respectively and are parameters for most of the variograms, and ${\sigma }^{2}$ is the variance specified by var.
${\mathbf{cov}}=\mathrm{Nag_VgmSymmStab}$
Symmetric stable variogram
 $γx = σ2 exp - x′ ν ,$
where
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$,
• $\nu ={\mathbf{params}}\left[2\right]$, $0<\nu \le 2$.
${\mathbf{cov}}=\mathrm{Nag_VgmCauchy}$
Cauchy variogram
 $γx = σ2 1 + x′ 2 -ν ,$
where
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$,
• $\nu ={\mathbf{params}}\left[2\right]$, $\nu >0$.
${\mathbf{cov}}=\mathrm{Nag_VgmDifferential}$
Differential variogram with compact support
 $γx = σ2 1 + 8 x′ + 25 x′ 2 + 32 x′ 3 1 - x′ 8 , x′<1 , 0 , x′ ≥ 1 ,$
where
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$.
${\mathbf{cov}}=\mathrm{Nag_VgmExponential}$
Exponential variogram
 $γx = σ2 exp-x′ ,$
where
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$.
${\mathbf{cov}}=\mathrm{Nag_VgmGauss}$
Gaussian variogram
 $γx = σ2 exp -x′ 2 ,$
where
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$.
${\mathbf{cov}}=\mathrm{Nag_VgmNugget}$
Nugget variogram
 $γx = σ2, x=0, 0, x≠0.$
No parameters need be set for this value of cov.
${\mathbf{cov}}=\mathrm{Nag_VgmSpherical}$
Spherical variogram
 $γx = σ2 1 - 1.5x′ + 0.5 x′ 3 , x′ < 1 , 0, x′ ≥ 1 ,$
where
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$.
${\mathbf{cov}}=\mathrm{Nag_VgmBessel}$
Bessel variogram
 $γx = σ2 2ν Γ ν+1 Jν x′ x′ ν ,$
where
• ${J}_{\nu }\left(·\right)$ is the Bessel function of the first kind,
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$,
• $\nu ={\mathbf{params}}\left[2\right]$, $\nu \ge 0$.
${\mathbf{cov}}=\mathrm{Nag_VgmHole}$
Hole effect variogram
 $γx = σ2 sinx′ x′ ,$
where
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$.
${\mathbf{cov}}=\mathrm{Nag_VgmWhittleMatern}$
Whittle-Matérn variogram
 $γx = σ2 21-ν x′ ν Kν x′ Γν ,$
where
• ${K}_{\nu }\left(·\right)$ is the modified Bessel function of the second kind,
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$,
• $\nu ={\mathbf{params}}\left[2\right]$, $\nu >0$.
${\mathbf{cov}}=\mathrm{Nag_VgmContParam}$
Continuously parameterised variogram with compact support
 $γx = σ2 21-ν x′ν Kν x′ Γν 1+8x′′+25x′′2+32x′′31-x′′8, x′′<1, 0, x′′≥1,$
where
• ${x}^{\mathrm{\prime \prime }}=‖\frac{{x}^{\prime }}{{\ell }_{1}{s}_{1}},\frac{{y}^{\prime }}{{\ell }_{2}{s}_{2}}‖$,
• ${K}_{\nu }\left(·\right)$ is the modified Bessel function of the second kind,
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$,
• ${s}_{1}={\mathbf{params}}\left[2\right]$, ${s}_{1}>0$,
• ${s}_{2}={\mathbf{params}}\left[3\right]$, ${s}_{2}>0$,
• $\nu ={\mathbf{params}}\left[4\right]$, $\nu >0$.
${\mathbf{cov}}=\mathrm{Nag_VgmGenHyp}$
Generalized hyperbolic distribution variogram
 $γx=σ2δ2+x′2λ2δλKλκδKλκδ2+x′212,$
where
• ${K}_{\lambda }\left(·\right)$ is the modified Bessel function of the second kind,
• ${\ell }_{1}={\mathbf{params}}\left[0\right]$, ${\ell }_{1}>0$,
• ${\ell }_{2}={\mathbf{params}}\left[1\right]$, ${\ell }_{2}>0$,
• $\lambda ={\mathbf{params}}\left[2\right]$, no constraint on $\lambda$,
• $\delta ={\mathbf{params}}\left[3\right]$, $\delta >0$,
• $\kappa ={\mathbf{params}}\left[4\right]$, $\kappa >0$.
Constraint: ${\mathbf{cov}}=\mathrm{Nag_VgmSymmStab}$, $\mathrm{Nag_VgmCauchy}$, $\mathrm{Nag_VgmDifferential}$, $\mathrm{Nag_VgmExponential}$, $\mathrm{Nag_VgmGauss}$, $\mathrm{Nag_VgmNugget}$, $\mathrm{Nag_VgmSpherical}$, $\mathrm{Nag_VgmBessel}$, $\mathrm{Nag_VgmHole}$, $\mathrm{Nag_VgmWhittleMatern}$, $\mathrm{Nag_VgmContParam}$ or $\mathrm{Nag_VgmGenHyp}$.
9:    $\mathbf{norm}$Nag_NormTypeInput
On entry: determines which norm to use when calculating the variogram.
${\mathbf{norm}}=\mathrm{Nag_OneNorm}$
The 1-norm is used, i.e., $‖x,y‖=\left|x\right|+\left|y\right|$.
${\mathbf{norm}}=\mathrm{Nag_TwoNorm}$
The 2-norm (Euclidean norm) is used, i.e., $‖x,y‖=\sqrt{{x}^{2}+{y}^{2}}$.
Suggested value: ${\mathbf{norm}}=\mathrm{Nag_TwoNorm}$.
Constraint: ${\mathbf{norm}}=\mathrm{Nag_OneNorm}$ or $\mathrm{Nag_TwoNorm}$.
10:  $\mathbf{np}$IntegerInput
On entry: the number of parameters to be set. Different covariance functions need a different number of parameters.
${\mathbf{cov}}=\mathrm{Nag_VgmNugget}$
np must be set to $0$.
${\mathbf{cov}}=\mathrm{Nag_VgmDifferential}$, $\mathrm{Nag_VgmExponential}$, $\mathrm{Nag_VgmGauss}$, $\mathrm{Nag_VgmSpherical}$ or $\mathrm{Nag_VgmHole}$
np must be set to $2$.
${\mathbf{cov}}=\mathrm{Nag_VgmSymmStab}$, $\mathrm{Nag_VgmCauchy}$, $\mathrm{Nag_VgmBessel}$ or $\mathrm{Nag_VgmWhittleMatern}$
np must be set to $3$.
${\mathbf{cov}}=\mathrm{Nag_VgmContParam}$ or $\mathrm{Nag_VgmGenHyp}$
np must be set to $5$.
11:  $\mathbf{params}\left[{\mathbf{np}}\right]$const doubleInput
On entry: the parameters for the variogram as detailed in the description of cov.
Constraint: see cov for a description of the individual parameter constraints.
12:  $\mathbf{pad}$Nag_EmbedPadInput
On entry: determines whether the embedding matrix is padded with zeros, or padded with values of the variogram. The choice of padding may affect how big the embedding matrix must be in order to be positive semidefinite.
${\mathbf{pad}}=\mathrm{Nag_EmbedPadZeros}$
The embedding matrix is padded with zeros.
${\mathbf{pad}}=\mathrm{Nag_EmbedPadValues}$
The embedding matrix is padded with values of the variogram.
Suggested value: ${\mathbf{pad}}=\mathrm{Nag_EmbedPadValues}$.
Constraint: ${\mathbf{pad}}=\mathrm{Nag_EmbedPadZeros}$ or $\mathrm{Nag_EmbedPadValues}$.
13:  $\mathbf{corr}$Nag_EmbedScaleInput
On entry: determines which approximation to implement if required, as described in Section 3.
Suggested value: ${\mathbf{corr}}=\mathrm{Nag_EmbedScaleTraces}$.
Constraint: ${\mathbf{corr}}=\mathrm{Nag_EmbedScaleTraces}$, $\mathrm{Nag_EmbedScaleSqrtTraces}$ or $\mathrm{Nag_EmbedScaleOne}$.
14:  $\mathbf{lam}\left[{\mathbf{maxm}}\left[0\right]×{\mathbf{maxm}}\left[1\right]\right]$doubleOutput
On exit: contains the square roots of the eigenvalues of the embedding matrix.
15:  $\mathbf{xx}\left[{\mathbf{ns}}\left[0\right]\right]$doubleOutput
On exit: the points of the $x$-coordinates at which values of the random field will be output.
16:  $\mathbf{yy}\left[{\mathbf{ns}}\left[1\right]\right]$doubleOutput
On exit: the points of the $y$-coordinates at which values of the random field will be output.
17:  $\mathbf{m}\left[2\right]$IntegerOutput
On exit: ${\mathbf{m}}\left[0\right]$ contains ${M}_{1}$, the size of the circulant blocks and ${\mathbf{m}}\left[1\right]$ contains ${M}_{2}$, the number of blocks, resulting in a final square matrix of size ${M}_{1}×{M}_{2}$.
18:  $\mathbf{approx}$Integer *Output
On exit: indicates whether approximation was used.
${\mathbf{approx}}=0$
No approximation was used.
${\mathbf{approx}}=1$
Approximation was used.
19:  $\mathbf{rho}$double *Output
On exit: indicates the scaling of the covariance matrix. ${\mathbf{rho}}=1.0$ unless approximation was used with ${\mathbf{corr}}=\mathrm{Nag_EmbedScaleTraces}$ or $\mathrm{Nag_EmbedScaleSqrtTraces}$.
20:  $\mathbf{icount}$Integer *Output
On exit: indicates the number of negative eigenvalues in the embedding matrix which have had to be set to zero.
21:  $\mathbf{eig}\left[3\right]$doubleOutput
On exit: indicates information about the negative eigenvalues in the embedding matrix which have had to be set to zero. ${\mathbf{eig}}\left[0\right]$ contains the smallest eigenvalue, ${\mathbf{eig}}\left[1\right]$ contains the sum of the squares of the negative eigenvalues, and ${\mathbf{eig}}\left[2\right]$ contains the sum of the absolute values of the negative eigenvalues.
22:  $\mathbf{fail}$NagError *Input/Output
The NAG error argument (see Section 3.6 in the Essential Introduction).

## 6  Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 3.2.1.2 in the Essential Introduction for further information.
On entry, argument $〈\mathit{\text{value}}〉$ had an illegal value.
NE_ENUM_INT
On entry, ${\mathbf{np}}=〈\mathit{\text{value}}〉$.
Constraint: for ${\mathbf{cov}}=〈\mathit{\text{value}}〉$, ${\mathbf{np}}=〈\mathit{\text{value}}〉$.
NE_ENUM_REAL_1
On entry, ${\mathbf{params}}\left[〈\mathit{\text{value}}〉\right]=〈\mathit{\text{value}}〉$.
Constraint: dependent on cov, see documentation.
NE_INT_ARRAY
On entry, ${\mathbf{maxm}}=\left[〈\mathit{\text{value}}〉,〈\mathit{\text{value}}〉\right]$.
Constraint: the minimum calculated value for maxm are $\left[〈\mathit{\text{value}}〉,〈\mathit{\text{value}}〉\right]$.
Where the minima of ${\mathbf{maxm}}\left[\mathit{i}-1\right]$ is given by ${2}^{k}$, where $k$ is the smallest integer satisfying ${2}^{k}\ge 2\left({\mathbf{ns}}\left[\mathit{i}-1\right]-1\right)$, for $\mathit{i}=1,2$.
On entry, ${\mathbf{ns}}=\left[〈\mathit{\text{value}}〉,〈\mathit{\text{value}}〉\right]$.
Constraint: ${\mathbf{ns}}\left[0\right]\ge 1$, ${\mathbf{ns}}\left[1\right]\ge 1$.
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.
See Section 3.6.6 in the Essential Introduction for further information.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 3.6.5 in the Essential Introduction for further information.
NE_REAL
On entry, ${\mathbf{var}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{var}}\ge 0.0$.
NE_REAL_2
On entry, ${\mathbf{xmin}}=〈\mathit{\text{value}}〉$ and ${\mathbf{xmax}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{xmin}}<{\mathbf{xmax}}$.
On entry, ${\mathbf{ymin}}=〈\mathit{\text{value}}〉$ and ${\mathbf{ymax}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{ymin}}<{\mathbf{ymax}}$.

## 7  Accuracy

If on exit ${\mathbf{approx}}=1$, see the comments in Section 3 regarding the quality of approximation; increase the values in maxm to attempt to avoid approximation.

## 8  Parallelism and Performance

nag_rand_field_2d_predef_setup (g05zrc) is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
nag_rand_field_2d_predef_setup (g05zrc) 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.

None.

## 10  Example

This example calls nag_rand_field_2d_predef_setup (g05zrc) to calculate the eigenvalues of the embedding matrix for $25$ sample points on a $5$ by $5$ grid of a two-dimensional random field characterized by the symmetric stable variogram (${\mathbf{cov}}=\mathrm{Nag_VgmSymmStab}$).

### 10.1  Program Text

Program Text (g05zrce.c)

### 10.2  Program Data

Program Data (g05zrce.d)

### 10.3  Program Results

Program Results (g05zrce.r)

The two plots shown below illustrate the random fields that can be generated by nag_rand_field_2d_generate (g05zsc) using the eigenvalues calculated by nag_rand_field_2d_predef_setup (g05zrc). These are for two realizations of a two-dimensional random field, based on eigenvalues of the embedding matrix for points on a $100$ by $100$ grid. The random field is characterized by the exponential variogram (${\mathbf{cov}}=\mathrm{Nag_VgmExponential}$) with correlation lengths both equal to $0.1$.