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Chapter Introduction
NAG Toolbox

# NAG Toolbox: nag_specfun_bessel_i1_scaled_vector (s18ct)

## Purpose

nag_specfun_bessel_i1_scaled_vector (s18ct) returns an array of values of the scaled modified Bessel function e|x|I1(x)${e}^{-|x|}{I}_{1}\left(x\right)$.

## Syntax

[f, ifail] = s18ct(x, 'n', n)
[f, ifail] = nag_specfun_bessel_i1_scaled_vector(x, 'n', n)

## Description

nag_specfun_bessel_i1_scaled_vector (s18ct) evaluates an approximation to e|xi|I1(xi)${e}^{-|{x}_{i}|}{I}_{1}\left({x}_{i}\right)$, where I1${I}_{1}$ is a modified Bessel function of the first kind for an array of arguments xi${x}_{\mathit{i}}$, for i = 1,2,,n$\mathit{i}=1,2,\dots ,n$. The scaling factor e|x|${e}^{-|x|}$ removes most of the variation in I1(x)${I}_{1}\left(x\right)$.
The function uses the same Chebyshev expansions as nag_specfun_bessel_i1_real_vector (s18at), which returns an array of the unscaled values of I1(x)${I}_{1}\left(x\right)$.

## References

Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications

## Parameters

### Compulsory Input Parameters

1:     x(n) – double array
n, the dimension of the array, must satisfy the constraint n0${\mathbf{n}}\ge 0$.
The argument xi${x}_{\mathit{i}}$ of the function, for i = 1,2,,n$\mathit{i}=1,2,\dots ,{\mathbf{n}}$.

### Optional Input Parameters

1:     n – int64int32nag_int scalar
Default: The dimension of the array x.
n$n$, the number of points.
Constraint: n0${\mathbf{n}}\ge 0$.

None.

### Output Parameters

1:     f(n) – double array
e|xi|I1(xi)${e}^{-|{x}_{i}|}{I}_{1}\left({x}_{i}\right)$, the function values.
2:     ifail – int64int32nag_int scalar
${\mathrm{ifail}}={\mathbf{0}}$ unless the function detects an error (see [Error Indicators and Warnings]).

## Error Indicators and Warnings

Errors or warnings detected by the function:

Cases prefixed with W are classified as warnings and do not generate an error of type NAG:error_n. See nag_issue_warnings.

W ifail = 1${\mathbf{ifail}}=1$
Constraint: n0${\mathbf{n}}\ge 0$.

## Accuracy

Relative errors in the argument are attenuated when propagated into the function value. When the accuracy of the argument is essentially limited by the machine precision, the accuracy of the function value will be similarly limited by at most a small multiple of the machine precision.

None.

## Example

```function nag_specfun_bessel_i1_scaled_vector_example
x = [0; 0.5; 1; 3; 6; 10; 1000; -1];
[f, ifail] = nag_specfun_bessel_i1_scaled_vector(x);
fprintf('\n    X           Y\n');
for i=1:numel(x)
fprintf('%12.3e%12.3e\n', x(i), f(i));
end
```
```

X           Y
0.000e+00   0.000e+00
5.000e-01   1.564e-01
1.000e+00   2.079e-01
3.000e+00   1.968e-01
6.000e+00   1.521e-01
1.000e+01   1.213e-01
1.000e+03   1.261e-02
-1.000e+00  -2.079e-01

```
```function s18ct_example
x = [0; 0.5; 1; 3; 6; 10; 1000; -1];
[f, ifail] = s18ct(x);
fprintf('\n    X           Y\n');
for i=1:numel(x)
fprintf('%12.3e%12.3e\n', x(i), f(i));
end
```
```

X           Y
0.000e+00   0.000e+00
5.000e-01   1.564e-01
1.000e+00   2.079e-01
3.000e+00   1.968e-01
6.000e+00   1.521e-01
1.000e+01   1.213e-01
1.000e+03   1.261e-02
-1.000e+00  -2.079e-01

```