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NAG C Library Manual

# NAG Library Function Documentnag_bessel_i1 (s18afc)

## 1  Purpose

nag_bessel_i1 (s18afc) returns a value for the modified Bessel function ${I}_{1}\left(x\right)$.

## 2  Specification

 #include #include
 double nag_bessel_i1 (double x, NagError *fail)

## 3  Description

nag_bessel_i1 (s18afc) evaluates an approximation to the modified Bessel function of the first kind, ${I}_{1}\left(x\right)$.
The function is based on Chebyshev expansions.
For large $x$, the function must fail because ${I}_{1}\left(x\right)$ cannot be represented without overflow.

## 4  References

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

## 5  Arguments

1:     xdoubleInput
On entry: the argument $x$ of the function.
2:     failNagError *Input/Output
The NAG error argument (see Section 3.6 in the Essential Introduction).

## 6  Error Indicators and Warnings

NE_REAL_ARG_GT
On entry, ${\mathbf{x}}=〈\mathit{\text{value}}〉$.
Constraint: $\left|{\mathbf{x}}\right|\le 〈\mathit{\text{value}}〉$.
$\left|{\mathbf{x}}\right|$ is too large and the function returns the approximate value of ${i}_{0}\left(x\right)$ at the nearest valid argument.

## 7  Accuracy

Let $\delta$ and $\epsilon$ be the relative errors in the argument and result respectively.
If $\delta$ is somewhat larger than the machine precision (i.e., if $\delta$ is due to data errors etc.), then $\epsilon$ and $\delta$ are approximately related by $\epsilon \simeq \left|\left({xI}_{0}\left(x\right)-{I}_{1}\left(x\right)\right)/{I}_{1}\left(x\right)\right|\delta$.
However, if $\delta$ is of the same order as machine precision, then rounding errors could make $\epsilon$ slightly larger than the above relation predicts.
For small $x$, $\epsilon \simeq \delta$ and there is no amplification of errors.
For large $x$, $\epsilon \simeq x\delta$ and we have strong amplification of errors. However, the function must fail for quite moderate values of $x$ because ${I}_{1}\left(x\right)$ would overflow; hence in practice the loss of accuracy for large $x$ is not excessive. Note that for large $x$, the errors will be dominated by those of the math library function exp.

None.

## 9  Example

The following program reads values of the argument $x$ from a file, evaluates the function at each value of $x$ and prints the results.

### 9.1  Program Text

Program Text (s18afce.c)

### 9.2  Program Data

Program Data (s18afce.d)

### 9.3  Program Results

Program Results (s18afce.r)