Integer, Intent (In)	::	m, n, ldp
Integer, Intent (Inout)	::	ifail
Real (Kind=nag_wp), Intent (In)	::	x(m), s, t(n), sigma, r, q
Real (Kind=nag_wp), Intent (Inout)	::	p(ldp,n)
Character (1), Intent (In)	::	calput

C Header Interface

#include <nag.h>

void	s30qcf_ (const char calput, const Integer m, const Integer n, const double x[], const double s, const double t[], const double sigma, const double r, const double q, double p[], const Integer ldp, Integer *ifail, const Charlen length_calput)

The routine may be called by the names s30qcf or nagf_specfun_opt_amer_bs_price.

3 Description

s30qcf computes the price of an American option using the closed form approximation of Bjerksund and Stensland (2002). The time to maturity,

T

, is divided into two periods, each with a flat early exercise boundary, by choosing a time

t \in [0, T]

, such that

t = \frac{1}{2} (\sqrt{5} - 1) T

. The two boundary values are defined as

\tilde{x} = \tilde{X} (t)

\tilde{X} = \tilde{X} (T)

with

\tilde{X} (τ) = B_{0} + (B_{\infty} - B_{0}) (1 - \exp \{h (τ)\}),

where

h (τ) = - (b τ + 2 σ \sqrt{τ}) (\frac{X^{2}}{(B_{\infty} - B_{0}) B_{0}}),

B_{\infty} \equiv \frac{β}{β - 1} X, B_{0} \equiv \max \{X, (\frac{r}{r - b}) X\},

β = (\frac{1}{2} - \frac{b}{σ^{2}}) + \sqrt{{(\frac{b}{σ^{2}} - \frac{1}{2})}^{2} + 2 \frac{r}{σ^{2}}} .

with

b = r - q

, the cost of carry, where

r

is the risk-free interest rate and

q

is the annual dividend rate. Here

X

is the strike price and

σ

is the annual volatility.

The price of an American call option is approximated as

\begin{matrix} P_{call} & = & α (\tilde{X}) S^{β} - α (\tilde{X}) ϕ (S, t | β, \tilde{X}, \tilde{X}) + \\ ϕ (S, t | 1, \tilde{X}, \tilde{X}) - ϕ (S, t | 1, \tilde{x}, \tilde{X}) - \\ X ϕ (S, t | 0, \tilde{X}, \tilde{X}) + X ϕ (S, t | 0, \tilde{x}, \tilde{X}) + \\ α (\tilde{x}) ϕ (S, t | β, \tilde{x}, \tilde{X}) - α (\tilde{x}) Ψ (S, T | β, \tilde{x}, \tilde{X}, \tilde{x}, t) + \\ Ψ (S, T | 1, \tilde{x}, \tilde{X}, \tilde{x}, t) - Ψ (S, T | 1, X, \tilde{X}, \tilde{x}, t) - \\ X Ψ (S, T | 0, \tilde{x}, \tilde{X}, \tilde{x}, t) + X Ψ (S, T | 0, X, \tilde{X}, \tilde{x}, t), \end{matrix}

where

α

ϕ

and

Ψ

are as defined in Bjerksund and Stensland (2002).

The price of a put option is obtained by the put-call transformation,

P_{put} (X, S, T, σ, r, q) = P_{call} (S, X, T, σ, q, r) .

The option price

P_{i j} = P (X = X_{i}, T = T_{j})

is computed for each strike price in a set

X_{i}

i = 1, 2, \dots, m

, and for each expiry time in a set

T_{j}

j = 1, 2, \dots, n

4 References

Bjerksund P and Stensland G (2002) Closed form valuation of American options Discussion Paper 2002/09 NHH Bergen Norway

Genz A (2004) Numerical computation of rectangular bivariate and trivariate Normal and

t

probabilities Statistics and Computing 14 151–160

5 Arguments

1: $calput$ – Character(1) Input

On entry: determines whether the option is a call or a put.

$calput ='C'$: A call; the holder has a right to buy.
$calput ='P'$: A put; the holder has a right to sell.

Constraint:

calput ='C'

'P'

2: $m$ – Integer Input

On entry: the number of strike prices to be used.

Constraint:

m \geq 1

3: $n$ – Integer Input

On entry: the number of times to expiry to be used.

Constraint:

n \geq 1

4: $x (m)$ – Real (Kind=nag_wp) array Input

On entry:

x (i)

must contain

X_{i}

, the

i

th strike price, for

i = 1, 2, \dots, m

Constraint:

x (i) \geq z ​ and ​ x (i) \leq 1 / z

, where

z = x02amf ()

, the safe range parameter, for

i = 1, 2, \dots, m

5: $s$ – Real (Kind=nag_wp) Input

On entry:

S

, the price of the underlying asset.

Constraint:

s \geq z ​ and ​ s \leq \frac{1}{z}

, where

z = x02amf ()

, the safe range parameter and

s^{β} < \frac{1}{z}

where

β

is as defined in Section 3.

6: $t (n)$ – Real (Kind=nag_wp) array Input

On entry:

t (i)

must contain

T_{i}

, the

i

th time, in years, to expiry, for

i = 1, 2, \dots, n

Constraint:

t (i) \geq z

, where

z = x02amf ()

, the safe range parameter, for

i = 1, 2, \dots, n

7: $sigma$ – Real (Kind=nag_wp) Input

On entry:

σ

, the volatility of the underlying asset. Note that a rate of 15% should be entered as

0.15

Constraint:

sigma > 0.0

8: $r$ – Real (Kind=nag_wp) Input

On entry:

r

, the annual risk-free interest rate, continuously compounded. Note that a rate of 5% should be entered as

0.05

Constraint:

r \geq 0.0

9: $q$ – Real (Kind=nag_wp) Input

On entry:

q

, the annual continuous yield rate. Note that a rate of 8% should be entered as

0.08

Constraint:

q \geq 0.0

10: $p (ldp, n)$ – Real (Kind=nag_wp) array Output

On exit:

p (i, j)

contains

P_{i j}

, the option price evaluated for the strike price

x_{i}

at expiry

t_{j}

for

i = 1, 2, \dots, m

and

j = 1, 2, \dots, n

11: $ldp$ – Integer Input

On entry: the first dimension of the array p as declared in the (sub)program from which s30qcf is called.

Constraint:

ldp \geq m

12: $ifail$ – Integer Input/Output

On entry: ifail must be set to

0

- 1

1

to set behaviour on detection of an error; these values have no effect when no error is detected.

A value of

0

causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of

- 1

means that an error message is printed while a value of

1

means that it is not.

If halting is not appropriate, the value

- 1

1

is recommended. If message printing is undesirable, then the value

1

is recommended. Otherwise, the value

0

is recommended. 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, $calput = 〈value〉$ was an illegal value.

$ifail = 2$: On entry, $m = 〈value〉$ .
Constraint: $m \geq 1$ .

$ifail = 3$: On entry, $n = 〈value〉$ .
Constraint: $n \geq 1$ .

$ifail = 4$: On entry, $x (〈value〉) = 〈value〉$ .
Constraint: $x (i) \geq 〈value〉$ and $x (i) \leq 〈value〉$ .

$ifail = 5$: On entry, $s = 〈value〉$ .
Constraint: $s \geq 〈value〉$ and $s \leq 〈value〉$ .

$ifail = 6$: On entry, $t (〈value〉) = 〈value〉$ .
Constraint: $t (i) \geq 〈value〉$ .

$ifail = 7$: On entry, $sigma = 〈value〉$ .
Constraint: $sigma > 0.0$ .

$ifail = 8$: On entry, $r = 〈value〉$ .
Constraint: $r \geq 0.0$ .

$ifail = 9$: On entry, $q = 〈value〉$ .
Constraint: $q \geq 0.0$ .

$ifail = 11$: On entry, $ldp = 〈value〉$ and $m = 〈value〉$ .
Constraint: $ldp \geq m$ .

$ifail = 14$: On entry, $s = 〈value〉$ and $β = 〈value〉$ .
Constraint: $s^{β} < 〈value〉$ .

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 399$: Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

The accuracy of the output will be bounded by the accuracy of the cumulative bivariate Normal distribution function. The algorithm of Genz (2004) is used, as described in the document for g01haf, giving a maximum absolute error of less than

5 \times 10^{- 16}

. The univariate cumulative Normal distribution function also forms part of the evaluation (see s15abf and s15adf).

8 Parallelism and Performance

s30qcf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.

s30qcf 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

None.

10 Example

This example computes the price of an American call with a time to expiry of

3

months, a stock price of

110

and a strike price of

100

. The risk-free interest rate is

8 %

per year, there is an annual dividend return of

12 %

and the volatility is

20 %

per year.

s30qc: FL CL CPP AD

NAG FL Interfaces30qcf (opt_​amer_​bs_​price)

▸▿ 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

NAG FL Interface
s30qcf (opt_amer_bs_price)