1. Introduction
This paper deals with the valuation of American-style basket options. Basket options constitute a popular type of financial derivatives and possess a payoff depending on a weighted average of different assets. In general, exact valuation formulas for such options are not available in the literature in semi-closed analytic form. Therefore, the development and analysis of efficient approximation methods for their fair values is of much importance.
In this paper, we consider the valuation of American basket options through partial differential complementarity problems (PDCPs). If
d denotes the number of different assets in the basket, then the pertinent PDCP is
d-dimensional. In this paper, we are interested in the situation where
d is medium or large, say
. It is well-known that this renders the application of standard discretisation methods for PDCPs impractical, due to the curse of dimensionality. For European- and Bermudan-style basket options, leading to high-dimensional partial differential equations (PDEs), an effective approach has been introduced by Reisinger and Wittum [
1] and next studied in, e.g., Reisinger and Wissmann [
2,
3,
4] and In ’t Hout and Snoeijer [
5]. This approach is based on a principal component analysis (PCA) and yields an approximation formula for the value of the basket option that requires the solution of a limited number of only low-dimensional PDEs. In the literature, an alternative useful approach has been investigated that employs the idea of comonotonicity. For European basket options, this comonotonic approach has been developed notably by Kaas et al. [
6], Dhaene et al. [
7,
8], Deelstra et al. [
9,
10] and Chen et al. [
11,
12]. Recently, an extension to American basket options has been presented by Hanbali and Linders [
13], who consider a comonotonic approximation formula that requires the solution of just two one-dimensional PDCPs. In the present paper we shall study and compare the PCA-based and comonotonic approaches for the effective valuation of American basket options. To our knowledge, this is the first paper where these two, different but related, approaches are jointly investigated. In our subsequent analysis, we shall include also the (simpler) case of European basket options.
A
European-style basket option is a financial contract that gives the holder the right to buy or sell a prescribed weighted average of
d assets at a prescribed maturity date
T for a prescribed strike price
K. We assume in this paper the well-known Black–Scholes model. Thus, the asset prices
(
) evolve according to a multidimensional geometric Brownian motion, which is given (under the risk-neutral measure) by the system of stochastic differential equations (SDEs)
Here
is time, with
representing the time of inception of the option,
is the given risk-free interest rate,
(
) are the given volatilities and
(
) is a multidimensional standard Brownian motion with given correlation matrix
. Further, the initial asset prices
(
) are given. In essentially all financial applications, the correlation matrix is full.
Let
be the fair value of a European basket option if at time till maturity
the
i-th asset price equals
(
). Financial mathematics theory yields that
u satisfies the
d-dimensional time-dependent PDE
whenever
. The PDE (
2) is also satisfied if
for any given
i, thus at the boundary of the spatial domain. At maturity time of the option its fair value is known and specified by the particular option contract. If
is the given payoff function of the option, then one has the initial condition
whenever
.
An
American-style basket option is a financial contract that gives the holder the right to buy or sell a prescribed weighted average of
d assets for a prescribed strike price
K at any given single time up to and including a prescribed maturity time
T. The fair value function
u of an American basket option satisfies the (nonlinear)
d-dimensional time-dependent PDCP
whenever
. The PDCP (4a–c) is provided with the same initial condition (
3). Further, (4a–c) also holds if
for any given
i.
In this paper, we shall consider the class of basket put options. These have a payoff function given by
with prescribed weights
(
) such that
.
An outline of our paper is as follows.
Following Reisinger and Wittum [
1], in
Section 2.1 a convenient coordinate transformation is applied to the PDE (
2) for European basket options by means of a spectral decomposition of the covariance matrix. This way, a
d-dimensional time-dependent PDE for a transformed option value function is obtained in which each coefficient is directly proportional to one of the eigenvalues. In
Section 2.2, this feature is exploited to derive a principal component analysis (PCA) based approximation. The key property of this approximation is that it is determined by just a limited number of one- and two-dimensional PDEs. The presentation in
Section 2.1 and
Section 2.2 follows largely that in [
5]. In
Section 2.3, the PCA-based approximation approach is extended to American basket options. This gives rise to an approximation that is defined by a limited number of one- and two-dimensional PDCPs. In
Section 3.1, an efficient discretisation of the one- and two-dimensional PDEs for European basket options is described, which employs finite differences on a nonuniform spatial grid followed by the Brian and Douglas Alternating Direction Implicit (ADI) scheme on a uniform temporal grid. This discretisation is adapted in
Section 3.2 to the pertinent PDCPs for American basket options, where the basic explicit payoff (EP) approach as well as the more advanced Ikonen–Toivanen (IT) splitting technique are considered.
Section 4 collects results from the literature on the comonotonic approach for valuing European and American basket options. We consider the same comonotonic approximation as Hanbali and Linders [
13], which is determined by just two one-dimensional PDEs (for the European basket) or PDCPs (for the American basket).
Section 5 contains the main contribution of our paper. In this section, we perform ample numerical experiments and obtain the positive result that the PCA-based and comonotonic approaches yield approximations to the option value that always lie close to each other for both European and American basket put options. We next study in detail the error in the discretisation described in
Section 3 for the PCA-based and comonotonic approximations and observe a favourable, near second-order convergence behaviour. The final
Section 6 presents our conclusions and outlook.
4. Comonotonic Approach
In a variety of papers in the literature, the concept of comonotonicity has been employed for arriving at efficiently computable approximations as well as upper and lower bounds for option values. For European-style basket options, relevant references to the comonotonic approach are, notably, Kaas et al. [
6], Dhaene et al. [
7,
8], Deelstra et al. [
9,
10] and Chen et al. [
11,
12]. Recently, an extension to American-style basket options has been considered by Hanbali and Linders [
13]. In this section, we review results obtained with the comonotonic approach and applied in loc. cit. Here the assumption has been made that the payoff function
is convex, which is satisfied by (
5), and that all correlations in the SDE system (
1) are nonnegative.
It follows from [
6] that an upper bound for the European basket option value function
u is acquired by setting all correlations in (
1) equal to one, i.e.,
for all
. Denote this upper bound by
. Consider the same coordinate transformations as in
Section 2.1 and denote the obtained transformed functions by
and
. The pertinent covariance matrix
has single nonzero eigenvalue
. Hence, the function
satisfies the one-dimensional PDE
whenever
,
. Next, the function
satisfies the one-dimensional PDE
whenever
,
. The same initial and boundary conditions apply as in
Section 2.1, using the pertinent function
.
It turns out that the upper bound above is, in general, rather crude. In the comonotonic approach, accurate lower bounds for the European basket option value have been derived, however. We consider here the lower bound chosen in [
13], which has been motivated by results obtained in [
6,
9]. Let
be given by
The lower bound is acquired upon replacing the volatility
by
for
and subsequently setting in (
1) all correlations equal to one. Denote this bound by
and the corresponding transformed functions by
and
. Then, with
, the function
satisfies the one-dimensional PDE
whenever
,
. Next, the function
satisfies the one-dimensional PDE
whenever
,
. The same initial and boundary conditions apply as in
Section 2.1, using the pertinent function
.
Clearly, the comonotonic upper as well as lower bound can be viewed as obtained upon replacing in the PDE (
2) the covariance matrix
by a certain matrix of rank one. For the lower bound, this rank-one matrix is given by
with (eigen)vector
and single nonzero eigenvalue
.
Based on a result by Vyncke et al. [
19], a specific linear combination of the comonotonic lower and upper bounds has been considered in [
13], which approximates the value of a European basket option. This
comonotonic approximation reads
where
is given by
with
In [
13], the authors next proposed (
25) as an approximation to the value of an American basket option, where
and
are now defined via the solutions
and
to the PDCP (16a–c) with
replaced by
and
, respectively, and function
replaced by
and
, respectively. We remark that, to our knowledge, it is an open question in the literature at present whether these functions
and
form actual lower and upper bounds for the American basket option value.
For the numerical solution of the pertinent PDEs and PDCPs, in [
13] a finite difference method was applied in space and the explicit Euler method in time, with the EP approach for American basket options. In the following, we shall employ the spatial and temporal discretisations described in
Section 3. In particular this allows for much less time steps than is required, in view of stability, by the explicit Euler method.
5. Numerical Experiments
In this section, we perform ample numerical experiments. Our main aims are to determine whether the PCA-based and comonotonic approaches define approximations to European and American basket put option values that lie close to each other, and next, to gain insight into the error of the discretisations described in
Section 3 in computing these approximations.
We consider two parts of experiments, depending on the parameter sets chosen for the basket option and underlying asset price model. In the first part we choose the same six parameter sets A–F as considered in [
5]. In the second part we shall select parameter sets similar to those in [
13].
Commencing with the first part, Set A is taken from Reisinger and Wittum [
1]. Here
,
,
,
and
The corresponding covariance matrix
has eigenvalues
and it is clear that
is dominant.
Sets B and C are obtained from Jain and Oosterlee [
20] and possess dimensions
and
, respectively. Here
,
,
and
,
,
for
. Sets B and C have
and
, respectively, and
. Hence,
is also dominant for these parameter sets.
Sets D, E, F possess dimensions
, respectively, where
,
,
and
,
,
for
with
. The relevant correlation structure has been considered in for example Reisinger and Wissmann [
2] and yields eigenvalues that decrease rapidly. Sets D, E, F have in particular
respectively.
It can be verified that for all Sets A–F the pertinent matrix of eigenvectors
Q satisfies the assumption from
Section 2.1.
Our first numerical experiment concerns the two adaptations of the temporal discretisation scheme to PDCPs by the EP and IT approaches as described in
Section 3.2 for American-style options. Consider Set A and
. For a fixed number of spatial grid points, given by
, we study the absolute error in the two pertinent discretisations of the PCA-based and comonotonic approximations
and
in function of the number of time steps
.
Figure 1 displays the obtained errors with respect to the values computed for a large number of time steps,
. Note that these errors do not contain the error due to spatial discretisation, but only due to the temporal discretisation.
Figure 1 clearly illustrates that, in the PCA-based as well as the comonotonic case, the IT approach yields a (much) smaller error than the EP approach for any given
N. Further, the observed order of convergence for IT is approximately 1.5, whereas for EP it is only approximately 1.0. The better performance of IT compared to EP is well-known in the literature, see, e.g., [
14,
17,
18]. Accordingly, in the following, we shall always apply the IT approach.
Let
as above.
Table 1 displays our reference values for the PCA-based and comonotonic approximations
and
, respectively, as well as the lower bound
for the European basket put option. These values have been obtained by applying the PDE discretisation from
Section 3 with
spatial and temporal grid points. Clearly, the positive result holds that, for each given set, the two approximations and the lower bound lie close to each other.
Similarly,
Table 2 shows our reference values for
,
,
for the American basket put option. These values have been obtained by applying the PDCP discretisation from
Section 3 and
. We find the favourable result that also in the American case, for each given set, the PCA-based and comonotonic approximations lie close to each other. Recall that, at present, it is not clear whether
forms an actual lower bound in this case.
We next study, for European and American basket put options and Sets A–F, the absolute error in the discretisation described in
Section 3 of the PCA-based and comonotonic approximations
and
in function of
. To determine the error of the discretisation for the PCA-based and comonotonic approximations, the corresponding reference values from
Table 1 and
Table 2 are used.
Figure 2 and
Figure 3 display for Sets A, B, C and D, E, F, respectively, the absolute error in the discretisation of
and
versus
, where the left column concerns the European option and the right column the American option.
As a main observation,
Figure 2 and
Figure 3 clearly indicate (near) second-order convergence of the discretisation error in all cases, that is, for all Sets A–F, for both the European and American basket options, and for both the PCA-based and comonotonic approximations. This is a very favourable result. Additional experiments indicate that the error stems essentially from the spatial discretisation (and not the temporal discretisation).
For the European option and Sets A and D, we remark that the error drop in the (less important) region corresponds to a change of sign. Besides this, in the case of the European basket option, the behaviour of the discretisation error is always seen to be regular.
For the American option, it is found that the discretisation error often behaves somewhat less regular, with oscillations occurring. A similar phenomenon has recently been observed and studied in [
5] for Bermudan basket options and is attributed to the spatial nonsmoothness of the exact option value function at the early exercise boundary.
In the following we consider the second part of experiments and choose parameter sets inspired by those from [
13]. Here a basket put option with
equally weighted underlying assets is taken and
. Next, the strike
and the maturity time
. For the interest rate we choose
(this differs from [
13] where the rate
is taken, but then American option values are often close to their European counterpart, which is less interesting). Finally, the volatilities are given by
with
. We select correlation
for all
. Then, for the pertinent two covariance matrices, the first eigenvalue is dominant. In particular, there holds
Further, the relevant matrices of eigenvectors
Q satisfy the assumption from
Section 2.1.
Table 3 and
Table 4 show our reference values for
,
,
for the European and American basket put option, respectively, which have been obtained in the same way as above. Again, we find the favourable result that, for each given parameter set and each given (European or American) option, these three values lie close to each other.
Figure 4 displays, analogously to
Figure 2 and
Figure 3, the absolute error in the discretisation of
and
for the (representative) three parameter sets given by
,
,
. The outcomes again indicate a favourable, second-order convergence result. The regularity of the error behaviour is seen to decrease as the maturity time
T increases. We note that for
this behaviour is partly explained from a (near) vanishing error when
.
6. Conclusions
The valuation of American basket options via
d-dimensional PDCPs constitutes a notoriously challenging task whenever the number of assets
d is medium or large. In this paper, we have studied an extension of the PCA-based approach by Reisinger and Wittum [
1] to valuate American basket options. This approximation approach is highly effective, as the numerical solution of only a limited number of low-dimensional PDCPs is required. In addition, we have considered the comonotonic approach, which was developed for basket options notably in [
6,
7,
8,
9,
10,
11,
12,
19]. We have studied the comonotonic approximation formula for American basket option values recently examined in Hanbali and Linders [
13]. The comonotonic approach is also highly effective, since it requires the numerical solution of just two one-dimensional PDCPs. To our knowledge, the present paper is the first in the literature where these two, different but related, approaches are jointly investigated.
For the discretisation of the pertinent PDCPs, we apply finite differences on a nonuniform spatial grid followed by the Brian and Douglas ADI scheme on a uniform temporal grid and selected the Ikonen–Toivanen (IT) technique [
15,
16,
17] to efficiently handle the complementarity problem in each time step.
As a first main result, we find in ample numerical experiments that the PCA-based and comonotonic approaches always yield approximations to the value of an American (as well as European) basket option that lie close to each other.
As a next main result, we observe near second-order convergence of the discretisation error in all numerical experiments for both the PCA-based and comonotonic approaches for American (as well as European) basket options.
At this moment it is still open which (if any) of the two approaches, PCA-based or comonotonic, is to be preferred for the approximate valuation of American basket options on assets. In particular, whereas in our experiments the two approaches always define approximations that lie close to each other, it is not clear at present which approach (if any) generally yields the smallest error with respect to the exact option value. The comonotonic approach requires less computational work than the PCA-based approach, but both are computationally cheap.
A further investigation into the PCA-based and comonotonic approaches, both experimental and analytical, will be the subject of future research. This concerns the open question above as well as their fundamental properties, such as convergence, and their range of applications.
The following abbreviations are used in this manuscript: