In this work, a new fourth-order finite difference (FD) approximation (for both structured and unstructured grid of nodes) is contributed and equipped with the fourth-order Runge–Kutta scheme to tackle the financial nonlinear partial differential equation (PDE) of Black–Scholes. This timedependent PDE problem is converted to a set of ordinary differential equations (ODEs). It is proved that under several criteria the procedure is time stable. Computational illustrations presented here, show that our approach is fast and accurate. The proposed technique reduces the computational cost, when more accurate results are requested.
1.
Introductory notes
In financial mathematics, options have become famous, so that in many cases more money is invested in them than in the underlying assets, both for hedging and for speculation [14]. There is a systematic procedure to obtain how much they are worthwhile, and hence they can be bought and sold with some confidence [36].
Options are basically traded directly among large financial institutes – also known as over-the-counter (i.e., OTC deals). These options often have nonstandard features that are tailored to the special requirements of the parties involved.
An asset is any financial object such as shares in a company, whose valuation is provided now, but is liable to modification in the future [10]. To discuss further, a call option of European type furnishes its owner the right (but without any obligation) to buy from the writer a specific asset for a specific price at a specific time in the future.
A fundamental option model is the following Black–Scholes partial differential equation (PDE) excluding the costs of transaction in a complete market [8]:
wherein r is the rate of interest without risk, q is the yield of dividend, and the initial conditions as comes next:
by considering E as the price of strike. The price of option, the underlying asset price and the 2nd derivative of V in terms of S are denoted by V, S and VSS, respectively.
Although several disadvantages are existed in (1.1), it is widely considered by traders and investors because of its simplicity, and also because, in several situations, it provides a fair valuation for price of options in the complete markets, see also the discussions of [21,33].
The most significant drawbacks of (1.1) are the followings. It considers a geometric Brownian motion (GBM), where the market might deviate from that assumption, as well as it assumes constant volatility, while the market is characterized by changing (stochastic) volatility regimes [28].
Therefore in practice, there are several caveats on the PDE of Black–Scholes (1.1), see [4] for more discussions. Theoretical models sometimes take a situation into account, wherein a portfolio optimization is exercised without the costs of transaction. Accordingly, more practical models at which the costs of transaction are considered, should be taken into account, see for example [18] and the references cited there. Noting that, the transaction costs reduce the expected return and thus cannot be ignored.
Two major resources for transaction costs are basically exist–fixed and impact costs. The first one is various types of fees (bank fee and so on) and generally they are functions with linear or piece-wise linear features of transaction size. The costs of impact are more challenging to model and generally have nonlinearity. Technically speaking, these costs are modifications in the considered asset's price that are caused by our own trading of that specific asset [7].
Pricing European-style option under the transaction costs, which are modeled by nonlinear PDEs is a the main topic of this research, because of the importance of considering transaction costs into (1.1).
It is recalling that for some time ago the Monte Carlo simulations were only employed for pricing European options while there is no such thing in recent years and they can successfully be applied for options with nonlinear volatilities [22]. However, the problem in pricing such options is that a model with time-dependent coefficients must be tackled, and this strategy relies on a several conditional expectations which are hard to compute. Thus numerical schemes for PDEs such as second-order finite difference (FD) or local/global RBF meshfree methods should be investigated, refer to [20,34].
In literature, different other techniques have been proposed, see e.g., [1] and a common note is the employment of high-order discretization schemes. To be more precise, the author in [1] discussed several merits of a 4th-rate compact scheme compared to the traditional solvers. To see some solvers for tackling the dynamic of financial models when come out as stochastic differential equations (SDEs), refer to [29,30]. See [16] for further details.
In this work, the derivatives involved in the PDE are estimated by higher order FD approximants contributed for both uniform and non-uniform grid of points. Then, a semi-discretization procedure [19] for discretizing on the asset variable is done to derive a set of (nonlinear) ordinary differential equations (ODEs).
After reviewing the preliminaries in Section 2, this paper presents a fully discretized high order FD method and compare its ability to simulate the governing equation of Black–Scholes under the transaction costs. We derive and propose new fourth-order FD discretization on any forms of discretization, i.e., for uniform and non-uniform discretizations in Section 3.
A classical 4th-rate explicit Runge–Kutta method is used along time in Section 4. The motivation in selecting such a scheme is that the fully discretized approach be of fourth-order accuracy in both time and space. An intensive numerical report will be furnished in Section 5 to validate the efficacy and usefulness of our scheme in option pricing under the costs of transaction. The simulation results confirmed that our fourth-order method is fruitful for studying the nonlinear Black–Scholes problem. Section 6 finalizes the work.
2.
The literature
The paper [23] discussed an improved volatility to develop the reliability of the option pricing PDE (1.1) as follows:
from a model of binomial at which
Herein, μ is the proportional cost of transaction, Δt is the period of transaction, σ0 is the initial constant of volatility.
Authors in [15] presented a correction of constant volatility of the following form:
wherein
and θ, ζ are nonnegative constants representing the the risk premium measure and transaction cost measure, respectively. One may refer to the work [22] for several further related discussions.
Barles–Soner [7] proposed another practical model for the implied volatility as follows:
wherein the maturity time is T, a=μ√γN, with factor of risk of aversion γ and N number of options for solding.
Computing the solution Z(⋅) based on the following first-order ODE which is nonlinear makes the overall pricing procedure challenging:
The implicit solution of (2.6) can be written as comes next [9]:
In this work, we use the exact solution (2.7) whenever required. Recalling that Z(x) is an increasing function to (−1,∞). To find the exact solution based on (2.7) algebraic nonlinear equations must be solved. In fact, for different values of x, corresponding nonlinear equations must be solved using iterative methods. Here the Newton's method without memory which has already been coded inside the built-in function Chop[FindRoot[]] is used. A Chop[] command has already been imposed so as to replace values smaller than 10−10 with zero in order to accelerate the use of the problem and avoid accumulation and rood-off errors.
The improved nonlinear Black–Scholes model (1.1) & (2.5) has a unique viscosity solution [7].
Here by employing U(S,τ)=V(S,t), τ=T−t, and considering (2.5), we present a novel computational scheme for the following PDE problem:
The problem (2.8) with its nonlinearity does not have a theoretical solution and this makes it requisite to present a robust scheme for pricing in the presence of transaction costs.
The parabolic PDE (2.8) is on the unbounded domain [27] below:
Now we need to truncate (2.9) into a bounded domain for the sake of numerical computation. Thus, we have
wherein smax, is a positive constant large enough for minimizing the error of truncating the domain.
It is recalled that for an option of call type, the boundary conditions are defined by [4]:
3.
Constructing a fourth-order scheme
Here, by employing a similar spirit as in [12,chapters 3-4] or [2,5] but with further terms in the expansions of Taylor, a 4th-rate FD scheme is built on unstructured grid of nodes.
In fact, in unstructured nodes and to preserve the 4th rate of speed on such grids, five and six points are required for estimating the 1st and 2nd derivatives of a given smooth function.
Let us assume that g(s) is sufficiently smooth and we have the following grid of points:
Consider the following five adjacent nodes:
and compute the interpolatory polynomial p(z) passing through the adjacent points, and thence its first differentiation p′(z).
By employing z=si and several symbolic computations, one obtains the following FD formulation on unstructured grids:
wherein the maximum spacing of local grid is h, one obtains
using ψl,q=sl−sq and
The sided FD formulas must be constructed similarly and used for the points on the boundaries and only the one near to the boundaries to have a unified fourth order of convergence. It is noted that in case of applying these formulas on structured grids, then the fourth order of convergence is preserved.
In a similar manner, FD approximants for the 2nd derivative terms can be obtained applying a similar methodology as above. So, let us consider the following set of points:
and calculate the 2nd-derivative interpolatory function p″(z) based on z. At this moment, by considering z=si and a symbolic language like Wolfram Mathematica [37], one obtains
where
Here, we have
To obtain (3.9), basically the procedure is to write the Taylor expansion of (3.7) up to fourth order and ignoring the larger errors terms. This can be done using a computer algebra system (CAS), here Mathematica, so as to obtain the weights properly and to keep the fourth order of convergence.
The procedure for obtaining the weights for the points {s1,s2,s3,sm−1,sm} to keep the fourth convergence order should be investigated by the six adjacent points as described above but for that specific node. The relations (3.3) and (3.7) are applicable on both non-uniform and uniform meshes and in case of equidistant discretion nodes, can yield in more accurate formulations.
4.
Constructing the set of equations and time stepping
Considering all the weights of the diffusion and convection terms along with the structure of (2.8), we can build a system of semi–discretized ODEs, see [17] for more details. This is pursued in this section using the notion of differentiation matrices.
By matrix of differentiation, we typically mean a matrix that interlinks a function derivative evaluated at the nodes to the function of the function values at these nodes. To illustrate further, we have:
wherein u_ is the function values vector and DS is the differentiation matrix. They are required when tackling PDEs by RBF–like or FD–like approximations.
Stencil weights are a special case of matrix of differentiation. Applying the novel approximations in (3.3) & (3.7), the differentiation matrices for the 1st and 2nd derivative of the function could be constructed and filled.
The set of ODEs after incorporating the boundaries can be given in the following compact matrix form:
where the time dependent coefficient matrix Γ(τ) is continuous on an interval in R.
Now, the system (4.2) is solved by the fourth–order explicit Runge–Kutta scheme as a temporal solver (considering the time discretization size ψτ), refer to [31] for an implementation. This is considered to have a fourth order discretization for both the spatial and temporal variables.
Consider uι as an estimate to the true solution u(τι), then we can construct our final (explicit) method. Select k+1 temporal nodes, a time step size Δτ=Tk>0, τι+1=τι+Δτ, 0≤ι≤k and u0=p(s), then the fourth–order RK method (RK4) [13] is provided as follows:
where
The quartic order refers to the localized error of truncation to be of O(Δτ5), and the total accumulated error is O(Δτ4). The following theorem addresses a criterion at which the fourth–order proposed scheme is time stable when a=0. A remark for the case when a≠0 will be given after the proof.
Theorem 4.1. As long as (4.2) is linear and reads the Lipschitz condition, then the iterative time–stepping scheme (4.3) is stable conditionally along time.
Proof. Here the Lipschitz condition gives (4.2) an existence and uniqueness of the solution. Besides, reminding that methods are A–stable as long as there are no stability restrictions for the following ODE problem:
This concept was furnished by Dahlquist for linear multi–step schemes (see for details [13,pages 40-50]). Incorporating the time–stepping method (4.3) on the system of ODEs (4.2) yields:
The explicit method (4.9) is time–stable if the matrix eigenvalues are:
have modulus less or equal than one. Viewing (4.9) as an iterative map, it would be clear that the eigenvalues of this matrix are
where ωi are the eigenvalues of matrix Γ. Thus, the A–stability is reduced to
Therefore, our proposed method is time–stable if the time step size Δτ reads (4.12). For similar strategy in order to find the stability of condition of our solver one may also refer to the discussions given in [32]. The stability function in (4.12) shows a conditional stable behavior for (4.3). The proof is complete now.
Theorem 1 is mostly useful for the linear case of (2.8). For the nonlinear case, the information on the stability of the method is difficult to cope with. As such, stability criterion in (4.12) is in fact a requisite criterion for the stability of nonlinear problems but it is not sufficient, see for more [11].
To establish the convergence of the proposed scheme which is the combination of the adaptive fourth-order FD scheme with a sixth-order RK method, it is now enough to recall the Dahlquist theorem [26,Chapter 6.7]. It states that as long as we assume that the starting values are correct and there is no round-off/cancellation errors, then the a multistep method (4.3) is convergent if and only if it is stable and consistent. This is valid as long as the stability condition (4.12) on the temporal step size holds and the the maximum spacing for spatial and temporal sizes tend to zero.
5.
Simulation reports
Here, we study the usefulness of our procedure for solving the time–dependent PDE problem (2.8) applying two different types of nodes, i.e., uniform nodes denoted by PMU and the nonuniform nodes denoted by PMNU.
The non–equidistant distribution of the discretization points for S can be used based on [6] as follows:
whereas m≫1 and xi are structured points on [0,1] and are denser around the strike E. We choose ξs=0.04915 in (5.1).
Some other notes are as follows:
● Traditional structured 2nd-rate FD discretization scheme equipped with the explicit Euler's method discussed in [9] is also used for comparisons and denoted by SFD.
● Here, we consider smin=0, smax=3E.
● Mathematica 11.0, [35] is employed for calculations.
● The specifications of our machine are 16.00 GB of RAM and Windows 7 with Intel(R) Core(TM) i5–2430M CPU 2.40GHz processor.
● The absolute errors (AE) and the l2 norm of the errors are reported for error compariosns.
Although three different methods have been compared here but the results are in agreement and also can compte the numerical results have recently been obtained in [3]. There are several other competitive and efficient schemes of higher orders in literature for doing option pricing such as the ones in [24,25] that we do not compare our scheme with them since they mainly tackle models without transaction costs.
Test 5.1. [7] This problem compares the effectiveness of various approaches for solving (2.8) in case of a vanilla call option having the following conditions:
The results are provided in Table 1 for the Test 5.1. The reference prices are obtained from the exact solution. Computational workouts show that by increasing the number of spatial discretization, PMNU and specially PMNU perform much better and produces accurate results in contrast to those of SFD.
To illustrate convergence rate of the whole scheme, we report the approximated real rate of convergence (ROC), applying the following formula
where AE(2m) is the solution of based on 2m nodes. If we use the obtained AE in l2 from Table 1, we obtain the following ROCs, {1.39,1.98} for SFD, {1.77,1.98} for PMU and {2.20,4.00} for PMNU. The ROCs show that despite there is a kink for the initial condition in option pricing under transaction costs, but the main proposed scheme PMNU quickly reaches the fourth order of convergence. This too upholds the theoretical discussions of Section 3.
Test 5.2. We assume same parameters as in Test 5.1 but with a=0.01. The choice of a small value for a is based on the real situations from the market as discussed in [7].
The reference solution here are: u(E+10,1)≃22.2960, u(E,1)≃14.6457, u(E−10,1)≃8.4032. The numerical results are furnished in Table 2 for a=0.01.
Because the costs of transaction are imposed for this experiment, the numerical solution is plotted for our fourth order procedures in Figure 1 so as to not only emphasize that the uniform and nonuniform discretizations applied in this work with fourth order are useful, but also to show the non-oscillatory positive behavior of the PMNU and PMNU.
Results in Table 2 show that once a≠0, viz the nonlinearity involved, the pricing procedure is getting harder due to non-constant system matrix in (4.2) with nonlinear entries.
In addition, the approximate solutions (option prices) become higher than the linear case in Table 1 with a=0 illustrating the effect of transaction costs.
6.
End notes
In this paper, after introducing the governing nonlinear equation in Section 2 for an important financial model, the details for the construction of new non-uniform fourth-order FD estimates of the 1st and 2nd derivatives of a given function are discussed in Section 3. Applying a semi-discretization approach, we have constructed an ODE system, which is nonlinear as long as the transaction cost is involved.
To march along time and to have a fully discretized fourth order convergent method, we have implemented the famous explicit Runge–Kutta scheme of fourth order. It was proved that under several conditions for the linear case (non–time–varying system (4.2)) the proposed method is time stable and subsequently a finer step size should be chosen for the time–varying case (nonlinear PDE model).
The emphasis in this paper was on proposing a fully fourth order method not only on the uniform grid of points but also for the non-equidistant distribution of the nodes with more concentration around the non-differentiable area of the initial condition.
The theoretical findings have been supported via several numerical investigations in Section 5 and it was illustrated that our scheme is fast, accurate as well as reduces the computational time for option pricing under transaction costs.
Future works could focus on applying this high order approach for American option pricing under transaction costs when the free boundary value problem is tackled with a penalty approach.
Acknowledgements
We would like to thank three anonymous referees whose insightful comments and suggestions helped us improve the first version of this work.
Conflicts of interest
The writers state that they have no conflicts of interest.