Option Pricing under Double Heston Model with Approximative Fractional Stochastic Volatility

. We establish double Heston model with approximative fractional stochastic volatility in this article. Since approximative fractional Brownian motion is a better choice compared with Brownian motion in ﬁnancial studies, we introduce it to double Heston model by modeling the dynamics of the stock price and one factor of the variance with approximative fractional process and it is our contribution to the article. We use the technique of Radon–Nikodym derivative to obtain the semianalytical pricing formula for the call options and derive the characteristic functions. We do the calibration to estimate the parameters. The calibration demonstrates that the model provides the best performance among the three models. The numerical result demonstrates that the model has better performance than the double Heston model in ﬁtting with the market implied volatilities for diﬀerent maturities. The model has a better ﬁt to the market implied volatilities for long-term options than for short-term options. We also examine the impact of the positive approximation factor and the long-memory parameter on the call option prices.


Introduction
We propose a double Heston model with approximative fractional stochastic volatility in this article. ough the Black and Scholes [1] model has been a benchmark in European option pricing studies, the existence of the volatility smile made the stochastic volatility model more proper for application. e Hull and White [2] model, the Scott [3] model, the Wiggins [4] model, the Stein and Stein [5] model, and the Lewis [6] model are developed, but most of these stochastic volatility models are not proper for application. e Heston [7] model which is determined by a Cox-Ingersoll-Ross (CIR) [8] process displays the property of mean reversion effect and the character of stationarity. It is the pioneering work in the history of stochastic volatility model studies since Heston technically derived the closed-form formula for the option prices by using characteristic functions. Bakshi et al. [9] empirically showed that stochastic volatility model has better performance than the Black-Scholes [1] model. However, the one-factor stochastic volatility model has some restrictions on exploring the correlation between the volatility and the smile. For instance, one-factor stochastic volatility model cannot explain some specific phenomena that the figure of the smile is unconnected with the volatility. Christoffersen et al. [10] developed a double Heston model for exploring the correlation between the volatility and the smile.
eir study showed that a two-factor model can explain the correlation between the volatility and the figure of the smile and has more flexibleness to present the volatility term structure. One factor having greater mean-reverting level explains the correlation between short-period returns and the square of volatility. e other one having smaller meanreverting level explains the correlation between long-period returns and the square of volatility. e financial market owns the properties of self-similarity and long-range dependence. One should consider it in financial studies. Fractional Brownian motion is centered Gaussian process and it also owns the two properties. erefore, compared with Brownian motion, it is a better financial tool. Hu and Øksendal [11] developed Wick products to analyze it because it is not a Markov process, or semimartingale, and therefore, the traditional Itô theory is not appropriate for the application of it. Its biggest disadvantage is that it cannot be economically explained very well [12]. If some specific conditions hold, Brownian motion can be expressed equivalently by using the mixed fractional Brownian motion [13], and therefore, one can use the latter for financial application [14][15][16]. Approximative fractional Brownian motion can also be used to replace fractional Brownian motion [17]. Two terms multiplied by a function can be used to express fractional Brownian motion; one can only concern the second term since it has long-range dependence and the first term has continuous trajectories [17]. Approximative fractional Brownian motion is the approximation of the second term and it is a semimartingale [17]. ao [17] has proved that when the dynamics of the stock price is driven by approximative fractional Brownian motion, it has no arbitrage opportunity. Several papers have used it as an efficient tool in financial modeling [18,19]. Several papers presented empirical studies on option pricing models with approximative fractional Brownian motion. Mrázek et al. [20] developed an approximative fractional stochastic volatility model for option pricing application. ey applied the Lewis formula [6] for obtaining the semi-closed-form pricing formula and the fundamental transform and used the optimization approach for calibration. ey considered two errors and five weights for calibration. Pospíšil and Sobotka [21] also developed an approximative fractional stochastic volatility model. ey used the Heston approach [7] for obtaining the semiclosed-form pricing formula and the characteristic functions and used the optimization approach for calibration. ey considered three errors and three weights for calibration. Several authors also developed an option pricing model with approximative fractional Brownian motion from a different point of view. Kang et al. [22] developed an approximative fractional stochastic volatility model to study foreign exchange (FX) options. ey derived the pricing formula and used some numerical approach to examine the impact of the long-memory parameter and the positive approximation factor on the option prices.
ere are two typical approaches to derive the formulae for the characteristic functions. e first approach is developed by Duffie et al. [23]. ey expended the Heston [7] model on the multifactor models which are called the affine jump-diffusion (AJD) models. Using the Feynman-Kac theorem, the closed form of characteristic functions can be derived. It is generally used to derive the characteristic functions for obtaining the pricing formula for the options. e advantage of the approach is that the solutions for the characteristic functions can be obtained when some specific conditions are met. e second one is introduced by Schöbel and Zhu [24] and developed by Zhu [25]. Schöbel and Zhu [24] used the expectation approach to derive the characteristic functions, and Zhu [25] extended it on the multifactor models and used the modular approach to derive the characteristic functions.
Specifically, the formulae for the characteristic functions can be expressed as a structure consisting of several modules, and the formulae for the modules are derived by the expectation approach. e advantage of this approach is that when the option pricing models are determined by several stochastic factors, one can obtain the characteristic functions by integrating the corresponding parts of the formulae for the characteristic functions to be a unified structure. Ahlip et al. [26] proposed a double Heston hybrid model with jumps. ey used expectation approach and martingale approach to obtain the pricing formula for the options. Meanwhile, another approach can also be used in several papers. For instance, He and Zhu [27] developed an option pricing model; they expressed the pricing formula for the options with the forward characteristic function. To obtain the formula for the options and corresponding characteristic function, they changed the dynamics of their model to the forward measure.
e loss function approach is generally used for the estimation of the parameters and there are two classes of the loss functions [28]. e first class is minimizing the error between the quoted and model prices. ere are two approaches generally used in the first class. e first one is to minimize the absolute value of the mean-squared error (MSE) between the quoted and model prices. Since this approach puts too much weight on long-term and in-themoney options, some authors prefer the second approach for estimating parameters [29]. e second one is to minimize the relative value of the mean-squared error (RVMSE) between the quoted and model prices. e second class is minimizing the error between the quoted and model implied volatilities. If one considers the denominator to be the weight attached to the loss function, there are several choices one can decide to be the weight. Christoffersen et al. [10] developed a loss function that they used the Black-Scholes sensitivity as the weight. e difference between the market prices of the bid and ask also can be taken as the weight [21]. ough there is no concern that which approach is better, the approach used for estimation and evaluation should be the same [29].
We develop double Heston model with approximative fractional stochastic volatility in this article. It is an extensive study based on the study of Ahlip et al. [26] and Kang et al. [22]. Our contribution is that we introduce approximative fractional Brownian motion to double Heston model by modeling the dynamics of the stock price and one factor of the variance with approximative fractional process and estimate the parameters under this model. We do some pioneering work to study this innovative model. We derive the semianalytical pricing formula. We do calibration to estimate the parameters and examine the impact of the longmemory parameter and the positive approximation factor on the call option prices.
We form the article as follows. We develop the model and then derive the semianalytical pricing formula in Section 2. We derive the characteristic functions in Section 3. We estimate the parameters and give some numerical study in Section 4. e conclusion is presented in Section 5.

The Double Heston Model with Approximative Fractional Stochastic Volatility
Let (Ω, F, P) be a probability space; F � (F t ) t∈[0,T] is the filtration and P is the risk-neutral probability measure. e stock price process T] are given by the following dynamic system: First, we give some preliminaries about fractional Brownian motion (B H t ) t≥0 and the approximation of it. It is a Gaussian process including Hurst index H ∈ (0, 1). It has zero mean and the covariance equation it is a standard Brownian motion. We usually assume H ∈ ((1/2), 1) for financial application since it displays longrange dependence in that case [19]. It can be decomposed as dW s , W t denotes a standard Brownian motion, and Γ denotes the gamma function. Since Z t has continuous trajectories, we can just consider the second term in the bracket which is B t � t 0 (t − s) H− (1/2) dW s and it has long-range dependence property [17,[19][20][21]. B t can be approximated by approximative fractional Brownian motion B ε,H t which is given by is semimartingale [17]. e dynamics of it can be given by [17] where H represents a long-memory parameter, we consider H ∈ ((1/2), 1) since we only concern the long-range dependence property [20,21], ε is an approximation factor, and ψ t is a stochastic process given by Some assumptions are given by the following statements: T] are mutually independent Brownian motions e call option price formula at time t ∈ [0, T] with strike K under P is given by e forward price for the stock F(t, T) at time t can be defined by F(t, T): � (S t /e − r(T− t) ), and then, we can have that S T � F(T, T), and C t (T, K) can be expressed as Lemma 1. e dynamics of F(t, T) under P can be given by the following equation: where (σ F (u, T)) t∈[0,T] and W � (W t ) t∈[0,T] are R 3 -valued processes given by Proof. We can obtain the following equation using Itô's lemma: We can obtain the expression for the dynamics of F(t, T) by applying Itô's lemma, and the proof is complete. erefore, we can obtain the following equation: □ Definition 1. We define the forward measure P T which is equivalent to P on (Ω, F T ): erefore, the formula for the call option price can be expressed as follows: where x t � ln F(t, T). where ), and f 1 (ϕ) and f 1 (ϕ) are the characteristic functions. erefore, we only need to derive the formulae for the two characteristic functions for obtaining the formula for the call option price.

The Derivation of the Characteristic Functions
We derive the formulae for the characteristic functions in this section.
Proof. We can obtain the following equation: erefore, we can obtain that We insert c t � exp(iϕx t ), α � 1 + iϕ, and β � iϕ into the above equation, and then we can obtain that We decompose W S into two parts: where W � (W t ) t∈[0,T] is a Brownian motion under P and not correlated with W S , W S , W v , W v , and W ψ . Consequently, we can obtain that We denote τ � T − t and we can obtain the following equations according to the dynamics of the model: Mathematical Problems in Engineering 5 We insert the above equations into the above expression for f 1 (ϕ) and rearrange it, and then we can obtain that We can rearrange it to obtain that erefore, we can obtain the following equation: where s 1 , s 2 , s 3 , and s 4 are given by equation (15). We can obtain the formula for f 2 (ϕ) by applying the same method used for deriving the formula for f 1 (ϕ): Inserting β � iϕ and decomposition of W S into the above equation, then we can obtain that We denote c � 1 − iϕ and rearrange it to obtain that Moreover, erefore, we can obtain the following equation: where q 1 , q 2 , q 3 , and q 4 are given by equation (16).
To complete the formulae for f 1 (ϕ) and f 2 (ϕ), we present the calculation of the expectations in the formulae for f 1 (ϕ) and f 2 (ϕ).

Lemma 2. If the processes v and v follow the dynamics in the model with independent Brownian motions
then the formula for it is given by Mathematical Problems in Engineering

Calibration
In this section, we use the loss function to estimate the parameters in the Heston model, double Heston model, and the model we developed and compare the performance among the three models. e market data offered on May 10, 2012, we use are the implied volatilities of the put options on Dow Jones Industrial Average ETF [28]. Since there are four maturities, we consider 0.001 as the risk-free interest rate. We set ε �1e − 5. We need to estimate the following parameters: As we mentioned before, there are two approaches generally used for calibration, to minimize the absolute value of the mean-squared error (MSE) and the relative value of the mean-squared error (RVMSE) between the quoted and model prices. We consider the first approach for calibration.
To be specific, first, we choose to use the bisection algorithm [28] to obtain the market prices since we only have the quoted implied volatilities data. Second, we use the market prices and the first approach to estimate the parameters. Suppose that the total number of the quotes is N, for a set number of N T maturities τ t (t � 1, . . . , N T ), there are a set number of N K strikes K k (k � 1, . . . , N K ). For each (τ t , K k ), the corresponding market price is C t,k and the model price is C Θ t,k . We consider the following loss function: (41) Table 1 shows the calibration results. It demonstrates that the model provides the smallest MSE and IVMSE. e estimation of the parameters also accords with the study based on Christoffersen et al. [10]. For both double Heston model and the model, one factor of the variance offers a higher mean-reverting level and the other factor of the variance offers a lower mean-reverting level. Since double Heston model offers better performance on MSE and IVMSE compared with the Heston model under the loss function we use, we plot the implied volatilities based on the former and the model using the estimated parameters, and the market implied volatilities for different maturities. Figure 1 shows the result. It demonstrates the model gives better performance than double Heston model on fitting with the market implied volatilities for different maturities. Compared with short-term options, the model has a better fit to the market implied volatilities on long-term options.
We also give some numerical results to examine the impact of the approximation factor ε and the long-memory parameter H on the call option prices. We set the values of parameters as follows: S � 100, K � 100, r � 0.005, and ξ � 0.5. e values of the other parameters are set in Table 2. Table 3 shows the result. It demonstrates the impact of the parameters ε and H on the prices of the call options.    When the value of ε is fixed, the call option price decreases as H increases. When the value of H is fixed, the call option price decreases as ε decreases.

Conclusion
We developed a double Heston model with approximative fractional stochastic volatility in this article. It is an extensive study based on the study of Ahlip et al. [26] and Kang et al. [22]. Since approximative fractional Brownian motion can replace fractional Brownian motion, we introduced it to double Heston model by modeling the dynamics of the stock price and one factor of the variance with approximative fractional process and it is our contribution to this article. We did some pioneering work to study this innovative model. We derived the pricing formula for the options and used the expectation approach to derive the characteristic functions. We did the calibration to estimate the parameters using the market data. e calibration demonstrates that the model we developed provides the smallest MSE and IVMSE.
In both double Heston model and the model, one factor of the variance has a higher mean-reverting level and the other factor of the variance has a lower mean-reverting level. e result accords with the study of Christoffersen et al. [10]. Since double Heston model offers better performance on MSE and IVMSE compared with the Heston model under the loss function we use, we plotted the implied volatilities based on the former and the model using the estimated parameters, and the market implied volatilities for different maturities. It demonstrates the model gives better performance than double Heston model on fitting with the market implied volatilities for different maturities. Compared with short-term options, the model has a better fit to the market implied volatilities on long-term options. We also examined the impact of the positive approximation factor and the long-memory parameter on the call option prices. e result demonstrates that when the value of the positive approximation factor is fixed, the call option price decreases as the long-memory parameter increases. When the value of the long-memory parameter is fixed, the call option price decreases as the positive approximation factor decreases.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.