Hindawi Publishing Corporation International Journal of Stochastic Analysis Volume 2012, Article ID 971212, 15 pages doi:10.1155/2012/971212 Research Article The First Passage Time and the Dividend Value Function for One-Dimensional Diffusion Processes between Two Reflecting Barriers Chuancun Yin1 and Huiqing Wang1, 2 1 2 School of Mathematical Sciences, Qufu Normal University, Shandong 273165, China School of Economics and Management, Southeast University, Nanjing 211189, China Correspondence should be addressed to Chuancun Yin, ccyin@mail.qfnu.edu.cn Received 26 July 2012; Accepted 24 September 2012 Academic Editor: Enzo Orsingher Copyright q 2012 C. Yin and H. Wang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We consider the general one-dimensional time-homogeneous regular diffusion process between two reflecting barriers. An approach based on the Itô formula with corresponding boundary conditions allows us to derive the differential equations with boundary conditions for the Laplace transform of the first passage time and the value function. As examples, the explicit solutions of them for several popular diffusions are obtained. In addition, some applications to risk theory are considered. 1. Introduction and the Model Diffusion processes with one or two barriers appear in many applications in economics, finance, queueing, mathematical biology, and electrical engineering. Among queueing system applications, reflected Ornstein-Uhlenbeck and reflected affine processes have been studied as approximations of queueing systems with reneging or balking 1, 2. Motivated by Ward and Glynn’s one-sided problem, Bo et al. 3 considered a reflected Ornstein-Uhlenbeck process with two-sided barriers. In this paper, we consider the expectations of some random variables involving the first passage time and local times for the general one-dimensional diffusion processes between two reflecting barriers. Let X {Xt , t ≥ 0} be a one-dimensional time-homogeneous reflected diffusion process with barriers a and b, which is defined by the following stochastic differential equation: dXt μXt dt σXt dBt X0 x ∈ a, b, dLt − dUt , 1.1 2 International Journal of Stochastic Analysis where Bt is a Brownian motion in R, L {Lt , t ≥ 0} and U {Ut , t ≥ 0} are the regulators of point a and b, respectively. Further, the processes L and U are uniquely determined by the following properties see, e.g., 4: 1 both t → Lt and t → Ut are continuous nondecreasing processes with L0 U0 0, t 2 L and U increase only when X a and X b, respectively, that is, 0 I{Xs a} dLs Lt t and 0 I{Xs b} dUs Ut , for t ≥ 0. It is well known that under certain mild regularity conditions on the coefficients μx and σx, the SDE 1.1 has a unique strong solution for each starting point see, e.g., 5. The solution Xt is a time-homogeneous strong Markov process with infinitesimal generator Afx 1 2 σ xf x 2 μxf x, x ∈ a, b, 1.2 acting on functions on a, b subject to boundary conditions: f a f b 0. Define the first passage time τy inf t ≥ 0 : Xt y , 1.3 where τy ∞ if Xt never reaches y. For λ > 0, η > 0, θ > 0, we consider the Laplace transform ϕ, and the value functions ψ, ψ1 , and ψ2 on x ∈ a, b: ϕx Ex e−λτy , 1.4 ∞ ∞ −λt ψx Ex η e dUt − θ e−λt dLt , 0 ψ1 x Ex ∞ 1.5 0 e−λt dUt , 1.6 e−λt dLt . 1.7 0 ψ2 x Ex ∞ 0 The rest of the paper is organized as follows. Section 2 studies the Laplace transform of the first passage time. Section 3 deals with the value function. Some applications in risk theory are considered in Section 4. 2. Laplace Transform Bo et al. 3 consider the Laplace transform Ex e−λτy for a reflected Ornstein-Uhlenbeck process with two-sided barriers. In this section we consider the Laplace transform of the first passage time for the general reflected diffusion process X defined by 1.1. International Journal of Stochastic Analysis 3 Theorem 2.1. Let x ∈ a, b, λ > 0, and assume that f1 y, f2 y satisfy the following equations, respectively: Af1 y λf1 y , y ∈ a, b, f1 a 0, Af2 y λf2 y , 2.1 y ∈ a, b, f2 b 0. 0 for y ∈ x, b and f2 y / 0 for y ∈ a, x, then If f1 y / f1 x Ex e−λτy , f1 y y ∈ x, b, f2 x , Ex e−λτy f2 y y ∈ a, x. 2.2 Proof. Applying the Itô formula for semimartingales to ht, Xt e−λt fXt with f ∈ Cb2 a, b we obtain t ht, Xt h0, X0 0 t t ∂h hs, Xs ds ∂s 0 ∂h hs, Xs dXs ∂x 2 ∂h hs, Xs dXs , Xs 2 0 ∂x t t fX0 −λe−λs fXs ds e−λs f Xs dXs 1 2 1 2 t fx 0 0 0 2.3 e−λs f Xs σ 2 Xs ds t e−λs A − λfXs ds 0 f a t e−λs dLs − f b 0 t t e−λs f Xs dBs 0 e−λs dUs . 0 Since τy < ∞ is a stopping time and x ∈ a, b, it follows from the optional sampling theorem that Ex e−λτy f Xτy fx τy Ex e−λs A − λfXs ds 0 f aEx τy 0 e−λs dLs − f bEx τy 0 e−λs dUs . 2.4 4 International Journal of Stochastic Analysis By the definitions of τy , U, and L we have τy Ex e−λs dLs 0, τy y ∈ a, x; Ex 0 e−λs dUs 0, y ∈ x, b; X τy y. 2.5 0 Substituting them into 2.4 one gets Ex e−λτy f y fx τy Ex e−λs A − λfXs ds 0 f aEx τy e−λs dLs , y ∈ x, b. 0 Ex e−λτy f y fx 2.6 τy Ex e −λs A − λfXs ds 0 − f bEx τy e−λs dUs , y ∈ a, x. 0 The result follows. Remark 2.2. Although neither f1 · nor f2 · in the Theorem 2.1 is unique, but each of their ratios is unique. As illustrations of Theorem 2.1, we consider some examples. Example 2.3. Bessel process: dXt d − 1/2Xt dt dBt , where d > 1 is a real number. We consider the differential equation 1/2ψ x d − 1/2xψ x λψx, λ > 0. It is well known that the increasing and decreasing solutions are, respectively: ψ x x−ν Iν 2λx , ψ− x x−ν Kν 2λx , 2.7 where ν d − 2/2 and Iν and Kν are the usual modified Bessel functions. Then, we can give f1 x, f2 x as follows: f1 x C1 ψ x C2 ψ− x, f2 x C3 ψ x C4 ψ− x, 2.8 where the constants C1 , C2 and C3 , C4 can be derived from f1 a 0 and f2 b 0, respectively. We can obtain their ratios, respectively: √ √ √ 2λaKν 2λa − νKν 2λa C1 M − √ , √ √ C2 2λaIν 2λa − νIν 2λa √ √ √ 2λbKν 2λb − νKν 2λb C3 − √ . N √ √ C4 2λbIν 2λb − νIν 2λb 2.9 International Journal of Stochastic Analysis 5 Substituting them into 2.2, we get Ex e−λτy Ex e−λτy √ Mx−ν Iν 2λx x−ν Kν 2λy y−ν Kν 2λx x−ν Kν 2λy y−ν Kν √ My−ν Iν √ Nx−ν Iν √ Ny−ν Iν √ 2λx , √ 2λy y ∈ x, b, 2.10 √ 2λx , √ 2λy y ∈ a, x. Example 2.4. The Ornstein-Uhlenbeck process 6 is as follows: dXt vk − Xt dt σdBt , v, σ > 0, k ∈ a, b. 2.11 In mathematical finance, the Ornstein-Uhlenbeck process above is known as Vasicek model for the short-term interest rate process 7. We consider the differential equation 1/2σ 2 ψ x vk − xψ x λψx. In the case k 0, σ 1, the two independent solutions to 1/2ψ x−vxψ x λψx are √ √ 2 ψ x H−λ/v − vx 2−λ/2v e1/2vx D−λ/v − 2vx , ψ− x H−λ/v √ vx 2−λ/2v e1/2vx Dλ/v 2 √ 2vx , 2.12 where Hν · and Dν · are, respectively, the Hermite and parabolic cylinder functions 8. Then, as the way used in Example 2.3, we obtain the ratios of the constants C1 , C2 and C3 , C4 , respectively: √ √ √ 2vDλ/v 2va vaDλ/v 2va C1 M − , √ √ √ C2 vaD−λ/v 2va − 2vD−λ/v 2va N vbDλ/v C3 − C4 vbD −λ/v √ √ √ 2vb 2vDλ/v 2vb . √ √ √ 2vb 2vb − 2vD−λ/v 2.13 Substituting them into 2.2, we get √ 2 Me1/2vx D−λ/v − 2vx Ex e−λτy √ 2 Me1/2vy D−λ/v − 2vy √ 2 Ne1/2vx D−λ/v − 2vx Ex e−λτy √ 2 Ne1/2vy D−λ/v − 2vy e1/2vx Dλ/v 2 e1/2vy Dλ/v 2 e1/2vx Dλ/v 2 e1/2vy Dλ/v 2 √ 2vx , √ 2vy y ∈ x, b, 2.14 √ 2vx , √ 2vy y ∈ a, x. 6 International Journal of Stochastic Analysis For the general k and σ, the two independent solutions are, respectively √ √ v 2v −λ/2v 1/2v/σ 2 x−k2 ψ x H−λ/v − e D−λ/v − x − k 2 x − k , σ σ √ √ v 2v −λ/2v 1/2v/σ 2 x−k2 ψ− x H−λ/v e D−λ/v x − k 2 x − k . σ σ 2.15 Then, as the way used in Example 2.3, we obtain the ratios of the constants C1 , C2 and C3 , C4 , respectively √ √ √ √ va − kD−λ/v 2v/σ a − k 2v/σ a − k 2σD−λ/v C1 −√ , M √ √ √ C2 − 2v/σ a − k va − kD−λ/v − 2v/σ a − k − 2σD−λ/v √ √ √ √ vb − kD−λ/v 2v/σ b − k 2σD−λ/v 2v/σ b − k C3 −√ . N √ √ √ C4 − 2v/σ b − k vb − kD−λ/v − 2v/σ b − k − 2σD−λ/v 2.16 Substituting them into 2.2, we get √ hx MD−λ/v − 2v/σ x − k Ex e−λτy √ h y MD−λ/v − 2v/σ y − k √ hx ND−λ/v − 2v/σ x − k −λτy Ex e √ h y ND−λ/v − 2v/σ y − k D−λ/v D−λ/v D−λ/v D−λ/v √ 2v/σ x − k √ 2v/σ y − k , y ∈ x, b, , y ∈ a, x, √ 2v/σ x − k √ 2v/σ y − k 2.17 where hx e1/2v/σ 2 x−k2 . Remark 2.5. If we take k 0, σ 1, a 0 and substitute the series forms of Hν · and Dν · into the above result, then it is the same as Bo et al. 3. Example 2.6. The square root process of Cox et al. 9: dXt vk − Xt dt σ Xt dBt , v, σ > 0, k ∈ a, b. 2.18 Now consider the differential equation 1 2 σ xψ x 2 vk − vxψ x λψx, λ > 0. 2.19 International Journal of Stochastic Analysis 7 If 2v/σ 2 k is not an integer, the two linear independent solutions are ψ x M λ 2v 2v , 2 k, 2 x , v σ σ ψ− x U λ 2v 2v , 2 k, 2 x , v σ σ 2.20 where M and U are the confluent hypergeometric functions of the first and second kinds, respectively. Then, as the way used in Example 2.3, we obtain the ratios of the constants C1 , C2 and C3 , C4 , respectively: M1 U λ/v, 2v/σ 2 k, 2v/σ 2 a C1 , − C2 M λ/v, 2v/σ 2 k, 2v/σ 2 a 2.21 U λ/v, 2v/σ 2 k, 2v/σ 2 b C3 . − N C4 M λ/v, 2v/σ 2 k, 2v/σ 2 b Substituting them into 2.2, we get M1 M λ/v, 2v/σ 2 k, 2v/σ 2 x Ex e−λτy M1 M λ/v, 2v/σ 2 k, 2v/σ 2 y NM λ/v, 2v/σ 2 k, 2v/σ 2 x Ex e−λτy NM λ/v, 2v/σ 2 k, 2v/σ 2 y U λ/v, 2v/σ 2 k, 2v/σ 2 x , U λ/v, 2v/σ 2 k, 2v/σ 2 y U λ/v, 2v/σ 2 k, 2v/σ 2 x U λ/v, 2v/σ 2 k, 2v/σ 2 y , y ∈ x, b. y ∈ a, x. 2.22 Example 2.7. The Gompertz Brownian motion process 10 is as follows: dXt vXtlnk − lnXt dt σXt dBt , v, σ > 0, k ∈ a, b. 2.23 Now consider the differential equation 1 2 2 σ x ψ x 2 vxln k − ln xψ x λψx, λ > 0. 2.24 The increasing and decreasing solutions are, respectively: ⎛ λ 1 v ψ x M⎝ , , 2 2v 2 σ ⎛ ψ− x U⎝ λ 1 v , , 2v 2 σ 2 x ln k ln x k σ2 2v 2 ⎞ ⎠, 2 ⎞ σ ⎠, 2v 2 2.25 8 International Journal of Stochastic Analysis where M and U, as in Example 2.6, are the first and second Kummer’s functions, respectively. Then, as the way used in Example 2.3, we obtain the ratios of the constants C1 , C2 and C3 , C4 , respectively: 2 U λ/2v, 1/2, v/σ 2 lna/k σ 2 /2v C1 M1 − , C2 M λ/2v, 1/2, v/σ 2 lna/k σ 2 /2v2 N U λ/2v, 1/2, v/σ 2 lnb/k C3 − C4 M λ/2v, 1/2, v/σ 2 lnb/k σ 2 /2v 2.26 2 σ 2 /2v2 . Substituting them into 2.2, we get 2 M1 M λ/2v, 1/2, v/σ 2 lnx/k σ 2 /2v −λτy Ex e M1 M λ/2v, 1/2, v/σ 2 lnx/k σ 2 /2v2 2 NM λ/2v, 1/2, v/σ 2 lnx/k σ 2 /2v −λτy Ex e NM λ/2v, 1/2, v/σ 2 lnx/k σ 2 /2v2 where A denotes λ/2v, 1/2, v/σ 2 lnx/k UA y ∈ x, b, , UA 2.27 UA , UA y ∈ a, x, 2 σ 2 /2v . Remark 2.8. For a certain choice of parameters for a and b in Examples 2.3–2.7, we get the Laplace transform of the first passage time of one-dimensional diffusion with one-sided barrier. For example, letting a → −∞ or b → ∞ in Example 2.4, one gets the Laplace transform of the first passage time of the Ornstein-Uhlenbeck process with one-sided barrier; see Nobile et al. 11, Ricciardi and Sato 12, Alili et al. 13, and Ditlevsen 6. 3. The Value Function In this section we study the value functions 1.5–1.7. Using Itô’s formula, we derive differential equation with boundary conditions for ψ. Theorem 3.1. The function ψ defined by 1.5 satisfies the differential equation 1 2 σ xψ x 2 μxψ x λψx, with the boundary conditions ψ a θ, ψ b η. a < x < b, 3.1 International Journal of Stochastic Analysis 9 Proof. Applying the Itô’s formula for semimartingales to ht, Xt e−λt ψXt with ψ ∈ Cb2 a, b we obtain t ht, Xt h0, X0 0 t t ∂h hs, Xs ds ∂s 0 ∂h hs, Xs dXs ∂x ∂2 h hs, Xs dXs , Xs 2 0 ∂x t t ψX0 −λe−λs ψXs ds e−λs ψ Xs dXs 1 2 t 1 2 0 ψx 0 0 e−λs ψ Xs σ 2 Xs ds t e−λs A − λψXs ds 0 3.2 ψ a t e −λs dLs − ψ b t 0 t e−λs ψ Xs dBs 0 e−λs dUs , 0 where we have used that ψ Xt dLt ψ adLt and ψ Xt dUt ψ bdUt . From 3.2 we have Ex e−λt ψXt − ψx Ex t e−λs A − λψXs ds 0 Ex ψ a t e −λs dLs − ψ b 0 t e −λs 3.3 dUs . 0 Let f be a solution of Afx λfx, x ∈ a, b, f a 0, 3.4 f b 1. In place of 3.3, we have Ex e−λt fXt − fx − t e−λs dUs . 3.5 0 Letting t → ∞, we get ∞ 0 e−λs dUs fx < ∞. 3.6 10 International Journal of Stochastic Analysis Likewise ∞ e−λs dLs < ∞. 3.7 0 Letting t → ∞ in 3.3 and noting 3.6 and 3.7, we get the desired result. ∞ Corollary 3.2. The function ψ1 x Ex 0 e−λt dUt is solution to the differential equation 1 2 σ xψ1 x 2 μxψ1 x λψ1 x, a < x < b, 3.8 with the boundary conditions ψ1 a 0, ψ1 b 1. ∞ Corollary 3.3. The function ψ2 x Ex 0 e−λt dLt is solution to the differential equation 1 2 σ xψ2 x 2 μxψ2 x λψ2 x, a < x < b, 3.9 with the boundary conditions ψ2 a −1, ψ2 b 0. For diffusions in Examples 2.3–2.7 we can obtain the explicit expressions for ψ, ψ1 , and ψ2 . Now we consider the Ornstein-Uhlenbeck process only. Example 3.4. The Ornstein-Uhlenbeck process is as follows: dXt vk − Xt dt σdBt , v, σ > 0, k ∈ a, b. 3.10 From Example 2.4, the two independent solutions of differential equation 1 2 σ ψ x 2 vk − xψ x λψx 3.11 are, respectively, √ √ v 2v −λ/2v 1/2v/σ 2 x−k2 ψ x H−λ/v − e D−λ/v − x − k 2 x − k , σ σ √ √ v 2v −λ/2v 1/2v/σ 2 x−k2 ψ− x H−λ/v e D−λ/v x − k 2 x − k . σ σ 3.12 The general solution of 3.11 is of the form ψx C1 ψ x C2 ψ− x, 3.13 International Journal of Stochastic Analysis 11 where the constants C1 and C2 are determined by the boundary conditions ψ a θ, ψ b η. They are C1 θψ b − ηψ a , b − ψ aψ− b ψ− aψ 3.14 ηψ− a − θψ− b C2 . ψ− aψ b − ψ aψ− b 4. Applications to Risk Theory Let Xt denote the surplus of the company. If no dividends were paid, the surplus process follows the stochastic differential equation dXt μXt dt σXt dBt , t ≥ 0, X0 x, 4.1 where B is a Brownian motion and μ and σ are Lipschitz-continuous functions. The company will pay dividends to its shareholders according to barrier strategy with parameter b > 0. Whenever the surplus is about to go above the level b, the excess will be paid as dividends, and when the surplus is below b nothing is paid out. Let Dt denote the aggregate dividends by time t. Thus the resulting surplus process Y is given by dYt μYt dt σYt dBt − dDt, t ≥ 0. 4.2 Let T inf{t ≥ 0 : Yt 0} be the time of ruin. Note that when b < ∞ ruin is certain, that is, P T < ∞ 1. We are interested in the Laplace transform of T . This model can be found in Paulsen 14, and some important special cases can be found in Gerber and Shiu 15, Cai et al. 16. It follows from Theorem 2.1 that, for 0 < x < b and λ > 0, Ex e−λT hx, where h is the solution of Ahx λhx, h b 0, x ∈ 0, b, 4.3 h0 1. Assume that an insurance company is not allowed to go bankrupt and the beneficiary of the dividends is required to inject capital into the insurance company to keep its risk process stays nonnegative. Under such a dividend policy the controlled risk process with initial reserve x > 0 satisfies t μ X t dt dX t dBt σ X dLt − dDt , t ≥ 0, 4.4 12 International Journal of Stochastic Analysis where Lt and Dt are local times at 0 and b, respectively. Lt ensures the insurance company will not ruin and Dt is the aggregate amount of paid dividends by time t. We consider the total expected discounted dividends minus the total expected discounted costs of injected capital: V x; b Ex ∞ e −λt dDt − k ∞ 0 e−λt dLt , 4.5 0 where λ > 0 is discounted factor and k is the cost per unit injected capital. Avram et al. 17 consider the problem in a Levy processes setting. According to Theorem 3.1, we obtain the formula V x, b, as long as we solve the equation 1 2 σ xV x 2 μxV x λV x, 0 < x < b, 4.6 with the boundary conditions V 0 k, V b 1. t x} be the first time when the surplus reaches the For 0 < x < b, let Tx inf{t | X level x. It follows from Theorem 2.1 that, for 0 < x < b and λ > 0, Ex e−λTx gx, where g is the solution of Agx λgx, g 0 0, x ∈ 0, b, 4.7 gb 1. We now give two examples. Example 4.1. In this example we consider the uncontrolled surplus of insurance company satisfying Xt x μt σWt , where Wt is Brownian motion. The controlled surplus process X at time t follows the equation t x X μt σWt 4.8 Lt − Dt . Now we consider the differential equation 1 2 σ V x 2 μV x λV x, 0 < x < b, 4.9 with the boundary conditions V 0 k, V b 1. Then V x, b kesb − 1 rx kerb − 1 sx e − e , rb sb s e −e r erb − esb 0 < x < b, 4.10 International Journal of Stochastic Analysis 13 where r and s are the positive root and negative root of the equation σ 2 /2ξ2 respectively, that is, r −μ μ2 2σ 2 λ σ2 , −μ − s μ2 2σ 2 λ σ2 . μξ − λ 0, 4.11 For 0 < x < b and λ > 0, Ex e−λTx : gx is the solution of 1 2 σ g x 2 μg x λgx, 0 < x < b, 4.12 with the boundary conditions g 0 0, gb 1. Solving it gives Ex e−λTx resx − serx , resb − serb 0 < x < b. 4.13 Example 4.2. In this example we consider the Ornstein-Uhlenbeck-type model. The company’s surplus evolves according to t x X t μ s ds ρX σBt Lt − Dt . 4.14 0 The model is considered in Cai et al. 16 for the special case where Lt 0. The diffusion and drift coefficients are σx ≡ σ, μx μ ρx. We consider the differential equation 1 2 σ V x, b 2 μ ρx V x, b λV x, b, 0 < x < b, 4.15 with the boundary conditions V 0 k, V b 1. In Cai et al. 16, they pointed out that the solution is given by V x, b Ht C1 −t1−c et M1 − a, 2 − c; −t C2 et Uc − a, c; −t 4.16 for certain coefficients C1 and C2 , with c 1/2, a −λ/2ρ, t −1/ρσ 2 μ ρx2 . Here M and U are called the confluent hypergeometric functions of the first and second kinds, respectively. For more details on confluent hypergeometric functions, see Abramowitz and Stegun 8. It follows from 3.7 in Cai et al. 16 that H t Ht − C1 1 − c−t−c et M1 − a, 2 − c; −t − C1 1−a −t1−c et M2 − a, 3 − c; −t 2−c C2 c − aet Uc − a 1, c 1; −t. 4.17 14 International Journal of Stochastic Analysis The conditions C1 and C2 can be determined by V 0 k, V b 1, where V x −2μ ρx/σ 2 H t. Solving it gives σ 2 C/ μ C1 2 2 ρx eμ ρb /ρσ − kD/μ eμ AD − BC σ 2 kB/μ eμ C2 2 2 /ρσ 2 2 − A/ μ ρb eμ AD − BC 2 /ρσ 2 , 4.18 ρb2 /ρσ 2 , where ⎛ 1/2 −1/2 ⎞ 2 2 μ2 μ μ 1 ⎠ ⎝ M 1 − a, 2 − c; 2 A − 2 ρσ 2 ρσ 2 ρσ 1−a − 2−c ⎛ μ B⎝ − 1−a 2−c μ2 ρσ 2 μ ρb ρσ 2 μ2 C U c − a, c; 2 ρσ D U c − a, c; μ ρb ρσ 2 μ2 M 2 − a, 3 − c; 2 ρσ 2 1/2 ρb ρσ 2 1/2 1 − 2 2 1/2 μ ρb ρσ 2 2 −1/2 M 2 − a, 3 − c; c − aU c − a 2 , 1, c ⎞ μ ⎠M 1 − a, 2 − c; 2 μ 2 4.19 ρb , ρσ 2 μ2 1; 2 , ρσ c − aU c − a ρb ρσ 2 1, c 1; μ ρb ρσ 2 2 . Acknowledgments The authors thank the reviewer for valuable insights and suggestions that largely contributed to the improvement of the paper. 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