Due to its practical importance, more and more people use the ruin probability, an important leading indicator, to judge whether an insurance company can survive or not. The classical risk model was introduced by Gerber  . The function is of the form
where is the insurance company’s surplus at time t. c represents the risk-free rate. is the total compensation by time t; is the i-th compensation; means the frequency of compensation. represents the initial investment for an insurance company. In order to make the classical risk model more realistic, more and more factors have been taken into account. The aggregate premium process taking a linear function of time was considered by Zou, Gao and Xie  , in which the explicit solutions of ruin time were obtained under assumption that the claim amount obeyed exponential distribution. Yao, Yang and Wang  studied a dual risk model with fixed transaction cost, and solved the optional control problem by the techniques of quasi-variational inequalities. Yuen, Zhou and Guo  discussed the compound Poisson risk model with debit interest and dividend payments, and the explicit expressions for the Gerber-Shiu function and the optimal barrier with exponential claim amounts were given. When the dividend strategy was considered, Yin and Yuen  considered a dividend policy aiming to maximize the expected discounted value of dividend until ruin. Taking the threshold dividend strategy into account, two integro-differential equations for the Gerber-Shiu discounted penalty function were obtained by Lin and Pavlova  . The ruin model with multiple thresholds was discussed by Lin and Sendova  . For more about the related risk model, we can refer to      and cited there in. From all the literatures mentioned above, they have a common assumption that the claim sizes obey exponential distribution, i.e. . However, in fact, different dividend levels always reflect the difference of the economic environment, which results in the changes of the claim sizes. In detail, when the surplus u is more than the threshold b, the claim sizes obey an exponential distribution where the strength is ; while , the claim sizes obey an exponential distribution with strength . Therefore, the exponential distribution is noted like
is an indicator function, such that and if . In order to make it more general, we set the risk model with the claim sizes obeying a switched exponential distribution as follows
where p and q are two suitable positive constants to be chosen later. Clearly, when we choose and , (3) is the standard exponential distribution applied in  .
The main aim of this letter is to compute the ruin probability under assumption that the claim sizes obey a switched exponential distribution (3).
The rest of this letter is organized as follows. In Section 2, we firstly get the Gerber-Shiu discounted penalty functions with different surplus. Then, the ruin probability is calculated by assuming the claim sizes obey a switched exponential distribution.
2. Closed Form Absolute Ruin Probability
In this letter, we consider the risk model with constant interest rate and debit interest . Besides, the studied risk model includes a general threshold dividend strategy. When the surplus is over threshold dividend b, we assume the insurance company gets insurance premium at a constant rate and earns interest at a constant rate r. If the surplus is between zero and b, it will collect insurance premium at a constant rate and earns interest at a constant rate r.
When the surplus is between and zero, the insurer can borrow an amount
of money equal to the deficit at a debit force . Meanwhile, the insurer will repay the debts continuously from his premium income. We denote the surplus of the insurer at time t with the credit interest r and debit interest by which is the solution to
where is the cumulative damages in time interval ,
represents the constant rate of premium, a Poisson process with intensity , which counts the claim numbers in the interval , and (representing the size of claims and independent of ) is a sequence of independent and identically distributed nonnegative variables with common distribution function that satisfies and has a positive mean . Let be a filtered probability space containing all processes and random variables in this letter, satisfying the usual conditions, i.e. is right continuous and P-complete. In order to obtain the expression of the ruin probability, we should work for two things. First of all, we should get the Gerber-Shiu discounted penalty function. Then, we calculate the ruin probability. So, let’s firstly define the Gerber-Shiu discounted penalty function .
where T is the time of bankrupting, is an indicator function, which means , if the time of bankrupting is finite value. Else if
. is a non-negative measurable function satisfying
. is the surplus immediately before the
company bankrupts. Next, we give the integro-differential equations for .
Theorem 2.1. Let be the Gerber-Shiu discounted penalty function by the surplus u and the threshold b for model (4). Then
the boundary conditions are given like
Proof. When , define the cumulative value
Note that the Gerber-Shiu discounted penalty functions have different expression when the surplus lies within different ranges, we discuss that with the following four cases:
1) When , the amount of compensation is . Let , its counterpart in the Gerber-Shiu discounted penalty function is ;
2) When , ;
3) When , ;
4) When , .
Therefore, the Gerber-Shiu discounted penalty function is given like,
For , by using Taylor expansion, we get
Insert (11) into (10) and divide it by t. Let , we get the desired result. Since (7) and (8) can be proved by using the same method, the detail is omitted. □
Next, we prove the main result on the absolute ruin probability. For convenience, we denote
, , , . The hyper-geometric function used later is introduced as follows
where the Gamma function is .
In view of (6), let and in the Gerber-Shiu discounted penalty function , we can get the following theorem on the ruin probability.
Theorem 2.2. In model (4), the closed-form ruin probability are given like
where is solution of .
Proof. By Theorem 2.1, we get
The boundary conditions are
In order to obtain the expression of ruin probabilities, we employ the confluent hyper-geometric function. In (16), make auxiliary function where a is a constant to be determined later and then
Define , we have
By choosing ,
Define , we have by (21)
Let , (22) can be rewritten as
By using the first kind of confluent hypergeometric function, we obtain that
where and are constants. Without extra claims, are constants for in the following.
Next, the same method can be used to calculate (17) and (18). In (17), set
, , , and , then we have
The solution of (25) has the form
Similarly, the solution of (18) is
In the sequel, we turn to compute the expression of ruin probability. Inserting
and into (24) yields that
Denote , then (28) yields that
Define , we have by (29)
Then we can use the same method to solve (26) and (27) like that
To calculate the coefficients of the ruin probability , we firstly rewrite (30)-(32) to be
By inserting boundary conditions into (30), (31) and (32), we get
which can be simplified as . The proof is complete. □
Remark 1. Let and , Theorem 2.1 reduces to the related results given in  . In this sense, the previously-known results are generalized. Further, Theorem 2.2 shows us the closed-form solution of the ruin probability which is valuable in theoretical applications.
In this letter, we discuss the ruin probability of the risk model under assumption that the claim amounts obey the switched exponential distribution. The closed-form expressions of the ruin probability are obtained by using the first kind of hypergeometric function. The obtained results may give us guidance in facing up to the economic crisis.
 Zou, W., Gao, J. and Xie, J. (2014) On the Expected Discounted Penalty Function and Optimal Dividend Strategy for a Risk Model with Random Incomes and Interclaim-Dependent Claim Sizes. Journal of Computational and Applied Mathematics, 255, 270-281.
 Yao, D., Yang, H. and Wang, R. (2011) Optimal Dividend and Capital Injection Problem in the Dual Model with Proportional and Fixed Transaction Costs. European Journal of Operational Research, 211, 568-576.
 Yin, C. and Yuen, K. (2011) Optimality of the Threshold Dividend Strategy for the Compound Poisson Model. Statistics and Probability Letters, 81, 1841-1846.
 Lin, X. and Pavlova, K.P. (2006) The Compound Poisson Risk Model with a Threshold Dividend Strategy. Insurance: Mathematics and Economics, 38, 57-80.
 Fu, K.-A. (2016) On Joint Ruin Probability for a Bidimensional Levy-Driven Risk Model with Stochastic Returns and Heavy-Tailed Claims. Journal of Mathematical Analysis and Applications, 442, 17-30.
 Zhao, Y. and Yao, J. (2005) Actuarial Module of Enterprise-Complemented Pension Scheme While Taking Vasicek as Stochastic Interest Rate Module. Journal of University of Shanghai for Science and Technology, 27, 268-270. (In Chinese)
 Chadjiconstantinidis, S. and Papaioannou, A.D. (2013) On a Perturbed by Diffusion Compound Poisson Risk Model with Delayed Claims and Multi-Layer Dividend Strategy. Journal of Computational and Applied Mathematics, 253, 26-50.
 Lu, D. and Zhang, B. (2016) Some Asymptotic Results of the Ruin Probabilities in a Two-Dimensional Renewal Risk Model with Some Strongly Subexponential Claims. Statistics and Probability Letters, 114, 20-29.