Advances in Mathematical Physics

Volume 2017 (2017), Article ID 4514935, 13 pages

https://doi.org/10.1155/2017/4514935

## Dynamics of a Computer Virus Propagation Model with Delays and Graded Infection Rate

^{1}School of Management Science and Engineering, Anhui University of Finance and Economics, Bengbu 233030, China^{2}Department of Computer, Liaocheng College of Education, Liaocheng 252004, China

Correspondence should be addressed to Zizhen Zhang; moc.361@adiahzzz

Received 14 July 2016; Accepted 1 September 2016; Published 4 January 2017

Academic Editor: Xiao-Jun Yang

Copyright © 2017 Zizhen Zhang and Limin Song. 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.

#### Abstract

A four-compartment computer virus propagation model with two delays and graded infection rate is investigated in this paper. The critical values where a Hopf bifurcation occurs are obtained by analyzing the distribution of eigenvalues of the corresponding characteristic equation. In succession, direction and stability of the Hopf bifurcation when the two delays are not equal are determined by using normal form theory and center manifold theorem. Finally, some numerical simulations are also carried out to justify the obtained theoretical results.

#### 1. Introduction

In recent years, with the fast development and popularization of computer technologies and network, Internet has offered numerous functionalities and facilities to the world. Meanwhile, Internet has also become a powerful mechanism for propagating computer viruses. Computer viruses are computer programs which have serious effects on individual and corporate computer systems in the network, such as modifying data and formatting disks [1, 2].

In order to analyze the propagation laws of computer viruses in the network, many epidemiological models have been borrowed to depict the spread of computer viruses because of the high similarity between the computer viruses and the biological viruses [3–5]. In [6–11], Mishra et al. proposed SIRS computer virus models in different forms. Yuan and Chen presented the SEIR computer virus propagation model in [12] and they studied the stability of the model. Based on the the work in [12], Dong et al. proposed the SEIR computer virus model with time delay in [13] and they investigated the Hopf bifurcation of the model. There are also some other different computer virus models which have been proposed by other scholars in recent years and one can refer to [14–18]. However, all the computer virus models above which incorporate the latent status of the viruses assume that the latent computers have no infection ability. This is not consistent with the reality, because an infected computer which is in latency can also infect other computers through file copying or file downloading. Based on this fact, Yang et al. established a computer virus propagation model with graded infection rate in [19]:where , , , and are the percentages of susceptible computers, latent computers, active computers, and recovered computers on the Internet, at time , respectively. is the rate at which external computers are connected to the Internet and it is also the rate at which internal computers are disconnected from the Internet; is the infected rate of the susceptible computers by the latent computers; is the infected rate of the susceptible computers by the active computers; is the rate at which the recovered computers become susceptibly virus-free again; is the rate at which the latent computers break out; and is the rate at which the active computers are cured by the antivirus software.

As pointed out in [9], one of the typical features of computer viruses is their latent characteristic. Therefore, they need a period to become active computers for the latent ones. Likewise, the antivirus software needs a period to clean the viruses in the active computers. Based on this and motivated by the work about the dynamical system with delay in [20–24], we incorporate two delays into system (1) and obtain the following delayed computer virus model:where is the latent period of the computer viruses and is the period that the antivirus software needs to clean the viruses in the active computers.

The rest of this paper is organized as follows. In Section 2, we present the existence of the viral equilibrium and conditions for the local stability of the viral equilibrium and existence of the Hopf bifurcation are derived. Direction and stability of the Hopf bifurcation are studied in Section 3 and some numerical simulations are performed in Section 4 to justify the obtained theoretical findings by taking some relevant values of the parameters in system (2) and using the Matlab software package. Finally, we end this paper with concluding remarks in Section 5.

#### 2. Existence of Local Hopf Bifurcation

By a simple computation, we know that if and , then system (2) has a unique viral equilibrium , where

Let , , , . Dropping the bars, system (2) becomeswhereThe linear system of system (4) isThe corresponding characteristic equation iswhere

*Case 1 (). *For , (7) becomeswhereThus, according to the Routh-Hurwithz theorem, we know that if conditions , , and hold, then viral equilibrium of system (2) without delay is locally asymptotically stable.

*Case 2 (, ). *For and , we can get the following from (7):whereWe assume that is a root of (11). Then,which implies thatwithLet ; then (14) becomes

Discussion about distribution of roots for (16) is similar to that in [25]. Therefore, we directly assume that (16) has at least one positive equilibrium .

If holds, we know that (14) has at least one positive root such that (11) has a pair of purely imaginary roots . For , where Differentiating both sides of (11) with respect to , one can obtain Thus, where and . Therefore, if condition holds, then . Based on the discussion above and according to the Hopf bifurcation theorem in [26], we obtain the following.

Theorem 1. *If conditions hold, then*(i)*viral equilibrium of system (2) is locally asymptotically stable for ;*(ii)*system (2) undergoes a Hopf bifurcation at viral equilibrium when and a family of periodic solutions bifurcate from .*

*Case 3 (). *For and , (7) becomeswhereLet be a root of (21). Then,It follows thatwithLet ; then, we have

*Similar to Case 2, we make the following assumption. (26) has at least one positive root . If condition holds, then there exists such that (21) has a pair of purely imaginary roots . For , where In addition, we have Further, where and . Therefore, if condition holds, then . Based on the discussion above and according to the Hopf bifurcation theorem in [26], we obtain the following.*

*Theorem 2. If conditions hold, then(i)viral equilibrium of system (2) is locally asymptotically stable for ;(ii)system (2) undergoes a Hopf bifurcation at viral equilibrium when and a family of periodic solutions bifurcate from .*

*Case 4 (). *For , we havewith Multiplying by , (31) becomes the following:Let be a root of (37); it is easy to getwhereIt leads to Thus, we can get the following equation with respect to :

*Next, we make the following assumption. (37) has at least one positive root . Then, for , we have Taking the derivative of with respect to , we obtain where Then, we can get that where*

*We assume that . Clearly, if condition holds, then we can conclude that . Therefore, according to the Hopf bifurcation theorem in [26], we obtain the following.*

*Theorem 3. If conditions hold, then(i)viral equilibrium of system (2) is locally asymptotically stable for ;(ii)system (2) undergoes a Hopf bifurcation at viral equilibrium when and a family of periodic solutions bifurcate from .*

*Case 5 (, ). *Let be the root of (7). Then,whereThus, one can get the following equation with respect to :

*Similar to Case 4, we assume that (45) has at least one positive root . Then, for , we have Differentiating (7) with respect to , we get withThen, we obtain where*

*We assume that . Thus, we know that , if condition holds. Therefore, according to the Hopf bifurcation theorem in [26], we obtain the following.*

*Theorem 4. If conditions hold and , then(i)viral equilibrium of system (2) is locally asymptotically stable for ;(ii)system (2) undergoes a Hopf bifurcation at viral equilibrium when and a family of periodic solutions bifurcate from .*

*3. Properties of the Hopf Bifurcation*

*3. Properties of the Hopf Bifurcation**In this section, we shall investigate direction of the Hopf bifurcation and stability of the bifurcating periodic solution of system (2) when and by using the center manifold theorem and the normal form theory which has been developed by Hassard et al. [26].*

*Let , , , , , . Then, is the Hopf bifurcation value of system (2) and system (2) can be rewritten aswhere and : and are given, respectively, by with Using Riesz representation theorem, there exists matrix such that In fact, choose For , define*

*Then, system (51) can be rewritten in the following form:For , , and bilinear formare defined, where , , and are adjoint operators.*

*Based on the discussion above, one can conclude that are common eigenvalues of and . The eigenvectors of and can be calculated corresponding to and , respectively. Let be the eigenvector of corresponding to and be the eigenvector of corresponding to . By some complex computations, we obtainwith From (59), we get such that . In what follows, we can obtain the coefficients by using the method introduced in [26]: with where with Thus, we can compute the following coefficients:*