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Research article Special Issues

Local porosity of the free boundary in a minimum problem

  • Received: 10 April 2023 Revised: 11 July 2023 Accepted: 20 July 2023 Published: 03 August 2023
  • Given certain set K and functions q and h, we study geometric properties of the set {xΩ:u(x)>0} for non-negative minimizers of the functional J(u)=Ω(1p|u|p+q(u+)γ+hu)dx over K, where ΩRn(n2) is an open bounded domain, p(1,+) and γ(0,1] are constants, u+ is the positive part of u and {xΩ:u(x)>0} is the so-called free boundary. Such a minimum problem arises in physics and chemistry for γ=1 and γ(0,1), respectively. Using the comparison principle of p-Laplacian equations, we establish first the non-degeneracy of non-negative minimizers near the free boundary, then prove the local porosity of the free boundary.

    Citation: Yuwei Hu, Jun Zheng. Local porosity of the free boundary in a minimum problem[J]. Electronic Research Archive, 2023, 31(9): 5457-5465. doi: 10.3934/era.2023277

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  • Given certain set K and functions q and h, we study geometric properties of the set {xΩ:u(x)>0} for non-negative minimizers of the functional J(u)=Ω(1p|u|p+q(u+)γ+hu)dx over K, where ΩRn(n2) is an open bounded domain, p(1,+) and γ(0,1] are constants, u+ is the positive part of u and {xΩ:u(x)>0} is the so-called free boundary. Such a minimum problem arises in physics and chemistry for γ=1 and γ(0,1), respectively. Using the comparison principle of p-Laplacian equations, we establish first the non-degeneracy of non-negative minimizers near the free boundary, then prove the local porosity of the free boundary.



    The concept of the hyperchaos was first put forward by Rössler [1] in 1979. Any system with at least two positive Lyapunov exponents is defined as hyperchaotic. Compared to chaotic attractors, hyperchaotic attractors have more complicated dynamical phenomena and stronger randomness and unpredictability. Hyperchaotic systems have aroused wide interest from more and more researchers in the last decades. A number of papers have investigated various aspects of hyperchaos and many valuable results have been obtained. For instance, in applications, in order to improve the security of the cellular neural network system, the chaotic degree of the system can be enhanced by designing 5D memristive hyperchaotic system [2]. For higher computational security, a new 4D hyperchaotic cryptosystem was constructed by adding a new state to the Lorenz system and well used in the AMr-WB G.722.2 codec to fully and partially encrypt the speech codec [3]. In fact, hyperchaos has a wide range applications such as image encryption [4], Hopfield neural network [5] and secure communication [6] and other fields [7,8]. Meanwhile, there are many hyperchaotic systems have been presented so far. Aimin Chen and his cooperators constructed a 4D hyperchaotic system by adding a state feedback controller to Lü system [9]. Based on Chen system, Z. Yan presented a new 4D hyperchaotic system by introducing a state feedback controller [10]. By adding a controlled variable, Gao et al. introduced a new 4D hyperchaotic Lorenz system [11]. Likewise, researchers also formulated 5D Shimizu-Morioka-type hyperchaotic system [12], 5D hyperjerk hypercaotic system [13] and 4D T hyperchaotic system [14] and so on.

    In [15], a chaotic Rabinovich system was introduced

    {˙x=hyax+yz,˙y=hxbyxz,˙z=cz+xy, (1.1)

    where (x,y,z)TR3 is the state vector. When (h,a,b,c)=(0.04,1.5,0.3,1.67), (1.1) has chaotic attractor[15,16]. System (1.1) has similar properties to Lorenz system, the two systems can be considered as special cases of generalized Lorenz system in [17]. Liu and his cooperators formulated a new 4D hyperchaotic Rabinovich system by adding a linear controller to the 3D Rabinovich system [18]. The circuit implementation and the finite-time synchronization for the 4D hyperchaotic Rabinovich system was also studied in [19]. Reference [20] formulated a 4D hyperchaotic Rabinovich system and the dynamical behaviors were studied such as the hidden attractors, multiple limit cycles and boundedness. Based on the 3D chaotic Rabinovich system, Tong et al. presented a new 4D hyperchaotic system by introducing new state variable [21]. The hyperchaos can be generated by adding variables to a chaotic system, which has been verificated by scholars [3,9,10,11,14]. In [18,19,20,21], the hyperchaotic systems were presented by adding a variable to the second equation of system (1.1). In fact, hyperchaos can also be generated by adding a linear controller to the first equation and second equation of system (1.1). Based on it, the following hyperchaotic system is obtained

    {˙x=hyax+yz+k0u,˙y=hxbyxz+mu,˙z=dz+xy,˙u=kxky, (1.2)

    where k0,h,a,b,d,k,k0,m are positive parameters. Like most hyperchaotic studies (see [14,22,23,24] and so on), the abundant dynamical properties of system (1.2) are investigated by divergence, phase diagrams, equilibrium points, Lyapunov exponents, bifurcation diagram and Poincaré maps. The results show that the new 4D Rabinovich system not only exhibit hyperchaotic and Hopf bifurcation behaviors, but also has the rich dynamical phenomena including periodic, chaotic and static bifurcation. In addition, the 4D projection figures are also given for providing more dynamical information.

    The rest of this paper is organized as follows: In the next section, boundedness, dissipativity and invariance, equilibria and their stability of (1.2) are discussed. In the third section, the complex dynamical behaviors such as chaos and hyperchaos are numerically verified by Lyapunov exponents, bifurcation and Poincaré section. In the fourth section, the Hopf bifurcation at the zero equilibrium point of the 4D Rabinovich system is investigated. In addition, an example is given to test and verify the theoretical results. Finally, the conclusions are summarized in the last section.

    Theorem 2.1. If k0>m, system (1.2) has an ellipsoidal ultimate bound and positively invariant set

    Ω={(x,y,z,u)|mx2+k0y2+(k0m)[zh(k0+m)k0m]2+k0mu2kM},

    where

    M={14h2d2(m+k0)2(k0m)a(da),(k0>m,d>a),14h2d2(m+k0)2(k0m)b(b+d),(k0>m,d>b),(m+k0)2h2k0m,(k0>m). (2.1)

    Proof. V(x,y,z,u)=mx2+k0y2+(k0m)[zh(k0+m)k0m]2+k0mu2k. Differentiating V with respect to time along a trajectory of (1.2), we obtain

    ˙V(x,y,z,u)2=amx2by2k0+dhmz+dhzk0+dmz2dz2k0.

    When ˙V(x,y,z,u)2=0, we have the following ellipsoidal surface:

    Σ={(x,y,z,u)|amx2+bk0y2+d(k0m)(zhm+hk02k02m)2=dh2(k0+m)24(k0m)}.

    Outside Σ that is,

    amx2+bk0y2+d(k0m)(zhm+hk02k02m)2<dh2(k0+m)24(k0m),

    ˙V<0, while inside Σ, that is,

    amx2+bk0y2+d(k0m)(zhm+hk02k02m)2>dh2(k0+m)24(k0m),

    ˙V>0. Thus, the ultimate bound for system (1.2) can only be reached on Σ. According to calculation, the maximum value of V on Σ is Vmax=14h2d2(m+k0)2(k0m)a(da),(k0>m,d>a) and

    Vmax=14h2d2(m+k0)2(k0m)b(b+d),(k0>m,d>b);Vmax=(m+k0)2h2k0m,(k0>m).

    In addition, ΣΩ, when a trajectory X(t)=(x(t),y(t),z(t),u(t)) of (1.2) is outside Ω, we get ˙V(X(t))<0. Then, limt+ρ(X(t,t0,X0),Ω)=0. When X(t)Ω, we also get ˙V(X(t))<0. Thus, any trajectory X(t) (X(t)X0) will go into Ω. Therefore, the conclusions of theorem is obtained.

    We can see that system (1.2) is invariant for the coordinate transformation

    (x,y,z,u)(x,y,z,u).

    Then, the nonzero equilibria of (1.2) is symmetric with respect to z axis. The divergence of (1.2) is

    W=˙xx+˙yy+˙zz+˙uu=(a+b+d),

    system (1.2) is dissipative if and only if a+b+d>0. It shows that each volume containing the system trajectories shrinks to zero as t at an exponential rate (a+b+d).

    For mk0, system (1.2) only has one equilibrium point O(0,0,0,0) and the Jacobian matrix at O is

    J=[ah0k0hb0m00d0kk00].

    Then, the characteristic equation is

    (d+λ)(λ3+s2λ2+s1λ+s0), (2.2)

    where

    s2=a+b,s1=abh2+km+kk0,s0=akm+bkk0+hkm+hkk0.

    According to Routh-Hurwitz criterion [25], the real parts of eigenvalues are negative if and only if

    d>0,a+b>0,(a+b)(abh2)+k(ak0+bmhmhk0)>0. (2.3)

    When m>k0, system (1.2) has other two nonzero equilibrium points E1(x1,y1,z1,u1) and E2(x1,y1,z1,u1), where

    x1=d(am+bk0+hm+hk0)mk0,y1=d(am+bk0+hm+hk0)mk0,
    z1=am+bk0+hm+hk0mk0,u1=a+b+2hmk0d(am+bk0+hm+hk0)mk0.

    The characteristic equation of Jacobi matrix at E1 and E2 is

    λ4+δ3λ3+δ2λ2+δ1λ+δ0=0,

    where

    δ3=a+b+d,δ2=x14d2+ab+ad+bdh2+km+kk0,
    δ1=1d[x12(3x12+adbd+kk0km)+k(m+k0)(d+h)]+abd+akm+bkk0dh2,
    δ0=3kk0x123kmx12+akmd+bkk0d+hkk0d+hkmd.

    Based on Routh-Hurwitz criterion [25], the real parts of eigenvalues are negative if and only if

    δ0>0,δ3>0,δ3δ2δ1>0,δ3δ2δ1δ21δ0δ23>0. (2.4)

    Therefore, we have:

    Theorem 2.2. (I) When mk0, system (1.2) only has one equilibrium point O(0,0,0,0) and O is asymptotically stable if and only if (2.3) is satisfied.

    (II) when m>k0, system (1.2) has two nonzero equilibria E1, E2, (2.4) is the necessary and sufficient condition for the asymptotically stable of E1 and E2.

    When (b,d,h,a,k0,k,m)=(1,1,10,10,0.8,0.8,0.8), system (1.2) only has zero-equilibrium point E0(0,0,0,0,0), its corresponding characteristic roots are: 1,0.230,5.232,16.462, E0 is unstable. The Lyapunov exponents are: LE1=0.199, LE2=0.083, LE3=0.000, LE4=12.283, system (1.2) is hyperchaotic. Figure 1 shows the hyperchaotic attractors on zxy space and yzu space. The Poincaré mapping on the xz plane and power spectrum of time series x(t) are depicted in Figure 2.

    Figure 1.  Hyperchaotic attractors of (1.2), (I) zxy space and (II) yzu space.
    Figure 2.  (I) Poincaré mapping on the xz plane and (II) power spectrum of time series x(t) for system (1.2).

    In the following, we fix b=1,d=1,h=10,a=10,k0=0.8,m=0.8, Figure 3 indicates the Lyapunov exponent spectrum of system (1.2) with respect to k[0.005,1.8] and the corresponding bifurcation diagram is given in Figure 4. These simulation results illustrate the complex dynamical phenomena of system (1.2). When k varies in [0.005,1.8], there are two positive Lyapunov exponents, system (1.2) is hyperchaoic as k varies.

    Figure 3.  Lyapunov exponents of (1.2) with m[0.005,1.8].
    Figure 4.  Bifurcation diagram of system (1.2) corresponding to Figure 3.

    Assume b=1,d=1,h=10,k=0.8,k0=0.8,m=0.8, the different Lyapunov exponents and dynamical properties with different values of parameter a are given in Table 1. It shows that system (1.2) has rich dynamical behaviors including periodic, chaos and hyperchaos with different parameters. The bifurcation diagram of system (1.2) with a[0.5,5] is given in Figure 5. Therefore, we can see that periodic orbits, chaotic orbits and hyperchaotic orbits can occur with increasing of parameter a. When a=1.08, Figure 6 indicates the (z,x,y,u) 4D surface of section and the location of the consequents is given in the (z,x,y) subspace and are colored according to their u value. The chaotic attractor and hyperchaotic attractor on yxz space and hyperchaotic attractor on yzu space are given in Figures 7 and 8, respectively. The Poincaré maps on xz plane with a=2 and a=4 are depicted in Figure 9.

    Table 1.  Lyapunov exponents of (1.2) with (b,d,h,k,k0,m)=(1,1,10,0.8,0.8,0.8).
    a LE1 LE2 LE3 LE4 Dynamics
    1.08 0.000 0.075 4.661 2.538 Periodic
    2 0.090 0.000 0.395 3.696 Chaos
    4 0.325 0.047 0.000 6.373 Hyperchaos

     | Show Table
    DownLoad: CSV
    Figure 5.  Phase diagram of (1.2) with a[0.55].
    Figure 6.  Phase diagram of system (1.2) with a=1.08.
    Figure 7.  Phase diagram of system (1.2) with a=2.
    Figure 8.  Phase diagrams of (1.2) with a=4 (I) yxz space, (II) yzu space.
    Figure 9.  Poincaré maps of (1.2) in xz plane with a=2 and a=4, respectively.

    Theorem 4.1. Suppose that ab>h2 and k0(ah)+m(bh)<0 are satisfied. Then, as m varies and passes through the critical value k=ah2+bh2a2bab2ak0+bmhmhk0, system (1.2) undergoes a Hopf bifurcation at O(0,0,0,0).

    Proof. Assume that system (1.2) has a pure imaginary root λ=iω,(ωR+). From (2.2), we get

    s2ω2s0=0,ω3s1ω=0,

    then

    ω=ω0=abh2+k(k0+m),
    k=k=(a+b)(h2ab)k0(ah)+m(bh).

    Substituting k=k into (2.2), we have

    λ1=iω,λ2=iω,λ3=d,λ4=(a+b).

    Therefore, when ab>h2, k0(ah)+m(bh)<0 and k=k, the first condition for Hopf bifurcation [26] is satisfied. From (2.2), we have

    Re(λ(k))|λ=iω0=h(k0+m)ak0bm2(s22+s1)<0,

    Thus, the second condition for a Hopf bifurcation [26] is also met. Hence, Hopf bifurcation exists.

    Remark 4.1. When abh2+k(k0+m)0, system (1.2) has no Hopf bifurcation at the zero equilibrium point.

    Theorem 4.2. When ab>h2 and k0(ah)+m(bh)<0, the periodic solutions of (1.2) from Hopf bifurcation at O(0,0,0,0) exist for sufficiently small

    0<|kk|=|k(a+b)(h2ab)k0(ah)+m(bh)|.

    And the periodic solutions have the following properties:

    (I) if δ1g>0 (resp., δ1g<0), the hopf bifurcation of system (1.2) at (0,0,0,0) is non-degenerate and subcritical (resp. supercritical), and the bifurcating periodic solution exists for m>m (resp., m<m) and is unstable (resp., stable), where

    δ1g=14dδ(k2δ01k02+ahδ01s1+kδ01k0s12dδ03δ05+2dδ04δ06),δ=ω0[(ab+h2)(akk0+hkk0hs1)+k(bmhk0)(ha+h2+kk0)k(akhm)(a2+hakk0+s1)],δ01=kω0(a2bm+a2hka2hk0+abhm+ah2kah2mah2k0+ak2k0bkmk0h3mhkmk0+hkk02+bms1hk0s1),δ03=ω0(a2bkm2a2bkk0+a2hk2a2hkk0+abhkm+2abhkk0+ah2k2ah2km+ah2kk0ak3k0+bk2mk0h3km2h3kk0+hk2mk0hk2k02+2a2bs12ah2s1bkms1+hkk0s1),δ04=(ab+h2)s12+(a3ba2h2+abh22abkm+2abkk0ahk2+2ahkk0bhkmh4+h2kmh2kk0)s1a2k3k0+abk2mk02abk2k02ahk3k0+ahk2mk0ahk2k02+bhk2mk0+h2k2mk0+h2k2k02,δ05=12d2+8s1[2ω0(kk0aω0+hkk0ω0hω0s1)+d(hω02ak2k02+kk0s1],δ06=12d2+8s1[d(kk0aω0+hkk0ω0hω0s1)2ω0(k2k02+ahs1kk0s1)].

    (II) The period and characteristic exponent of the bifurcating periodic solution are:

    T=2πω0(1+τ2ϵ2+O(ϵ4)),β=β2ϵ2+O(ϵ4),

    where ϵ=kkμ2+O[(kk)2] and

    μ2=ReC1(0)α(0)=(s22+s1)δ1gh(k0+m)ak0bm,
    τ2=δ2gω0δ1g(ams2+bk0s2+hms2+hk0s2+ms1+k0s1)s1(h(k0+m)ak0bm),
    β2=δ1g,
    δ2g=14dδ(k2δ02k02+ahδ02s1+kδ02k0s12δ03δ06d2δ04δ05d).

    (III) The expression of the bifurcating periodic solution is

    [xyzu]=[kk0ϵcos(2πtT)hω0ϵsin(2πtT)(kk0s1)ϵcos(2πtT)aω0ϵsin(2πtT)ϵ2[kk0(kk0s1)+hs1a2d+δ05δ06sin(4πtT)]k(a+h)ϵcos(2πtT)+kω0ϵsin(2πtT)]+O(ϵ3).

    Proof. Let k=k, by straightforward computations, we can obtain

    t1=[ihω0kk0kk0s1+iaω00(iω0+a+h)k],t3=[0010],t4=[ak0hmbmhk00h2ab],

    which satisfy

    Jt1=iω0t1,Jt3=dt3,Jt4=(a+b)t4.

    Now, we use transformation X=QX1, where X=(x,y,z,u)T, X1=(x1,y1,z1,u1)T, and

    Q=[kk0hω00akhmkk0s1aω00bmhk00010k(a+h)kω00h2ab],

    then, system (1.2) is transformed into

    {˙x1=ω0y1+F1(x1,y1,z1,u1),˙y1=ω0x1+F2(x1,y1,z1,u1),˙z1=dz1+F3(x1,y1,z1,u1),˙u1=(a+b)u1+F4(x1,y1,z1,u1), (4.1)

    where

    δ=ω0 [(ab+h2)(akk0+hkk0hs1)+k(bmhk0)(ha+h2+kk0)k(akhm) (a2+hakk0+s1)],

    F1(x1,y1,z1,u1) =1δz1(f11x1+f12y1+f13u1),

    f11=kk0ω0(abhak2h3+hkm)+ (kk0s1)ω0(a2bah2bkm+hkk0),

    f12=kk0ω0(abhak2h3+hkm)+ (kk0s1)ω0(a2bah2bkm+hkk0),

    f13=kk0ω0(abhak2h3+hkm)+ (kk0s1)ω0(a2bah2bkm+hkk0),

    F2(x1,y1,z1,u1)= 1δz1(f21x1+f22y1+f23u1),

    f21=(ab+h2)(2k2k022kk0s1+s12) k(a+h)(ak2k0bkmk0hkmk0+ hkk02+bms1hk0s1),

    f22=aω0[abk(mk0)hk(ak0bm)] +abs1h2s1]hω0[ak(ak+bk0+hkhm)h2k(m+k0)],

    f23=(h2ab)[hkk0(mk0)kk0(akbm)s1(bmhk0)]+k(a+h)[ak(ak2hm) +bm(bm2hk0)+h2(m2+k02)],

    F3(x1,y1,z1,u1)= [kk0x1hω0y1+(akhm)u1][(kk0s1)x1aω0y1+(bmhk0)u1],

    F4(x1,y1,z1,u1)= 1δz1(f41x1+f42y1+f43u1),

    f41=kk0 (hω0ka+h2ω0k+k2k0ω0) (kk0s1)kω0(a2+ahkk0+s1),

    f42= a3ω02k+a2ω02kh+h2ω02kaak2k0ω02+h3 ω02k+k2k0hω02+ akω02s1,

    f43=a2bkmω0a2hk2ω0+ a2hkk0ω0abhkmω0ah2k2 ω0+ah2kmω0+ah2kk0ω0ak3 k0ω0 +bk2mk0ω0+ h3kmω0+hk2mk0ω0hk2k02ω0bkm ω0s1+ hkk0ω0s1.

    Furthermore,

    g11=14[2F1x21+2F1y21+i(2F2x21+2F2y21)]=0,
    g02=14[2F1x212F1y2122F2x1y1+i(2F2x212F2y21+22F1x1y1)]=0,
    g20=14[2F1x212F1y21+22F2x1y1+i(2F2x212F2y2122F1x1y1)]=0,
    G21=18[3F1x31+3F2y31+3F1x1y21+3F2x21y1+i(3F2x313F2y31+3F2x1y213F1x21y1)]=0.

    By solving the following equations

    [d00(a+b)][ω111ω211]=[h111h211],[d2iω000(a+b)2iω0][ω120ω220]=[h120h220],

    where

    h111=12[(kk0+ah)s1k2k02],
    h211=14(2F4x21+2F4y21)=0,
    h120=12[k2k02hs1a+kk0s1+(hkk0ω0kk0aω0hω0s1)i],
    h220=14(2F4x212F4y212i2F4x1y1)=0,

    one obtains

    ω111=12d[hs1akk0(kk0s1)],ω211=0,ω220=0,
    ω120=12d2+8s1{2ω0(kk0aω0+hkk0ω0hω0s1)+d(hω02ak2k02+kk0s1)
    +[d(kk0aω0+hkk0ω0hω0s1)2ω0(k2k02+ahs1kk0s1)]i},
    G1110=12[(2F1x1z1+2F2y1z1)+i(2F2x1z12F1y1z1)]=12δ(δ01+δ02i),

    where

    δ01=kω0(a2bm+a2hka2hk0+abhm+ah2kah2mah2k0+ak2k0bkmk0h3mhkmk0+hkk02+bms1hk0s1),δ02=(abh2)s12+(a3ba2h2+abh22abkk0ahk2+bhkmh4+h2km+h2kk0)s1+k2k0(a2kbma+2abk0+ahkahm+ahk0bhmh2mh2k0),G2110=12[(2F1x1u1+2F2y1u1)+i(2F2x1u12F1y1u1)]=0,G1101=12[(2F1x1z12F2y1z1)+i(2F2x1z1+2F1y1z1)]=12δ[δ03+δ04i],

    where

    δ03=ω0(a2bkm2a2bkk0+a2hk2a2hkk0+abhkm+2abhkk0+ah2k2ah2km+ah2kk0ak3k0+bk2mk0h3km2h3kk0+hk2mk0hk2k02+2a2bs12ah2s1bkms1+hkk0s1),δ04=(ab+h2)s12+(a3ba2h2+abh22abkm+2abkk0ahk2+2ahkk0bhkmh4+h2kmh2kk0)s1a2k3k0+abk2mk02abk2k02ahk3k0+ahk2mk0ahk2k02+bhk2mk0+h2k2mk0+h2k2k02,
    G2101=12[(2F1x1u12F2y1u1)+i(2F2x1u1+2F1y1u1)]=0,
    g21=G21+2j=1(2Gj110ωj11+Gj101ωj20)=δ1g+δ2gi,

    where

    δ1g=14dδ(k2δ01k02+ahδ01s1+kδ01k0s12dδ03δ05+2dδ04δ06),
    δ2g=14dδ(k2δ02k02+ahδ02s1+kδ02k0s12δ03δ06d2δ04δ05d),
    δ05=12d2+8s1[2ω0(kk0aω0+hkk0ω0hω0s1)+d(hω02ak2k02+kk0s1)],
    δ06=12d2+8s1[d(kk0aω0+hkk0ω0hω0s1)2ω0(k2k02+ahs1kk0s1)].

    Based on above calculation and analysis, we get

    C1(0)=i2ω0(g20g112|g11|213|g02|2)+12g21=12g21,
    μ2=ReC1(0)α(0)=(s22+s1)δ1gh(k0+m)ak0bm,
    τ2=δ2gω0δ1g(ams2+bk0s2+hms2+hk0s2+ms1+k0s1)s1(h(k0+m)ak0bm),

    where

    ω(0)=ω0(ams2+bk0s2+hms2+hk0s2+ms1+k0s1)s1s22+s21,
    α(0)=h(k0+m)ak0bm2(s22+s1),β2=2ReC1(0)=δ1g.

    Note α(0)<0. From ab>h2 and k0(ah)+m(bh)<0, we obtain that if δ1g>0 (resp., δ1g<0), then μ2>0 (resp., μ2<0) and β2>0 (resp., β2<0), the hopf bifurcation of system (1.2) at (0,0,0,0) is non-degenerate and subcritical (resp. supercritical), and the bifurcating periodic solution exists for k>k (resp., k<k) and is unstable (resp., stable).

    Furthermore, the period and characteristic exponent are

    T=2πω0(1+τ2ϵ2+O(ϵ4)),β=β2ϵ2+O(ϵ4),

    where ϵ=kkμ2+O[(kk)2]. And the expression of the bifurcating periodic solution is (except for an arbitrary phase angle)

    X=(x,y,z,u)T=Q(¯y1,¯y2,¯y3,¯y4)T=QY,

    where

    ¯y1=Reμ,¯y1=Imμ,(¯y3,¯y4,¯y5)T=ω11|μ|2+Re(ω20μ2)+O(|μ|2),

    and

    μ=ϵe2itπT+iϵ26ω0[g02e4itπT3g20e4itπT+6g11]+O(ϵ3)=ϵe2itπT+O(ϵ3).

    By computations, we can obtain

    [xyzu]=[kk0ϵcos(2πtT)hω0ϵsin(2πtT)(kk0s1)ϵcos(2πtT)aω0ϵsin(2πtT)ϵ2[kk0(kk0s1)+hs1a2d+δ05δ06sin(4πtT)]k(a+h)ϵcos(2πtT)+kω0ϵsin(2πtT)]+O(ϵ3).

    Based on the above discussion, the conclusions of Theorem 4.2 are proved.

    In order to verify the above theoretical analysis, we assume

    d=2,h=1,k0=1,m=2,b=1.5,a=0.5.

    According to Theorem 4.1, we get k=1. Then from Theorem 4.2, μ2=4.375 and β2=0.074, which imply that the Hopf bifurcation of system (1.2) at (0,0,0,0) is nondegenerate and supercritical, a bifurcation periodic solution exists for k<k=1 and the bifurcating periodic solution is stable. Figure 10 shows the Hopf periodic solution occurs when k=0.999<k=1.

    Figure 10.  Phase portraits of (2.1) with (d,h,k0,m,b,a,k)=(2,1,1,2,1.5,0.5,0.999).

    In this paper, we present a new 4D hyperchaotic system by introducing a linear controller to the first equation and second equation of the 3D Rabinovich system, respectively. If k0=0, m=1 and the fourth equation is changed to ky, system (1.2) will be transformed to 4D hyperchaotic Rabinovich system in [18,19]. Compared with the system in [18], the new 4D system (1.1) has two nonzero equilibrium points which are symmetric about z axis when m>k0 and the dynamical characteristics are more abundant. The complex dynamical behaviors, including boundedness, dissipativity and invariance, equilibria and their stability, chaos and hyperchaos of (1.2) are investigated and analyzed. Furthermore, the existence of Hopf bifurcation, the stability and expression of the Hopf bifurcation at zero-equilibrium point are studied by using the normal form theory and symbolic computations. In order to analyze and verify the complex phenomena of the system, some numerical simulations are carried out including Lyapunov exponents, bifurcations and Poincaré maps, et al. The results show that the new 4D Rabinovich system can exhibit complex dynamical behaviors, such as periodic, chaotic and hyperchaotic. In the real practice, the hyperchaotic Rabinovich system can be applied to generate key stream for the entire encryption process in image encryption scheme [27]. In some cases, hyperchaos and chaos are usually harmful and need to be suppressed such as in biochemical oscillations [8] and flexible shaft rotating-lifting system [28]. Therefore, we will investigate hyperchaos control and chaos control in further research.

    Project supported by the Doctoral Scientific Research Foundation of Hanshan Normal University (No. QD202130).

    The authors declare that they have no conflicts of interest.



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