Hyers-Ulam stability for nonlocal differential equations

HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2020-0041 Natural Science, 2020, Volume 65, Issue 10, pp. 3-9 This paper is available online at HYERS-ULAM STABILITY FOR NONLOCAL DIFFERENTIAL EQUATIONS Nguyen Van Dac1 and Pham Anh Toan2 1Faculty of Computer Science and Engineering, Thuyloi University 2Nguyen Thi Minh Khai High School, Hanoi Abstract. In this paper, we present a result on Hyers-Ulam stability for a class of nonlocal differential equations in Hilbert spaces by using the theory of integral equations with completely positive kernels together with a new Gronwall inequality type. The paper develops some recent results on fractional differential equations to the ones involving general nonlocal derivatives. Instead of Mittag-Leffler functions, we must utilize the characterization of relaxation function. Keywords: nonlocal differential equation, mild solution, Hyers-Ulam stability. 1. Introduction Let H be a separable Hilbert space. Consider the following equation (k ∗ ∂tu) (t) + Au(t) = f(t, u(t)), t ∈ J := [0, T ]. (1.1) where the unknown function u takes values in H , the kernel k ∈ L1loc (R+), A is an inbounded linear operator, and f : J × H → H is a given function. Here the notation ∗ denotes the Laplace convolution, i.e., (k ∗ v)(t) = ∫ t 0 k(t− s)v(s)ds. In [1], authors introduced a result on the existence, regularity and stability for mild solutions to (1.1) where f depends only on u and the initial condition is given by u(0) = u0. (1.2) Our goal in this paper is to consider the Hyers-Ulam stability for (1.1). The Hyers-Ulam stability for functional equations was founded in 1940 by S.M Ulam in a talk at Wisconsin University (see [2]) and by D. H Hyers’ answer a year later for additive functions defined on Banach spaces (see [3]). However, the first result on the Hyers-Ulam stability of a differential equation was addressed by C.Alsina and R. Ger in 1998 (see [4]). In this remarkable work, they proved that if a differentiable function Received October 2, 2020. Revised October 23, 2020. Accepted October 30, 2020. Contact Nguyen Van Dac, e-mail address: nvdac@tlu.edu.vn 3 Nguyen Van Dac and Pham Anh Toan y : I → R satisfies |y′(t)− y(t)| ≤ ǫ for all t ∈ I , where ǫ > 0 is a given number and I is an open interval of R, then there exists a differentiable function g : I → R satisfying both g′(t) = g(t) and |y(t) − g(t)| ≤ 3ǫ for all t ∈ I . It then has attracted attention of mathematicians for decades (see [5-13]) to study this type of stability for differential equations systematically. In order to deal with (1.1), we use the following standing hypotheses: (A) The operator A : D(A) → H is densely defined, self-adjoint, and positively definite. (K) The kernel function k ∈ L1loc(R+) is nonnegative and nonincreasing, and there exists a function l ∈ L1loc(R+) such that k ∗ l = 1 on (0,∞). (F ) The continuous function f : J ×H → H is Lipschitzian , i.e , there is Lf > 0 such that ‖f(t, v1)− f(t, v2)‖ ≤ Lf‖v1 − v2‖, ∀t ∈ J, ∀v1, v2 ∈ H. 2. Preliminaries 2.1. The resolvent families and the Gronwall type inequality Consider the following scalar integral equations s(t) + µ(l ∗ s)(t) = 1, t ≥ 0, (2.1) r(t) + µ(l ∗ r)(t) = l(t), t > 0. (2.2) The existence and uniqueness of s and r were analyzed in [8]. Recall that the function l is called a completely positive kernel iff s(·) and r(·) take nonnegative values for every µ > 0. The complete positivity of l is equivalent to that (see [14]), there exist α ≥ 0 and k ∈ L1loc(R+) nonnegative and nonincreasing which satisfy αl + l ∗ k = 1. So the hypothesis (K) implies that l is completely positive. Denote by s(·, µ) and r(·, µ) the solutions of (2.1) and (2.1), respectively. As mentioned in [15], the functions s(·, µ) and r(·, µ) take nonnegative values even in the case µ ≤ 0. We collect some additional properties of these functions. Proposition 2.1. [1, 15] Let the hypothesis (K) hold. Then for every µ > 0, s(·, µ), r(·, µ) ∈ L1loc(R +). In addition, we have 1. The function s(·, µ) is nonnegative and nonincreasing. Moreover, s(t, µ) [ 1 + µ ∫ t 0 l(τ)dτ ] ≤ 1, ∀t ≥ 0, (2.3) hence if l 6∈ L1(R+) then lim t→∞ s(t, µ) = 0 for every µ > 0. 2. The function r(·, µ) is nonnegative and one has s(t, µ) = 1− µ ∫ t 0 r(τ, µ)dτ = k ∗ r(·, µ)(t), t ≥ 0, (2.4) 4 Hyers-Ulam stability for nonlocal differential equations so ∫ t 0 r(τ, µ)dτ ≤ µ−1, ∀t > 0. If l 6∈ L1(R+) then ∫∞ 0 r(τ, µ)dτ = µ−1 for every µ > 0. 3. For each t > 0, the functions µ 7→ s(t, µ) and µ 7→ r(t, µ) are nonincreasing. 4. Equation (2.1) is equivalent to the problem d dt [k ∗ (s− 1)] + µs = 0, s(0) = 1. 5. Let v(t) = s(t, µ)v0+(r(·, µ)∗g)(t), here g ∈ L∞loc(R+). Then v solves the problem d dt [k ∗ (v − v0)](t) + µv(t) = g(t), v(0) = v0. Let us mention that, the hypothesis (A) ensures the existence of an orthonormal basis of H consisting of eigenfunctions {en}∞n=1 of the operator A and we have Av = ∞∑ n=1 λnvnen, where λn > 0 is the eigenvalue corresponding to the eigenfunction en of A, D(A) = {v = ∞∑ n=1 vnen : ∞∑ n=1 λ2nv 2 n <∞}. We can assume that 0 < λ1 ≤ λ2 ≤ ... ≤ λn →∞ as n→∞. For γ ∈ R, one can define the fractional power of A as follows: D(Aγ) = { v = ∞∑ n=1 vnen : ∞∑ n=1 λ2γn v 2 n <∞ } , Aγv = ∞∑ n=1 λγnvnen. Let Vγ = D(Aγ). Then Vγ is a Banach space endowed with the norm ‖v‖γ = ‖A γv‖ = ( ∞∑ n=1 λ2γn v 2 n ) 1 2 . Furthermore, for γ > 0, we can identify the dual space V ∗γ of Vγ with V−γ . We now define the following operators: S(t)v = ∞∑ n=1 s(t, λn)vnen, t ≥ 0, v ∈ H, (2.5) R(t)v = ∞∑ n=1 r(t, λn)vnen, t > 0, v ∈ H. (2.6) 5 Nguyen Van Dac and Pham Anh Toan It is easily seen that S(t) and R(t) are linear. We collect some basic properties of these operators in the following lemma. Lemma 2.1. [1] Let {S(t)}t≥0 and {R(t)}t>0, be the families of linear operators defined by (2.5) and (2.6), respectively. Then 1. For each v ∈ H and T > 0, S(·)v ∈ C([0, T ];H) and AS(·)v ∈ C((0, T ];H). Moreover, ‖S(t)v‖ ≤ s(t, λ1)‖v‖, t ∈ [0, T ], (2.7) ‖AS(t)v‖ ≤ ‖v‖ (1 ∗ l)(t) , t ∈ (0, T ]. (2.8) 2. Let v ∈ H, T > 0 and g ∈ C([0, T ];H). Then R(·)v ∈ C((0, T ];H), R ∗ g ∈ C([0, T ];H) and A(R ∗ g) ∈ C([0, T ];V− 1 2 ). Furthermore, ‖R(t)v‖ ≤ r(t, λ1)‖v‖, t ∈ (0, T ], (2.9) ‖(R ∗ g)(t)‖ ≤ ∫ t 0 r(t− τ, λ1)‖g(τ)‖dτ, t ∈ [0, T ], (2.10) ‖A(R ∗ g)(t)‖− 1 2 ≤ (∫ t 0 r(t− τ, λ1)‖g(τ)‖ 2dτ ) 1 2 , t ∈ [0, T ]. (2.11) The following proposition shows a Gronwall type inequality. Proposition 2.2. Let v be a nonnegative continuous function satisfying v(t) ≤ C1 + C2 ∫ t 0 r(t− τ, µ)v(τ)dτ, t ∈ J, (2.12) for given nonnegative numbers C1, C2 and µ > 0. Then v(t) ≤ s(t,−C2)C1. Proof. From (2.2) and the positivity of r(·, µ) and l(·), we get r(t, µ) ≤ l(t), ∀t ∈ J, andµ > 0. Combining this inequality with (2.12) yield v(t) ≤ C1 + C2(l ∗ v)(t). (2.13) Consider the following equation ξ(t) = C1 + C2(l ∗ ξ)(t), t ∈ J. 6 Hyers-Ulam stability for nonlocal differential equations Obviously ξ(0) = C1 and the equation is equivalent to ξ(t)− C1 = C2(l ∗ ξ)(t). Taking the convolution with the kernel k gives us k ∗ (ξ − C1) = C2(1 ∗ ξ)(t). Then ξ is a solution to the following systems d dt [k ∗ (ξ − C1)] = C2ξ(t) ξ(0) = C1. So ξ(t) = s(t,−C2)C1. Therefore, we arrive at v(t) ≤ s(t,−C2)C1, ∀t ∈ J, thanks to the comparison principle. 2.2. Existence result to system (1.1) - (1.2) Definition 2.1. A function u ∈ C((0, T ];H) is said to be a mild solution to (1.1)-(1.2) on [0, T ] iff u(t) = S(t)u0 + ∫ t 0 R(t− τ)f(τ, u(τ))dτ, for t ∈ [0, T ]. Theorem 2.1. Let (A), (K) and (F) hold. Then the mild solution to (1.1)-(1.2) is unique. Proof. To get the result, we use the same arguments as in [1]. 3. Hyers-Ulam stability on [0, T ] We first define of Hyer-Ulam stability for nonlocal differential equation (1.1) and then we show our main result. We consider (1.1) and the following inequality ‖ (k ∗ ∂tv) (t) + Av(t)− f(t, v(t))‖ ≤ ǫ, t ∈ J, (3.1) where ǫ > 0 is given. We now give the definition of mild solution to the above inequality. Definition 3.1. A continuous funtion v : J → H is said to be a mild solution to (3.1) if there exists a function g ∈ L1loc(J,H) such that ‖g(t)‖ ≤ ǫ and v(t) = S(t)v(0) + ∫ t 0 R(t− τ)[f(τ, v(τ)) + g(τ)]dτ, t ∈ J. 7 Nguyen Van Dac and Pham Anh Toan Definition 3.2. Equation (1.1) is called Hyers-Ulam stable, with respect to s defined on J , if there exists a real number C > 0 such that for each ǫ > 0 and for every mild solution v of (3.1), there is a mild solution u of (1.1) with ‖v(t)− u(t)‖ ≤ Cǫs(t, ν), ∀t ∈ [0, T ], for some ν ∈ R. Definition 3.3. Equation (1.1) is called generalized Hyers-Ulam stable, with respect to s(t, ν), if there exists θ ∈ C (R+,R+) , θ(0) = 0 such that for each mild solution v of (3.1) there exists a mild solution u of (1.1) with ‖v(t)−u(t)‖ ≤ θ(ǫ)s(t, ν), for all t ∈ J . Remark 3.1. It is clear that if equation (1.1) is Hyers-Ulam stable then it is also generalized Hyers-Ulam stable. The following Theorem is the main result in this paper. Theorem 3.1. If (A), (K) and (F) hold, then the equation (1.1) is Hyers-Ulam stable. Proof. Let v be a mild solution to (3.1). By Theorem 2.1, the following problem (k ∗ ∂tu) (t) + Au(t) = f(t, u(t)), t ∈ J, u(0) = v(0), admits a unique mild solution given by u(t) = S(t)v(0) + ∫ t 0 R(t− τ)f(τ, u(τ))dτ, ∀t ∈ J. Therefore, we have ‖v(t)− u(t)‖ ≤ ∥∥∥ ∫ t 0 R(t− τ)[f(τ, v(τ)) + g(τ)− f(τ, u(τ))]dτ ∥∥∥ ≤ ǫ ∫ t 0 r(t− τ, λ1)dτ + Lf ∫ t 0 r(t− τ, λ1)‖v(τ)− u(τ)‖dτ ≤ ǫ 1 λ1 + Lf ∫ t 0 r(t− τ, λ1)‖v(τ)− u(τ)‖dτ, thanks to (F) and Proposition 2.1. It comes from the Gronwall type inequality stated in Proposition 2.2 that ‖v(t)− u(t)‖ ≤ ǫ λ1 s(t,−Lf ). The proof is complete. 8 Hyers-Ulam stability for nonlocal differential equations 4. Conclusions In this paper, the Hyers-Ulam stability has been discussed for a class of nonlocal evolution equations in Hilbert space. The result may be extended to more general models and concepts. It is very interesting to investigate these types of stabilities for nonlocal differential equations in Banach spaces, where the new methods and ideas are needed due to the lack of Hilbertian structrure on phase spaces. REFERENCES [1] T.D. Ke, N.N. Thang, L.T.P. Thuy, 2020. Regularity and stability analysis for a class of semilinear nonlocal differential equations in Hilbert spaces. J. Math. Anal. Appl., 483, No. 2, pp. 123655. [2] S.M. Ulam, 1960. A Collection of Mathematical Problems. Interscience, New York. [3] D.H. Hyers, 1941. On the stability of the linear functional equation. Proc. Natl. Acad. Sci. USA, 27, pp. 222-224. [4] C. Alsina, R. Ger, 1988. On some inequalities and stability results related to the exponential function. J. Inequal. Appl., 2, pp. 373-380. [5] S.M. Jung, 2004. Hyers-Ulam stability of linear differential equations of first order. Appl. Math. Lett., 17 (10), pp. 1135-1140. [6] S. Jung, 2010. A fixed point approach to the stability of differential equations y0 = F (x, y). Bull. Malays. Math. Sci. Soc., 33, pp. 47-56. [7] V. Kalvandi, N. Eghbali, J. Michael Rassias, 2019. Mittag-Leffler-Hyers-Ulam Stability of Fractional Differential Equations of Second Order. J. Math. Ext., 13, pp. 29-43. [8] R.K. Miller, 1968. On Volterra integral equations with nonnegative integrable resolvents. J. Math. Anal. Appl., 22, pp. 319-340. [9] T. Miura, S. Miyajima and S.E. Takahasi, 2003. A characterization of HyersUlam stability of first order linear differential operators. J. Math. Anal. Appl., 286 (1), pp. 136-146. [10] T. Miura, S. Miyajima and S.E. Takahasi, 2003. HyersUlam stability of linear differential operator with constant coefficients. Math. Nachr., 258 (1), pp. 90-96. [11] M. Oboza, 1993. Hyers stability of the linear differential equation. Rocznik Nauk.-Dydakt. Prace Mat., 13, pp. 259-270. [12] M. Oboza, 1997. Connections between Hyers and Lyapunov stability of the ordinary differential equations. Rocznik Nauk.-Dydakt. Prace Mat., 14, pp. 141-146. [13] J. Wang, M. Feckan, Y. Zhou, 2012. Ulams type stability of impulsive ordinary differential equations. J. Math. Anal. Appl., 395 (1), pp. 258-264. [14] Ph. Cle´ment, J. A. Nohel, 1981. Asymptotic behavior of solutions of nonlinear Volterra equations with completely positive kernels. SIAM J. Math. Anal., 12, pp. 514-535. [15] V. Vergara, R. Zacher, 2017. Stability, instability, and blowup for time fractional and other nonlocal in time semilinear subdiffusion equations. J. Evol. Equ., 17, pp. 599-626. 9