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Mathematical Analysis Vol. 1 (2020), No. 2, 71–80 https:nn maco.lu.ac.ir & Convex Optimization DOI: 10.29252/maco.1.2.7 Research Paper VARIATIONAL INEQUALITIES ON 2-INNER PRODUCT SPACES HEDAYAT FATHI∗ AND SEYED ALIREZA HOSSEINIOUN Abstract. We introduce variational inequality problems on 2-inner product spaces and prove several existence results for variational in- equalities defined on closed convex sets. Also, the relation between variational inequality problems, best approximation problems and fixed point theory is studied. MSC(2010): 58E35; 46B99; 47H10. Keywords: Variational inequality, 2-Inner product spaces, Fixed point theory, Best approximation. 1. introduction The theory of variational inequalities is an important domain of applied mathematics, introduced in the early sixties by Stampacchia and Hartman[10]. It developed rapidly because of its applications. A classical variational in- equality problem is to find a vector u∗ 2 K such that hv − u∗;T (u∗)i ≥ 0; v 2 K; where K ⊆ Rn is nonempty, closed and convex set and T is a mapping from Rn into itself. Later, variational inequality expanded to Hilbert and Banach spaces. So far, a large number of existing conditions have been established. The books [11] and [3] provide a suitable introduction to variational inequal- ity and its applications. For other generalizations of variational inequalities see [14],[15]. The concept of 2-inner product space was introduced by Diminnie, Gahler and A.White[5]. Definition 1.1. Let X be a real linear space with dimX ≥ 2. Suppose that (:; :j:) is a real-valued function defined on X3 satisfying the following conditions: (2I1) (x; xjz) ≥ 0 and (x; xjz) = 0 if and only if x and z are linearly dependent, Date: Received: September 17, 2020, Accepted: December 6, 2020. ∗Corresponding author. 71 72 H. FATHI AND S.A.R. HOSSEINIOUN (2I2) (x; xjz) = (z; zjx); (2I3) (y; xjz) = (x; yjz), (2I4) (αx; yjz) = α(x; yjz) for any scalar α 2 R, (2I5) (x + x0; yjz) = (x; yjz) + (x0; yjz): Then (:; :j:) is called a 2-inner product on X and (X; (:; :j:)) is called a 2-inner product space (or 2-pre-Hilbert space). 2-inner product spaces have been intensively studied by many authors in the last three decades. A systematic presentation of the recent results related to the theory of 2-inner product spaces as well as an extensive list of the related references can be found in the book [4]. Another concept which is closely related to 2-inner product is 2-norm, which is introduced by Gahler in 1965 [9]. Definition 1.2. Let X be a real linear space with dimX ≥ 2 and k :; : k: X2 ! [0; 1) a function satisfying the following conditions for all α 2 R and all x; y; z 2 X: (2N1) k x; y k= 0 , x and y linearly dependent, (2N2) k x; y k=k y; x k; (2N3) k αx; y k= jαj k x; y k; (2N4) k x + y; z ∥≤∥ x; z k + k y; z k; Then k :; : k is called 2-norm on X and (X; k :; : k) is called linear 2-normed space. The basic theory of 2-normed spaces can be found in [8]. Some basic properties of 2-inner product and 2-norm can be immediately obtained are as follows: (1) (0; yjz) = 0; (x; 0jz) = 0. (2) For x; y; z 2 X we have the Couchy-Schwartz inequality p p j(x; yjz)j ≤ (x; xjz) (y; yjz): (3) k x; y k is non-negative for all x; y 2 X. (4) k x; y + αx k=k x; y k for all x; y 2 X and α 2 R: The next theorem provides a relation between 2-norms and 2-inner products. p Theorem 1.3. [4] On any 2-inner product space (X; (:; :j:)), ka; bk = (a; ajb) defines a 2-norm for which 1 (1.1) (a; bjc) = (ka + b; ck2 − ka − b; ck2) 4 and (the 2-dimensional analogue of the Parallelogram Law) ka + b; ck2 + ka − b; ck2 = 2(ka; ck2 + kb; ck2): Conversely if Parallelogram Law established in linear 2-normed space (X; k :; : k) then (1.1) defines a 2-inner product on X. Whenever a 2-inner product space (X; (:; :j:)) is given, we consider it as a linear 2-normed space (X; k:; :k) with the 2-norm defined in Theorem (1.3). VARIATIONAL INEQUALITIES ON 2-INNER PRODUCT SPACES 73 For a fixed z 2 X, the function Pz(x) =k x; z k (x 2 X), is a seminorm on X. Since dimX > 1 for any y =6 0 there exists x 2 X such that x and y are linearly independent and hence Px(y) =6 0. So the class P = fPz : z 2 Xg forms a family of semi-norms which separates points in X and generates a locally convex topology on X, which is called the natural topology induced by the 2-norm k :; : k. Definition 1.4. Let (X; k:; :k) be a 2-normed space , and u 2 X. (1) A sequence fxng ⊆ X is said to be u-convergent to x 2 X and u denoted by xn −! x, if limn−!1 kxn − x; uk = 0. (2) A subset K of X is said u-closed, if fxng ⊆ K is u-convergent to x 2 X, then x 2 K: 2. Variational inequalities on 2-inner product spaces In this section, we give the natural generalization of variational inequality on 2-inner product spaces and then we prove our main theorem. Definition 2.1. Let K be an arbitrary nonempty subset of real 2-inner product space X, 0 =6 u 2 X and T : K 7−! X be a mapping. The set of variational inequality corresponds to T , K; and u, denoted by VI(T; K; u), is the set of all x0 2 K that apply to the following condition (x − x0;T (x0)ju) ≥ 0 8x 2 K: Some immediate consequences of definition 2.1 are listed below: (1) If there exists x0 such that T (x0) = αu for some α 2 R then x0 is a solution of VI(T; K; u): (2) If u 2 K and x0 2 K is a solution of VI(T; K; u), then (x0;T (x0)ju) ≤ 0: Before proving existence theorems we recall the following notions and prove analog of a Mintys lemma [13]. Definition 2.2. Let K ⊆ X be a closed and convex set, T : K 7−! X be a mapping and u 2 X (i) T is said to be u-monotone if (x − y; T (x) − T (y)ju) ≥ 0 (x; y 2 K): (ii) (iii) T is said to be strictly u-monotone if x =6 y implies that (x − y; T (x) − T (y)ju) > 0: (iv) (v) T is said to be u-pseudo monotone if (x − y; T (y)ju) ≥ 0 implies (x − y; T (x)ju) ≥ 0. 74 H. FATHI AND S.A.R. HOSSEINIOUN Lemma 2.3. Let u 2 X; K ⊆ X be a closed and convex set and T : K 7−! X be a continuous and u-pseudo monotone. Then an element x0 2 K is a solution of VI(T; K; u) if and only if (2.1) (x − x0;T (x)ju) ≥ 0 8x 2 K: Proof. Suppose that x0 2 K is a solution of VI(T; K; u). Then for any x 2 K, we have (x−x0;T (x0)ju) ≥ 0 and the u-pseudo monotonicity implies that (x − x0;T (x)ju) ≥ 0: Conversely, suppose x0 2 K holds in condition (2.1). In this case, if x 2 K, we define xt by xt = (1 − t)x0 + tx; t 2 (0; 1): Insert xt in (2.1), we have (xt − x0;T (xt)ju) ≥ 0; which implies that (t(xt − x0);T (xt)ju) ≥ 0 and finally (x − x0;T (xt)ju) ≥ 0: By taking the limit in the above inequality and by using the continuity of T , we get (x − x0;T (x0)ju) ≥ 0: i.e. x0 is a solution of VI(T; K; u): □ Remark 2.4. The set VI(T; K; u) is not necessarily single-valued. But when T is u-strictly monotone the uniqueness property holds. In fact if x; x0 2 K are two solutions of VI(T; K; u) then (y − x; T (x)ju) ≥ 0 y 2 K; (y − x0;T (x0)ju) ≥ 0 y 2 K: So, setting y = x0 in first inequality and y = x in second, we have (x − x0;T (x) − T (x0)ju) ≤ 0: Now strictly monotonicity implies that x = x0: For any subset S of X define u-orthogonal complement of S by ? f 2 j j 8 2 g Su = x X (x; y u) = 0; y S : Proposition 2.5. Let S ⊆ X be nonempty and closed under scaler multi- plication, u 2 X and T : S ! X be a mapping. Then x0 is solution to 2 ? VI(T; S; u) if and only if T (x0) Su : Proof. If x0 is a solution of VI(T; S; u) and x 2 S then (x−x0;T (x0)ju) ≥ 0. − 2 j 2 ? Now 0; x s implies that (x; T (x0) u) = 0: Conversely if T (x0) Su then clearly (x − x0;T (x0)ju) = 0 and the proof is complete. □ The following theorem is fundamental existence theorem for VI(T; K; u). VARIATIONAL INEQUALITIES ON 2-INNER PRODUCT SPACES 75 Theorem 2.6.
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