Int. J. Anal. Appl. (2022), 20:36 MϕA-h-Convexity and Hermite-Hadamard Type Inequalities Sanja Varošanec∗ Department of Mathematics, Faculty of Science, University of Zagreb, Bijenička 30, 10000 Zagreb, Croatia ∗Corresponding author: varosans@math.hr Abstract. We investigate a family of MϕA-h-convex functions, give some properties of it and several inequalities which are counterparts to the classical inequalities such as the Jensen inequality and the Schur inequality. We give the weighted Hermite-Hadamard inequalities for an MϕA-h-convex function and several estimations for the product of two functions. 1. Preliminaries It is known that the classical convexity can be generalized to an MN-convexity, where M and N are means which is described in [8]. The other direction of generalization leads to the concept of h-convexity, [13]. It is interesting to see properties of a function which definition combines some elements of MN-convexity and of h-convexity. Let M and N be two means in two variables. We say that a function f : I →R is MN-convex if f (M(x,y))≤ N(f (x), f (y)) for every x,y ∈ I. In this paper we will focus on a somewhat special type of means. Let ϕ be a continuous, strictly monotone function defined on the interval I. By Mϕ we denote a quasi-arithmetic mean: Mϕ(x,y;t,1− t) := ϕ−1(tϕ(x)+(1− t)ϕ(y)), x,y ∈ I,t ∈ [0,1]. It is obvious that the power mean Mp corresponds to ϕ(x)= xp if p 6=0 and to ϕ(x)= logx if p =0. Received: Jun. 26, 2022. 2010 Mathematics Subject Classification. 26A51, 26D15. Key words and phrases. the Hermite-Hadamard inequality; the Jensen inequality; MϕA-h-convex function; quasi- arithmetic mean; the Schur inequality. https://doi.org/10.28924/2291-8639-20-2022-36 ISSN: 2291-8639 © 2022 the author(s). https://doi.org/10.28924/2291-8639-20-2022-36 2 Int. J. Anal. Appl. (2022), 20:36 Let ϕ and ψ be two continuous, strictly monotone functions defined on intervals I and K respectively. Let h : J → R be a non-negative function, (0,1) ⊆ J and let f : I → K such that h(t)ψ(f (x))+ h(1− t)ψ(f (y))∈ ψ(K) for all x,y ∈ I,t ∈ (0,1). We say that a function f is MϕMψ-h-convex if f (Mϕ(x,y;t,1− t))≤ Mψ(f (x), f (y);h(t),h(1− t)) for all x,y ∈ I and all t ∈ (0,1). Especially, a function f : I →R is called MϕA-h-convex if f (Mϕ(x,y;t,1− t))≤ h(t)f (x)+h(1− t)f (y) (1.1) for all x,y ∈ I and t ∈ (0,1). The MϕMψ-h-concavity is defined on a natural way. Some particular cases of MϕMψ-h-convex functions are recently investigated. If h(t) = t, then the MϕMψ-h-convexity collapses to the MϕMψ-convexity which is described in [8]. If Mϕ, Mψ are an arithmetic mean (A), a geometric mean (G) or a harmonic mean (H), then we can find several results. For example, AA-h-convexity or simply h-convexity firstly appeared in [13]. An HA-h-convexity or harmonic-h-convexity is described in [2] and [10]. HG-h-convexity investigated in [10] and AG-h- convexity or log-h-convexity in [9]. AMp-h-convexity or (h,p)-convexity is described in [6] while some properties of MpA-h-convex functions are given in [4]. Also, we have to mention article [1] devoted to the MN-h-convexity where M,N ∈{A,G,H}. The aim of this paper is to give several statements about MϕA-h-convex functions primarly related to the Hermite-Hadamard inequality and the Jensen inequality. The following section is devoted to the properties of MϕA-h-convex functions. Also in that section we give counterparts to the Jensen and the Schur inequality and some related results. In the third section we prove several inequalities of Hermite-Hadamard type. 2. Properties of MϕA-h-convex functions and Jensen-type inequalities Proposition 2.1. Let ϕ be a continuous, strictly monotone function defined on the interval I. Let h be a non-negative function defined on the interval J, (0,1) ⊆ J. A function f is MϕA-h-convex (concave) on I if and only if the function f ◦ϕ−1 is h-convex (concave) on ϕ(I). Proof. Let us suppose thatf is MϕA-h-convex on I and let u,v ∈ ϕ(I), t ∈ (0,1). Since ϕ is continuous and strictly monotone on I, there exist x,y ∈ I such that u = ϕ(x),v = ϕ(y). Then (f ◦ϕ−1)(tu +(1− t)v) = (f ◦ϕ−1)(tϕ(x)+(1− t)ϕ(y))) = f (Mϕ(x,y;t,1− t))≤ h(t)f (x)+h(1− t)f (y) = h(t)f (ϕ−1(u))+h(1− t)f (ϕ−1(v)) = h(t)(f ◦ϕ−1)(u)+h(1− t)(f ◦ϕ−1)(v) which means that f ◦ϕ−1 is h-convex. The second case is proved similarly. � Int. J. Anal. Appl. (2022), 20:36 3 Proposition 2.2. Let ϕ be a continuous, strictly monotone function defined on the interval I. Let h,h1,h2 be non-negative functions defined on the interval J, (0,1)⊆ J. (i) Let h1 and h2 have a property h2(t)≤ h1(t), t ∈ (0,1). If f : I → [0,∞) is MϕA-h2-convex, then f is an MϕA-h1-convex function. (ii) If f ,g are MϕA-h-convex functions, λ > 0, then f +g and λf are MϕA-h-convex. (iii) Let f , : I → [0,∞) be similarly ordered functions on I, i.e. (f (x)− f (y))(g(x)−g(y))≥ 0, x,y ∈ I and h(t)+h(1− t)≤ c for all t ∈ (0,1), where h =max{h1,h2} and c is a fixed positive number. If f is MϕA-h1-convex and g is MϕA-h2-convex, then the product f g is MϕA-h-convex. Proof. The proof is based on the known results for h-convex functions and characterization given in Proposition 2.1. Let us prove part (i). If f is MϕA-h2-convex, then f ◦ϕ−1 is h2-convex. Then, using Proposition 8 from [13], we get that f ◦ϕ−1 is h1-convex, i.e. f is MϕA-h1-convex. Other parts are proved similarly by applying Propositions 9 and 10 from [13]. � The following theorem gives a counterpart of the Schur inequality. Theorem 2.1. Let h be a non-negative supermultiplicative function defined on the interval J, (0,1)⊆ J. Let ϕ be a continuous, strictly monotone function defined on the interval I. Let f : I → [0,∞) be MϕA-h-convex. If ϕ is increasing, then for any x1,x2,x3 ∈ I such that x1 < x2 < x3 and ϕ(x3)−ϕ(x2), ϕ(x3)−ϕ(x1), ϕ(x2)−ϕ(x1)∈ J the following holds h(ϕ(x3)−ϕ(x2))f (x1)−h(ϕ(x3)−ϕ(x1))f (x2)+h(ϕ(x2)−ϕ(x1))f (x3)≥ 0. (2.1) If ϕ is decreasing, then for any x1,x2,x3 ∈ I such that x1 < x2 < x3 and ϕ(x2)−ϕ(x3), ϕ(x1)−ϕ(x3), ϕ(x1)−ϕ(x2)∈ J the following holds h(ϕ(x2)−ϕ(x3))f (x1)−h(ϕ(x1)−ϕ(x3))f (x2)+h(ϕ(x1)−ϕ(x2))f (x3)≥ 0. (2.2) Proof. Let assume that ϕ is increasing. For x1,x2,x3 ∈ I such that x1 < x2 < x3 we have u1 := ϕ(xi) < u2 := ϕ(x2) < u3 := ϕ(x3). Since a function g := f ◦ϕ−1 is h-convex, using Proposition 16 from [13], we get h(u3 −u2)g(u1)−h(u3 −u1)g(u2)+h(u2 −u1)g(u3)≥ 0 and after appropriate substitutions we obtain inequality (2.1). Inequality (2.2) is proved in a similar way. � 4 Int. J. Anal. Appl. (2022), 20:36 The following theorem is a counterpart of the discrete Jensen inequality and its converse for an MϕA-h-convex function. Theorem 2.2. Let h : J → R be a non-negative supermultiplicative function, (0,1) ⊆ J. Let ϕ be a continuous, strictly monotone function defined on the interval I. Let f : I → [0,∞) be a MϕA- h-convex function. Let w1, . . . ,wn be non-negative real numbers such that Wn = ∑n i=1wi 6= 0 and wi Wn ∈ J, i =1, . . . ,n. (i) Then for all x1, . . . ,xn ∈ I the following holds f ( ϕ−1 ( 1 Wn n∑ i=1 wiϕ(xi) )) ≤ n∑ i=1 h ( wi Wn ) f (xi). (ii) Then for all x1, . . . ,xn ∈ (a,b)⊆ I the following holds n∑ i=1 h ( wi Wn ) f (xi) ≤ f (a) n∑ i=1 h ( wi Wn ) h ( ϕ(b)−ϕ(xi) ϕ(b)−ϕ(a) ) +f (b) n∑ i=1 h ( wi Wn ) h ( ϕ(xi)−ϕ(a) ϕ(b)−ϕ(a) ) . Proof. Since f is a MϕA-h-convex function, then f ◦ϕ−1 is h-convex on ϕ(I) and using the Jensen inequality for h-convex functions and its converse ( [13, Theorems 19 and 21]), we get the above results. � The following result is a property of subadditivity for an index set function. Let K be a finite non-empty set of positive integers. Let us define the index set function F by F(K)= h(WK)f ( ϕ−1 ( 1 WK ∑ i∈K wiϕ(xi) )) − ∑ i∈K h(wi)f (xi), where wi ∈ J, WK := ∑ i∈K wi ∈ J, xi ∈ I. Theorem 2.3. Let h : J → R be a non-negative supermultiplicative function and let M and K be finite non-empty sets of positive integers with M ∩ K = ∅. Let wi > 0, (i ∈ M ∪ K) be such that WK,WM,WM∪K ∈ J. Let ϕ be a continuous, strictly monotone function defined on the interval I. If f : I → [0,∞) is MϕA-h-convex, then the following inequality holds F(M ∪K)≤ F(M)+F(K). Furthermore, if Mk := {1, . . . ,k}, k =2, . . . ,n and WMk ∈ J, then F(Mn)≤ F(Mn−1)≤ . . . ≤ F(M2)≤ 0 Int. J. Anal. Appl. (2022), 20:36 5 and F(Mn) ≤ min 1≤i