EUROPEAN JOURNAL OF PURE AND APPLIED MATHEMATICS Vol. 17, No. 3, 2024, 1585-1601 ISSN 1307-5543 – ejpam.com Published by New York Business Global Differentiating Odd Dominating Sets in Graphs Mary Ann A. Carbero1∗, Gina A. Malacas1,2, Sergio R. Canoy, Jr.1,2 1 Department of Mathematics and Statistics, College of Science and Mathematics, MSU-Iligan Institute of Technology, 9200 Iligan City, Philippines 2 Center for Mathematical and Theoretical Physical Sciences, Premier Research Institute of Science and Mathematics, MSU-Iligan Institute of Technology, 9200 Iligan City, Philippines Abstract. Let G = (V (G), E(G)) be a simple and undirected graph. A dominating set S ⊆ V (G) is called a differentiating odd dominating set if for every vertex v ∈ V (G), |N [v] ∩ S| ≡ 1(mod 2) and NG[u] ∩ S ̸= NG[v] ∩ S for every two distinct vertices u and v in V (G). The minimum cardinality of a differentiating odd dominating set of G, denoted by γo D(G), is called the differentiating odd domination number. In this paper, we discuss differentiating odd dominating sets and give bounds or exact values of the differentiating odd domination numbers of some graphs. We give necessary and sufficient conditions for some graphs to admit a differentiating odd dominating set. Moreover, we characterize the differentiating odd dominating sets in graphs resulting from join, corona, and lexicographic product of some graphs and determine the differentiating odd domination numbers of these graphs. 2020 Mathematics Subject Classifications: 05C69 Key Words and Phrases: Differentiating, domination, odd dominating, differentiating-dominating, differentiating odd dominating 1. Introduction Domination is one of the most explored areas in Graph Theory. Indeed, numerous variations of domination have been introduced and investigated from various perspectives and approaches (see [1], [2], [5], [7], [10], [13], [14], and [18]). One prominent area of research in this domain is the investigation of differentiating-dominating sets in graphs, which are alternatively referred to as identifying codes in certain contexts. This research has roots dating back to 1998 when Karpovsky, Chakrabarty, and Levitin introduced identifying codes (see [4]) and have been investigated further by Frick et al in 2008 (see [6]). Furthermore, in the study of Canoy and Malacas [12], they characterized the differentiating-dominating sets in the join, corona, and lexicographic product of ∗Corresponding author. DOI: https://doi.org/10.29020/nybg.ejpam.v17i3.5213 Email addresses: maryann.carbero@g.msuiit.edu.ph (M. Carbero), gina.malacas@g.msuiit.edu.ph (G. Malacas), sergio.canoy@g.msuiit.edu.ph (S. Canoy) https://www.ejpam.com 1585 © 2024 EJPAM All rights reserved. M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1586 graphs and determined the bounds or the exact differentiating-domination numbers of the aforementioned graphs. Other studies related to the topic can be found in [8], [9], [11], [15], [16], and [17]. In 1989, Sutner introduced the concept of odd dominating set under the name “odd-parity cover” (see [19]). Specifically, he showed that every graph contains odd dominating set in the context of cellular automata (see [19]). However, this parameter has been studied very little. Previous studies on parity domination mainly focused on algorithmic problems and even dominating sets [3]. In this paper, we introduce the concept of differentiating odd dominating sets in graphs. Note that the concept of differentiating-dominating set may be used to model problems which involve protection in a given network where the goal is to specifically determine the exact location of an intruder (e.g. burglar or fire). When used in this case as a protection strategy, an element of a differentiating-dominating set may refer to a monitoring device or location (vertex) where a monitoring device is positioned or placed. When, in addition, the num- ber of these locations or monitors adjacent to a location (with or with no monitoring device) is required to be odd for every location, then the concept of odd dominating set is also imposed. 2. Terminologies and Notation Let G = (V (G), E(G)) be a simple and undirected graph. The open neighborhood of a vertex v of G is the set NG(v) = {u ∈ V (G) : uv ∈ E(G)} and its closed neighborhood is the set NG[v] = NG(v) ∪ {v}. The open neighborhood of a subset S of V (G) is the set NG(S) = ∪v∈SNG(v) and its closed neighborhood is the set NG[S] = NG(S) ∪ S. Vertex v is a leaf if degG(v) = 1 and the vertex u ∈ (V (G) ∩NG(v)) is called a support vertex. L(G) and S(G) denote the sets consisting of all leaves and support vertices in G, respectively. A graph G of order n ≥ 3 is point distinguishing if for any two distinct vertices u and v of G, NG[u] ̸= NG[v]. It is totally point determining if for any two distinct vertices u and v of G, NG(u) ̸= NG(v) and NG[u] ̸= NG[v]. A set S ⊆ V (G) is a dominating set (respectively, total dominating set) in G if NG[S] = V (G) (respectively, NG(S) = V (G)). The smallest cardinality of a dominating set in G, denoted by γ(G), is called the domination number in G. A dominating set in G with cardinality γ(G) is called a γ-set of G. A set of vertices S is called an odd dominating set (respectively, even dominating set) if for every vertex v ∈ V (G), |NG[v]∩S| ≡ 1(mod 2) (respectively, |NG[v]∩S| ≡ 0(mod 2)). The minimum cardinality of an odd dominating set is called the odd domination number in G (respectively, even domination number), denoted by γodd(G) (respectively, γeven(G)). Any odd dominating set with cardinality γodd(G) is called a γodd-set. A set S ⊆ V (G) is a differentiating set in a graph G if for every two distinct vertices u and v in G, NG[u] ∩ S ̸= NG[v] ∩ S. It is a strictly differentiating set if it is differentiating and NG[u] ∩ S ̸= S for all u ∈ V (G). A differentiating (respectively, strictly differentiating) subset S of V (G) which is also dominating is called a M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1587 differentiating-dominating (respectively, strictly differentiating-dominating) set in a graph G. The minimum cardinality of a differentiating-dominating (respectively, strictly differentiating-dominating) set in G, denoted by γD(G) (respectively, γSD(G)), is called the differentiating-domination (respectively, strictly differentiating-domination) number in G. Any differentiating-dominating (respectively, strictly differentiating-dominating) set with cardinality γD(G) (respectively, γSD(G)) is called a γD-set (respectively, γSD- set). A set S ⊆ V (G) is a differentiating odd dominating set (respectively, differentiating even dominating set) if it is both differentiating and odd dominating (respectively, both differentiating and even dominating). A set S ⊆ V (G) is a strictly differentiating odd dominating set (respectively, strictly differentiating even dominating set) if it is both strictly differentiating and odd dominating (respectively, both strictly differentiating and even dominating). The sets DOD(G) and DED(G) is the set of all differentiating odd dominating sets and the set of all differentiating even dominating sets, respectively, in G. The sets SDOD(G) and SDED(G) is the set of all strictly differentiating odd dominating sets and the set of all strictly differentiating even dominating sets, respectively, in G. The minimum cardinality of a differentiating odd dominating (respectively, differentiating even dominating) set in G, denoted by γoD(G) (respectively, γeD(G)), is called the differentiating odd domination number (respectively, differentiating even domination number in G. The minimum cardinality of a strictly differentiating odd dominating (respectively, strictly differentiating even dominating) set in G, denoted by γoSD(G) (respectively, γeSD(G)), is called the strictly differentiating odd domination number (respectively, strictly differentiating even domination number) in G. Any differentiating odd dominating (respectively, strictly differentiating odd dominating) set with cardinality γoD(G) (respectively, γoSD(G)) is called a γoD-set (respectively, γoSD-set). 3. Results Remark 1. Every differentiating odd dominating set in a connected graph G is an odd dominating set. Remark 2. Every differentiating odd dominating set in a connected graph G is a differentiating-dominating set. Theorem 1. Let G be a graph. Then G admits a differentiating-dominating set if and only if it is point distinguishing. Proof. Suppose G admits a differentiating set, say S. Suppose G is not point distinguishing. Then there exist distinct vertices a, b ∈ V (G) such that NG[a] = NG[b]. This implies that NG[a]∩S = NG[b]∩S, a contradiction. Thus, G is point distinguishing. For the converse, suppose that G is point distinguishing. Then S = V (G) is a differentiating-dominating set, showing that G has a differentiating-dominating set. Lemma 1. Let G be a connected graph of order m and let S be a differentiating odd dominating set in G. Then m ≤ 2|S|−1. In particular, m ≤ 2γ o D(G)−1, i.e., γoD(G) ≥ ln(m)+ln(2) ln(2) . M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1588 Proof. Let S be a differentiating odd dominating set in G and let k = |S|. Let D = {Q ⊆ S : |Q| is odd}. Then |D| = 2k−1. Since S is differentiating odd dominating, m ≤ 2k−1. If S is a γoD-set, then m ≤ 2γ o D(G)−1. This proves the assertion. Theorem 2. Let G be a point distinguishing connected graph. (i) If G has a support vertex v with |NG(v)| = 2, then G does not admit a differentiating odd dominating set. (ii) If S is a differentiating odd dominating set in G and v ∈ V (G) with |NG(v) ∩ L(G)| ≥ 2, then NG(v) ∩ L(G) ⊆ S and v /∈ S. Proof. Let w ∈ L(G)∩NG(v) and let z ∈ NG(v)\{w}. SupposeG has a differentiating odd dominating set S. If w ∈ S, then v /∈ S because S is an odd dominating set. Since S is a differentiating set, NG[w] ∩ S = {w} ̸= NG[v] ∩ S. This forces z ∈ S. However, the assumption would imply that NG[v] ∩ S = {w, z}, contradicting the fact that S is an odd dominating set. Thus, w /∈ S. Consequently, v ∈ S. Since S is odd dominating, z /∈ S. It follows that NG[w] ∩ S = {v} = NG[v] ∩ S, contrary to the assumption that S is a differentiating set. Therefore, G has no differentiating odd dominating set, showing that (i) holds. Next, suppose that S is a differentiating odd dominating set and v ∈ V (G) with |NG(v)∩L(G)| ≥ 2. Suppose v ∈ S. Since S is odd dominating, (NG(v)∩L(G))∩S = ∅. Let x, y ∈ NG(v) ∩ L(G) where x ̸= y. Then NG[x] ∩ S = NG[y] ∩ S = {v}, contrary to the assumption that S is a differentiating set. Therefore, v /∈ S. Since S is a dominating set, NG(v) ∩ L(G) ⊆ S. Thus, (ii) holds. The next results follow from the preceding ones. Corollary 1. For n ≥ 2, Kn and Pn do not admit a differentiating odd dominating set. Corollary 2. Let Sn = K1,n be a star of order n + 1 where n ≥ 3. Then Sn admits a differentiating odd dominating set if and only if n is odd. Moreover, if n is odd, then S = V (Sn) \ {v0} where degG(v0) = n, is the only differentiating odd dominating set in Sn. In particular, γoD(Sn) = n. Proof. Let V (Sn) = {v0, v1, · · · , vn}, where degG(v0) = n. Suppose Sn admits a differentiating odd dominating set, say S. By Theorem 2(ii), S = V (Sn) \ {v0}. Since S is odd dominating, |NSn [v] ∩ S| = |S| = n is odd. For the converse, suppose that n is odd. Then S = V (Sn) \ {v0} is a differentiating odd dominating set in Sn. Note that if n is odd, then S = V (Sn)\{v0} is the only differentiating odd dominating set in Sn by Theorem 2(ii). Hence, γoD(Sn) = n. Corollary 3. Let G be any non-trivial connected graph of order n and let m be a positive odd integer with m ≥ 3. Then there exists a connected graph H obtained from G such that γoD(H) = mn. Moreover, if every vertex in G has even degree, then γodd(H) = n. In particular, the difference γoD(G)− γodd(G) can be made arbitrarily large. M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1589 Proof. Let V (G) = {v1, v2, · · · , vn} and let V (Km) = {x1, x2, · · · , xm}. Let H be the graph obtained from G by adding the edges vixj for each i ∈ {1, 2, · · · , n} and for each j ∈ {1, 2, · · · ,m}. By Theorem 2(ii), S = V (H) \ V (G) is a γoD-set in H. Thus, γoD(H) = mn. Cleary, γodd(H) = n. Suppose now that |NG(v)| is even for every v ∈ V (G). Since V (G) is a minimum dominating set in H and |NH [x] ∩ V (G)| is odd for every x ∈ V (H), V (G) is a γodd-set in H. Thus, γodd(H) = n. Suppose γoD(G) = 1, say S = {v} is a γoD-set in G. If there exists w ∈ V (G) \ {v}, then NG[v] ∩ S = NG[w] ∩ S = {v}, contrary to the fact that S is a differentiating set. Thus, G = K1. We state this formally. Remark 3. Let G be a graph. Then γoD(G) = 1 if and only if G = K1. Remark 4. There exists no connected graph G with γoD(G) = 2. To see this, suppose that such a connected graphG exists. Then |V (G)| = 2 according to Lemma 1. Hence, G = K2. This, however, is not possible by Corollary 1. Theorem 3. Let G be a connected graph of order n ≥ 4. If G admits a differentiating odd dominating set, then max{γD(G), γodd(G), 3} ≤ γoD(G) ≤ n − |S(G)|. Moreover, γoD(G) = 3 if and only if G = K1,3. Proof. By Remarks 1, 2, 3, and 4, max{γD(G), γodd(G), 3} ≤ γoD(G). Next, let S be a γoD-set in G. Let v ∈ S(G) and let xv ∈ L(G) ∩ NG(v) be fixed. Since S is an odd dominating set, v ∈ S or xv ∈ S but not both. Let DG = {w ∈ V (G) \ S : w = v ∈ S(G) or w = xv}. Then |DG| = |S(G)| and S ⊆ V (G) \DG. It follows that γ o D(G) = |S| ≤ n− |S(G)|. For the second part, suppose that γoD(G) = 3. From Lemma 1, it follows that n = 4. It can easily be verified that among the connected graphs of order 4, only K1,3 satisfies the given property. Thus, G = K1,3. The converse is easy. Let Sp = K1,p and Sq = K1,q be stars with central vertices (support vertices) v0 and w0, respectively. Then the double star Sp,q is the graph obtained from Sp and Sq by adding the edge v0w0. Corollary 4. Let Tn be a tree of n ≥ 4. If Tn has a differentiating odd dominating set, then γoD(Tn) ≤ n− |S(Tn)| with equality holding if |NTn(v)∩L(Tn)| is odd and at least 3 for every v ∈ S(Tn). In particular, if Tn = Sp,q (a double star), where p ≥ 3 and q ≥ 3 and are odd, then γoD(Tn) = n− 2 = p+ q. M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1590 Proof. Suppose S is a γoD-set in Tn. By Theorem 3, γoD(Tn) ≤ n − |S(Tn)|. Next, suppose that |NTn(v) ∩ L(Tn)| is odd and at least 3 for every v ∈ S(Tn). By theorem 2(ii), it follows that NTn(v) ∩ L(Tn) ⊆ S and v /∈ S for every v ∈ S(Tn). Therefore, S = NTn(v)∩L(Tn) and γoD(Tn) = n−|S(Tn)|. From this, it follows that γoD(Tn) = p+q when Tn = Sp,q. Theorem 4. γoD(Cn) = n for n ≥ 4. Proof. Let Cn = [v1, v2, ..., vn, v1] and let S be a γoD-set in Cn. Suppose S ̸= V (Cn). Then there exists v ∈ V (Cn) \ S. Without loss of generality, we may assume that v = v1. Since S is odd dominating, v2 ∈ S or vn ∈ S but not both. Assume that v2 ∈ S. Then vn /∈ S. Since |NCn [v2] ∩ S| must be odd, v3 /∈ S. This implies that NCn [v1]∩S = NCn [v2]∩S = {v2}, contrary to the assumption that S is a differentiating set. Therefore, S = V (Cn) and γoD(Cn) = n. Theorem 5. [6] Let Cn be the cycle on n vertices. For n ≥ 3, γD(C2n) = n. Corollary 5. Let n be a positive integer such that n ≥ 3. Then there exists a connected graph G such that γoD(G) − γD(G) = n. In other words, the difference γoD − γD can be made arbitrarily large. Proof. Let G = C2n. By Theorem 5, γD(C2n) = n and by Theorem 4, γoD(C2n) = 2n. Therefore, γoD(G)− γD(G) = n. Theorem 6. Let G = Kn1,n2,...,nk be the complete k-partite graph with 2 ≤ n1 ≤ n2 ≤ · · · ≤ nk, where k ≥ 2. Then G admits a differentiating odd dominating set if and only if ∑ j ̸=t nj is even for every t ∈ {1, 2, . . . , k}. Moreover, in this case, γoD(G) = ∑k j=1 nj. Proof. Let Sn1 , Sn2 , . . . , Snk be the partite sets in G. Suppose G admits a differentiating odd dominating set S. Let j ∈ {1, 2, . . . , k} and let v ∈ Snj . Suppose v /∈ S. Note that since S is odd dominating, |NG[v] ∩ S| = |NG(v) ∩ S| is odd. Pick any w ∈ Snj \ {v}. Since S is differentiating and NG(w) ∩ S = NG(v) ∩ S, it follows that w ∈ S and NG[w] ∩ S = {w} ∪ (NG(v) ∩ S). Since |NG(v) ∩ S| is odd, |NG[w] ∩ S| is even, contrary to the assumption that S is an odd dominating set. Therefore, Snj ⊆ S for each j ∈ {1, 2, . . . , k}, i.e., S = V (G). Now, let t ∈ {1, 2, . . . , k} and let a ∈ Snt . Then NG[a]∩S = NG[a] = {a}∪ (∪j ̸=tSnj ). Since S is odd dominating, |NG[a]| = 1 + ∑ j ̸=t |Snj | = 1 + ∑ j ̸=t nj is odd. This implies that ∑ j ̸=t nj is even. For the converse, suppose that ∑ j ̸=t nj is even for every t ∈ {1, 2, . . . , k}. Let D = V (G) and let x, y ∈ V (G) with x ̸= y. Suppose first that x, y ∈ Sr for some r ∈ {1, 2, . . . , k}. Since y /∈ NG[x], NG[x] ∩ D = NG[x] ̸= NG[y] = NG[y] ∩ D. Next, suppose that x ∈ Sp and y ∈ Sq for p ̸= q, where p, q ∈ {1, 2, . . . , k}. Since V (Sq) \ {y} ⊆ NG[x] \ NG[y], NG[x] ∩ D = NG[x] ̸= NG[y] = NG[y] ∩ D. Hence, D is a differentiating set. Next, let w ∈ V (G) and let w ∈ St. Then, by assumption, M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1591 |NG[w] ∩D| = |NG[w]| = 1 + ∑ j ̸=t nj is odd. Therefore, D = V (G) is a differentiating odd dominating set in G. Whenever the given property is satisfied, we find that S = V (G) is the only differentiating odd dominating set in G. Thus, γoD(G) = |V (G)| = ∑k j=1 nj . The next result is immediate from Theorem 6. Corollary 6. Let Km,n be a complete bipartite graph such that m ≥ 2 and n ≥ 2. Then Km,n admits a differentiating odd dominating set if and only if m and n are both even. Moreover, γoD(Km,n) = m+ n. Theorem 7. Let G1, G2, · · · , Gk be the components of G. Then G admits a differentiating odd dominating set if and only if Gj admits a differentiating odd dominating set for each j ∈ {1, 2, . . . , k}. In this case, γoD(G) = k∑ j=1 γoD(Gj). Proof. Suppose G admits a differentiating odd dominating set, say S. Let Sj = S ∩ V (Gj) for each j ∈ {1, 2, . . . , k}. Since S is dominating, Sj is dominating in Gj for each j ∈ {1, 2, . . . , k}. Next, let j ∈ {1, 2, . . . , k} and let u, v, w ∈ V (Gj), where u ̸= v. Since S is differentiating odd dominating, NGj [u] ∩ Sj = NG[u] ∩ S ̸= NG[v] ∩ S = NGj [v] ∩ Sj and |NGj [w]∩Sj | is odd. This implies that Sj is a differentiating odd dominating set in Gj . For the converse, suppose that each component Gj admits a differentiating odd dominating set, say Dj . Then, clearly, S ′ = ∪k j=1Dj is a differentiating odd dominating set in G. Now, let S0 be a γoD-set in G. Then S′ j = S0 ∩ V (Gj) is a differentiating odd dominating set in Gj for each j ∈ {1, 2, . . . , k} and S0 = ∪k j=1S ′ j . Hence, γoD(G) = |S0| = k∑ j=1 |S′ j | ≥ k∑ j=1 γoD(Gj). On the other hand, ifD′ j is a γ o D-set inGj for each j ∈ {1, 2, . . . , k}, then S′ 0 = ∪k j=1D ′ j is a differentiating odd dominating set in G. It follows that γoD(G) ≤ |S′ 0| = k∑ j=1 |D′ j | = k∑ j=1 γoD(Gj). This proves the desired equality. Corollary 7. Let G be a graph. Then each of the following holds: M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1592 (i) If G = Kn, then γoD(G) = n. (ii) γoD(G) = 2 if and only if G = K2. (iii) γoD(G) = 3 if and only if G ∈ {K3,K1,3}. The join G+H of two graphs G and H is the graph with V (G+H) = V (G)∪V (H) (disjoint union) and E(G+H) = E(G) ∪ E(H) ∪ {uv : u ∈ V (G) and v ∈ V (H)}. Theorem 8. Let G be a non-trivial point distinguishing graph and let K1 = ⟨v⟩. Then S ⊆ V (K1 +G) is a differentiating odd dominating set in K1 +G if and only if one of the following holds: (i) S = {v} ∪ SG where SG = V (G) ∩ S ̸= ∅ satisfies the following: (a) |SG| is even and SG is a strictly differentiating set in G (b) |NG[u] ∩ SG| is even for all u ∈ V (G) (ii) S ⊆ V (G), |S| is odd, and S is a strictly differentiating odd dominating set in G. Proof. Let S be a differentiating odd dominating set in K1 + G. Let V (K1) = {v} and let SG = V (G) ∩ S. Consider the following cases: Case 1: v ∈ S. Then S = {v} ∪ SG. Since S is differentiating in K1 + G, SG ̸= ∅. Since S is odd dominating in K1+G, |NK1+G[v]∩S| = |{v}|+ |SG| is odd. It follows that |SG| is even. Suppose SG is not differentiating in G. Then there exist x, y ∈ V (G) and x ̸= y such that NG[x] ∩ SG = NG[y] ∩ SG. It follows that NK1+G[x] ∩ S = {v} ∪ (NG[x] ∩ SG) = NK1+G[y] ∩ S, a contradiction to the fact that S is differentiating in K1 + G. Therefore, SG is differentiating in G. Furthermore, suppose SG is not strictly differentiating in G. Then there exists w ∈ V (G) such that NG[w] ∩ SG = SG. It follows that NK1+G[w] ∩ S = {v} ∪ SG = NK1+G[v] ∩ S, a contradiction to the fact that S is differentiating in K1 + G. Thus, SG is strictly differentiating in G. This proves that (a) holds. Now, let u ∈ V (G). Since S is odd dominating, |NK1+G[u]∩S| = |{v}|+ |NG[u]∩SG| is odd. Thus, |NG[u] ∩ SG| is even. This shows that (b) holds. Hence, (i) holds. Case 2: v ̸∈ S. Then S ⊆ V (G). Since S is odd dominating, |NK1+G[v] ∩ S| = |S| is odd. Since S is odd dominating and v /∈ S, it follows that S is odd dominating in G. Moreover, as in Case 1, S is strictly differentiating in G. Therefore, (ii) holds. M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1593 Conversely, suppose (i) holds. Let x, y be distinct vertices in V (K1 + G). If x, y ∈ V (G), then NG[x] ∩ SG ̸= N [y] ∩ SG by (a). It follows that NK1+G[x] ∩ S = {v} ∪ [NG[x] ∩ SG] ̸= {v} ∪ [NG[y] ∩ SG] = NK1+G[y] ∩ S. Suppose x = v. Then NG[y] ∩ SG ̸= SG because SG is strictly differentiating. Since NK1+G[x] ∩ S = SG ∪ {v}, then NK1+G[x] ∩ S ̸= NK1+G[y] ∩ S. Since |SG| is even and (b) holds, it follows that |NK1+G[z] ∩ S| is odd for all z ∈ V (K1 +G). Therefore, S is a differentiating odd dominating set in K1 +G. Next, suppose (ii) holds. Since S is strictly differentiating-dominating set in G, S is differentiating-dominating in K1 + G. Let w ∈ V (K1 + G). If w = v, then |NK1+G[w] ∩ S| = |S| is odd, by assumption. If w ∈ V (G), then |NK1+G[w]∩S| = |NG[w]∩S| is odd because S is odd dominating in G. Therefore, S is a differentiating odd dominating set in K1 +G. The graphs G and K1 + G in Figure 1 illustrate the graphs described in Thereom 8(i). z d h c g x w u b f y a e G : z d h c g x w u b f y a e v K1 +G : Figure 1: Graphs G and K1 +G illustrating Theorem 8 (i) In the next results, we use the following parameters for any graph G′ admitting the given set: γeoD (G′) = min{|S| : |S| is even and S ∈ DOD(G′)} γeoSD(G ′) = min{|S| : |S| is even and S ∈ SDOD(G′)} γooSD(G ′) = min{|S| : |S| is odd and S ∈ SDOD(G′)} γeeSD(G ′) = min{|S| : |S| is even and S ∈ SDED(G′)} Corollary 8. Let G be a point distinguishing graph. M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1594 (i) If G admits a strictly differentiating odd dominating set with odd cardinality, then γoD(K1+G) ≤ γooSD(G) and equality holds if G does not admit a differentiating even dominating set. (ii) If G admits a strictly differentiating even dominating set with even cardinality, then γoD(K1 +G) ≤ γeeSD(G) + 1 and equality holds if G does not admit a differentiating odd dominating set with odd cardinality. Corollary 9. All fans Fn = K1+Pn of order n+1 have no differentiating odd dominating set for all n. Proof. Clearly, F1 and F2 do not admit a differentiating odd dominating set. Suppose now that n ≥ 3. Let K1 = ⟨v⟩ and G = Pn = [v1, v2, . . . , vn]. Suppose Fn has a differentiating odd dominating set, say S. Suppose S = {v} ∪ SG, where SG satisfies (a) and (b) in Theorem 8. If v1 ∈ SG, then v2 ∈ SG by property (b). Hence, by the same property (b), v3 /∈ SG. This implies that NG[v1]∩SG = NG[v2]∩SG = {v1, v2}, contradicting property (a). This forces v1 /∈ SG. By property (b), v2, v3 /∈ SG. Thus, NG[v1] ∩ SG = NG[v2] ∩ SG = ∅, contradicting (a). Therefore, S ⊆ V (Pn) and satisfies (ii) in Theorem 8. If v1 ∈ S, then v2, v3 /∈ S because S is odd dominating in G. This would imply that SG is not differentiating in G, contradicting (ii). Hence, v1 /∈ S. Since S is odd dominating in G, v2 ∈ S and v3 /∈ S. This implies that NG[v1] ∩ S = NG[v2] ∩ S = {v2}, contrary to the assumption that S is differentiating in G. Therefore, such S does not exist, i.e., Fn does not admit a differentiating odd dominating set. Corollary 10. The wheel Wn = K1 +Cn admits a differentiating odd dominating set if and only if n is odd and n ̸= 3. Moreover, if n is odd and n ≥ 5, then γoD(Wn) = n. Proof. Suppose Wn admits a differentiating odd dominating set, say S. Since W3 = K4 is not point distinguishing, n ̸= 3. LetK1 = ⟨v⟩ andG = Cn = [v1, v2, . . . , vn, v1]. Suppose S = {v} ∪ SG, where SG satisfies (a) and (b) in Theorem 8(i). If v1 ∈ SG, then v2 ∈ SG or vn ∈ SG but not both by (b). We may assume that v2 ∈ SG. Then vn /∈ SG. Again, by property (b), v3 /∈ SG. It follows that NG[v1] ∩ SG = NG[v2] ∩ SG = {v1, v2}, contradicting property (a). Consequently, v1 /∈ SG. By property (b), v2, v3, vn ∈ SG and v4 /∈ SG (hence, n ̸= 4). This implies that NG[v2] ∩ SG = NG[v3] ∩ SG = {v2, v3}, contradicting property (a). Therefore, v /∈ S. Thus, S ⊆ V (G) and satisfies (ii) of Theorem 8. Suppose S ̸= V (G). We may assume that v1 ∈ V (G) \ S. Since S is odd dominating, v2 ∈ S or vn ∈ S but not both. Assume that v2 ∈ S. Then vn /∈ S. Again, since S is odd dominating, v3 /∈ S. Therefore, NG[v1]∩S = NG[v2]∩S = {v2}, contrary to the fact that S is differentiating in G. Thus, S = V (G). Since S is odd dominating and v /∈ S, |NWn [v] ∩ S| = |V (Cn)| = n is odd. For the converse suppose that n ≥ 5 and is odd. By Theorem 8, S0 = V (Cn) is a differentiating odd dominating set in Wn. As seen earlier, if n is odd and n ≥ 5, then V (Cn) is the only differentiating odd dominating set in Wn. Accordingly, γ o D(Wn) = n. M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1595 The next result is due to Canoy and Malacas [12]. Theorem 9. [12] Let G and H be non-trivial graphs of orders m ≥ 2 and n ≥ 2, respectively. Then S ⊆ V (G + H) is a differentiating-dominating set in G + H if and only if SG = V (G) ∩ S and SH = V (H) ∩ S are differentiating sets in G and H, respectively, and either SG or SH is strictly differentiating. Theorem 10. Let G and H be non-complete graphs of order m ≥ 4 and n ≥ 4, respectively. Then S ⊆ V (G + H) is a differentiating odd dominating set in G + H if and only if S = SG ∪ SH , where SG and SH are differentiating sets in G and H, respectively, either SG or SH is strictly differentiating, and one of the following statements holds: (i) |SG| and |SH | are even, and SG and SH are both odd dominating sets in G and H, respectively. (ii) |SG| and |SH | are odd, and SG and SH are both even dominating sets in G and H, respectively. (iii) |SG| is odd, |SH | is even, SG is odd dominating in G, SH is even dominating set in H. (iv) |SG| is even, |SH | is odd, SG is even dominating in G, and SH is odd dominating sets in H. Proof. Let S ⊆ V (G + H) be a differentiating odd dominating set in G + H. Let SG = V (G) ∩ S and SH = V (H) ∩ S. By Theorem 9, SG and SH are differentiating- dominating sets in G and H, respectively, and either SG or SH is stricly differentiating. Now, since S is odd dominating in G + H, |NG+H [x] ∩ S| = |NG[x] ∩ SG| + |SH | and |NG+H [y]∩S| = |NH [y]∩SH |+ |SG| are odd for every x ∈ V (G) and for every y ∈ V (H). Hence, if |SG| is even (or |SH | is even), then |NH [y] ∩ SH | is odd (resp. |NG[x] ∩ SG| is odd). This implies that SH is odd dominating (resp. SG is odd dominating). If |SG| is odd (or |SH | is odd), then |NH [y] ∩ SH | is even (resp. |NG[x] ∩ SG| is even). Hence, SH is even dominating (resp. SG is even dominating). Therefore, (i), or (ii), or (iii), or (iv) holds. For the converse, suppose that S = SG ∪ SH , where SG and SH are differentiating- dominating sets in G and H, respectively, and either SG or SH is strictly differentiating. Then S is a differentiating set in G+H. Now let p ∈ V (G+H). Then |NG+H [p]∩S| = |NG[p]∩SG|+ |SH | if p ∈ V (G) and |NG+H [p]∩S| = |NH [p]∩SH |+ |SG| if p ∈ V (H). Hence, if one of (i), (ii), (iii), and (iv) holds, then S is an odd dominating set in G+H. Corollary 11. Let G and H be a non-complete graphs of order m ≥ 4 and n ≥ 4, respectively, such that G +H admits differentiating odd dominating set. If both G and H do not have an even dominating set and both G and H admit a strictly differentiating set then γoD(G+H) = min{γeoD (G) + γeoSD(H), γeoD (H) + γeoSD(G)}, M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1596 where we set γeoSD(G ′) = +∞ whenever SDOD(G′) = ∅, where G′ ∈ {G,H}. Example 1. Let G = Sp,q and H = Sr,t (double stars), where p, q, r, and t are odd numbers greater than 2. By Corollary 4, γoD(G) = γeoD (G) = γeoSD(G) = p + q and γoD(H) = γeoD (H) = γeoSD(H) = r + t. By Corollary 11, γoD(G+H) = p+ q + r + t. The corona G ◦ H of two graphs G and H is the graph obtained by taking one copy of G of order n and n copies of H, and then joining the ith vertex of G to every vertex in the ith copy of H. For every v ∈ V (G), we denote by Hv the copy of H whose vertices are attached one by one to the vertex v. Subsequently, we denote by v+Hv the subgraph of the corona G ◦H corresponding to the join ⟨v⟩+Hv, where v ∈ V (G). Theorem 11. [12] Let G (not necessarily point distinguishing) and let H be non-trivial connected graphs. Then C ⊆ V (G ◦ H) is a differentiating-dominating set in G ◦ H if and only if for every v ∈ V (G), one of the following is true: (i) v ∈ C, NG(v) ∩ C ̸= ∅, and C ∩ V (Hv) is a differentiating set in Hv; (ii) v ∈ C, NG(v) ∩ C = ∅, and C ∩ V (Hv) is a strictly differentiating set in Hv; (iii) v /∈ C, NG(v) ∩ C ̸= ∅, and C1 = V (Hv) ∩ C is a differentiating-dominating set in Hv; or (iv) v /∈ C, NG(v)∩C = ∅, and C1 = V (Hv)∩C is a strictly differentiating-dominating set in Hv. Theorem 12. Let G be a non-trivial connected graph and let H be any non-trivial graph such that G ◦H admits a differentiating odd dominating set. Then S ⊆ V (G ◦H) is a differentiating odd dominating set in G ◦H if and only if S = SG ∪ [∪v∈V (G)Sv], where SG ⊆ V (G) and Sv ⊆ V (Hv) for each v ∈ V (G), and satisfies the following conditions: (i) For each v ∈ SG with |NG(v) ∩ SG| ̸= 0, Sv is differentiating even dominating in Hv, and either (a) |NG(v) ∩ SG| and |Sv| are odd or (b) |NG(v) ∩ SG| and |Sv| are even. (ii) For each w ∈ V (G)\SG with |NG(w)∩SG| ≠ 0, Sw is differentiating odd dominating in Hw, and either (c) |NG(w) ∩ SG| is even and |Sw| is odd or (d) |NG(w) ∩ SG| is odd and |Sw| is even. (iii) For each v ∈ V (G) with |NG(v) ∩ SG| = 0, it holds that (e) Sv is a strictly differentiating even dominating set in Hv and |Sv| is even if v ∈ SG and M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1597 (f) Sv is strictly differentiating odd dominating and |Sv| is odd if v ∈ V (G) \SG. Proof. Suppose S is a differentiating odd dominating set in G ◦ H. Let SG = S ∩V (G) and let Sv = S ∩V (Hv) for each v ∈ V (G). Then S = SG∪ [∪v∈V (G)Sv]. Let v ∈ SG such that |NG(v) ∩ SG| ≠ 0. By Theorem 11, Sv is a differentiating set in Hv. Suppose first that |NG(v) ∩ SG| is odd. Since S is odd dominating in G ◦ H, |NG◦H [p] ∩ S| = |NHv [p] ∩ Sv| + |{v}| is odd for every p ∈ V (Hv). This implies that |NHv [p] ∩ Sv| is even for every p ∈ V (Hv). Thus, Sv is even dominating in Hv. Moreover, because |NG◦H [v]∩S| = |NG(v)∩SG|+ (|{v}|+ |Sv|) is also odd, |Sv| is odd. This shows that (i)(a) holds. Similarly, (i)(b) also holds. Therefore, (i) holds. Next, let w ∈ V (G) \ SG with |NG(w) ∩ SG| ≠ 0. Again, by Theorem 11, Sw is a differentiating set in Hw. Suppose |NG(w) ∩ SG| is even. Since S is odd dominating in G ◦H, |NG◦H [q] ∩ S| = |NHw [q] ∩ Sw| is odd for every q ∈ V (Hw). It follows that Sw is differentiating odd dominating in Hv. Since |NG◦H [w]∩S| = |NG(w)∩SG|+ |Sw| is odd and |NG(w)∩SG| is even, |Sw| is odd. This shows that (ii)(c) holds. Similar arguments will show that (ii)(d) holds. Thus, (ii) holds. Finally, let v ∈ V (G) such that |NG(v) ∩ SG| = 0. Suppose first that v ∈ SG. Then, by Theorem 8(i), |Sv| is even and Sv is a strictly differentiating even dominating set in Hv. If v ∈ V (G) \ SG, then Sv is odd and Sv is strictly differentiating odd dominating in Hv by Theorem 8(ii). Thus, (iii) holds. For the converse, suppose that S satisfies (i), (ii), and (iii). By Theorem 11, S is a differentiating-dominating set in G ◦ H. Let x ∈ V (G ◦ H) \ S and v ∈ V (G) be such that x ∈ V (v +Hv). Suppose |NG(x) ∩ SG| ≠ 0. If v ∈ SG, then |NG◦H [x] ∩ S| = |NG(x) ∩ SG|+ (|Sv|+ 1) if x = v; otherwise, |NG◦H [x] ∩ S| = |NHv [x] ∩ Sv| + 1. By (a) and (b), |NG◦H [x] ∩ S| is odd. Suppose v ∈ V (G) \ SG. Then |NG◦H [x] ∩ S| = |Sv| if x = v; otherwise, |NG◦H [x] ∩ S| = |NHv [x] ∩ Sv|. By (c) and (d), |NG◦H [x] ∩ S| is odd. Lastly, suppose that |NG(x) ∩ SG| = 0. Then parts of (e) and (f) would imply that |NG◦H [x] ∩ S| is odd. Therefore, S is a differentiating odd dominating set in G ◦H. Corollary 12. Let G be a non-trivial connected graph of order n and let H be a graph that admits a strictly differentiating odd dominating set with odd cardinality. Then γoD(G ◦ H) ≤ |V (G)|γooSD(H) and equality holds if H = Kp, where p is odd and at least 3. Proof. Let Sv ⊆ V (Hv) be a strictly differentiating odd dominating set with odd cardinality such that |Sv| = γooSD(H v) for each v ∈ V (G). Set S = ∪v∈V (G)Sv. By Theorem 12, S is a differentiating odd dominating set in G ◦ H. Thus, γoD(G ◦ H) ≤ |S| = nγooSD(H). If H = Kp, then γoSD(H) = p. Desired equality follows now from Theorem 2(ii). Corollary 13. Let G be a non-trivial connected graph of order n and let H be a graph that admits a differentiating even dominating set with even cardinality. If G is r-regular, where r is a positive even integer, then γoD(G ◦H) ≤ n+ nγeeD (H). M. Carbero, G. Malacas, S. Canoy, Jr. / Eur. J. Pure Appl. Math, 17 (3) (2024), 1585-1601 1598 The lexicographic product G[H] also has V (G[H]) = V (G) × V (H) as its vertex set, and u = (u1, u2) is adjacent with v = (v1, v2) whenever u1v1 ∈ E(G) or u1 = v1 and u2v2 ∈ E(H). Observe that any subset C of V (G) × V (H) (infact, any set of ordered-pairs) can be written as C = ∪v∈A({v} × Bv), where S ⊆ V (G) and Bv ⊆ V (H) for each v ∈ S. Henceforth, we shall use this form to denote any subset C of V (G)V (H). Theorem 13. [12] Let G (not necessarily point distinguishing) and H be non-trivial connected graphs. Then C = ∪x∈S({x}×Tx), where S ⊆ V (G) and Tx ⊆ V (H) for each x ∈ S, is a differentiating-dominating set in G[H] if and only if (i) S = V (G); (ii) Tx is a differentiating set in H for every x ∈ V (G); (iii) Tx or Ty is strictly differentiating in H whenever x and y are adjacent vertices of G with NG[x] = NG[y]; and (iv) Tx or Ty is (differentiating) dominating in H whenever x and y are distinct non-adjacent vertices of G with NG(x) = NG(y). Theorem 14. Let G be a non-trivial connected graph and let H be a non-trivial point distinguishing connected graph. Then S = ∪v∈A({v} × Bv), where A ⊆ V (G) and Bv ⊆ V (H) for each v ∈ A, is a differentiating odd dominating set in G[H] if and only if (i) A = V (G); (ii) Bv is a differentiating set in H for every v ∈ V (G); (iii) Bv or Bu is strictly differentiating in H whenever u and v are adjacent vertices of G with NG[u] = NG[v]; (iv) Bv or Bu is differentiating-dominating in H whenever u and v are distinct non-adjacent vertices of G with NG(u) = NG(v); and (v) For every v ∈ V (G) and for every p ∈ V (H), |NH [p]∩Bv|+ ∑ w∈NG(u) |Bw| is odd. Proof. Suppose S is a differentiating odd dominating set in G[H]. Then (i), (ii), (iii) and (iv) hold by Theorem 13. Let v ∈ V (G) and p ∈ V (H). Then NG[H][(v, p)] = [{v} × (NH [p] ∩Bv)] ∪ [∪w∈NG(v)({w} ×Bw)]. Since S is odd dominating in G[H], |NG[H][(v, p)] ∩ S| = |NH [p] ∩Bv|+ ∑ w∈NG(v) |Bw| is odd, showing that (v) holds. For the converse, suppose that S satifies the five conditions. Since (i), (ii), (iii), and (iv) hold, S is a differentiating-dominating set in G[H] by Theorem 13. By (v), it follows that S is odd dominating. REFERENCES 1599 Corollary 14. Let G be non-trivial totally point determining graph and let H be a point distinguishing connected graph. If H admits a differentiating odd dominating set with even cardinality, then γoD(G[H]) ≤ |V (G)|γeoD (H). Proof. For each v ∈ V (G), let Bv be a differentiating odd dominating set with even cardinality such that |Bv| = γeoD (H). Then S = ∪v∈V (G)({v} × Bv) satisfies the first four properties in Theorem 14. Now let v ∈ V (G) and p ∈ V (H). Since Bv is differentiating odd dominating in H, |NH [p] ∩ Bv| is odd. Moreover, since |Bw| is even for every w ∈ V (G), it follows that ∑ w∈NG(v) |Bw| is even. Therefore, |NH [p] ∩ Bv| + ∑ w∈NG(v) |Bw| is odd, showing that property (v) in Theorem 14 is also satisfied. Accordingly, S is a differentiating odd dominating set in G[H] and γoD(G[H]) ≤ |S| = |V (G)|γeoD (H). Conclusion The concept of differentiating odd dominating set has been introduced and initially investigated in this study. The differentiating odd domination number of a graph on at least four vertices is at least equal to the maximum of the odd domination number, the differentiating-domination number of the graph and, 3, and at most equal to the difference of the order of the graph and the number of its support vertices. As shown in this study, some graphs do not admit this kind of dominating set. The newly defined concept and parameter have been investigated for the join, corona, and lexicographic products of some classes of graphs. It may be interesting and worthwhile to find necessary and sufficient conditions for a graph to admit a differentiating odd dominating set, study the complexity of the decision problem involving the parameter, and investigate the parameter for some other families of graphs. 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