Consider a simple connected graph G=(V,E)\documentclass[12pt]{minimal}
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\begin{document}$$G = (V,E)$$\end{document} of order p and size q. For a bijection f:E→{1,2,…,q}\documentclass[12pt]{minimal}
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\begin{document}$$f : E \to \{1,2,\ldots,q\}$$\end{document}, let f+(u)=∑e∈E(u)f(e)\documentclass[12pt]{minimal}
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\begin{document}$$f^+(u) = \sum_{e\in E(u)} f(e)$$\end{document} where E(u)\documentclass[12pt]{minimal}
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\begin{document}$$E(u)$$\end{document} is the set of edges incident to u. We say f is a local antimagic labeling of G if for any two adjacent vertices u and v, we have f+(u)≠f+(v)\documentclass[12pt]{minimal}
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\begin{document}$$f^+(u) \ne f^+(v)$$\end{document}. The minimum number of distinct values of f+\documentclass[12pt]{minimal}
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\begin{document}$$f^+$$\end{document} taken over all local antimagic labeling of G is denoted by χla(G)\documentclass[12pt]{minimal}
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\begin{document}$$\chi_{la}(G)$$\end{document}. Let G[H]\documentclass[12pt]{minimal}
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\begin{document}$$G[H]$$\end{document} be the lexicographic product of graphs G and H. In this paper, we obtain sharp upper bound for χla(G[On])\documentclass[12pt]{minimal}
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\begin{document}$$\chi_{la}(G[O_n])$$\end{document} where On\documentclass[12pt]{minimal}
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\begin{document}$$O_n$$\end{document} is a null graph of order n≥3\documentclass[12pt]{minimal}
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\begin{document}$$n\ge 3$$\end{document}. Sufficient conditions for even regular bipartite and tripartite graphs G to have χla(G)=3\documentclass[12pt]{minimal}
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\begin{document}$$\chi_{la}(G)=3$$\end{document} are also obtained. Consequently, we successfully determined the local antimagic chromatic number of infinitely many (connected and disconnected) regular graphs that partially support the existence of an r-regular graph G of order p such that (i) χla(G)=χ(G)=k\documentclass[12pt]{minimal}
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\begin{document}$$\chi_{la}(G)=\chi(G)=k$$\end{document}, and (ii) χla(G)=χ(G)+1=k\documentclass[12pt]{minimal}
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\begin{document}$$\chi_{la}(G)=\chi(G)+1=k$$\end{document} for each possible r,p,k\documentclass[12pt]{minimal}
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\begin{document}$$r,p,k$$\end{document}.
