An upper bound for the semistrong chromatic index of Halin graphs

For a graph $ G $, a semistrong matching is a matching $ M $ such that every edge in $ M $ contains at least one endpoint of degree one in the induced subgraph $ G[V(M)] $. The semistrong chromatic index $ \chi_{s s}^{\prime}(G) $ denotes the minimum number of colors required for a proper edge-coloring where each color class induces a semistrong matching. We study this parameter for Halin graphs, which are planar graphs formed by connecting all leaves of a tree $ T $ (with no degree-two vertices) via an outer cycle $ C $. Our main result establishes that for any Halin graph $ G = T\cup C $ with maximum degree $ \Delta(G) $, the semistrong chromatic index satisfies $ \chi_{s s}^{\prime}(G) \leq \Delta(G)+4 $, with equality attained by the wheel graphs $ W_4 $ and $ W_7 $.