A set Formula: see text of vertices is a total dominating set of a graph Formula: see text if every vertex of Formula: see text has at least one neighbor in Formula: see text. The total domination number Formula: see text is the minimum cardinality among all total dominating sets of Formula: see text The graph Formula: see text is obtained by adding a new vertex Formula: see text for each edge Formula: see text and joining Formula: see text to the end vertices Formula: see text and Formula: see text of Formula: see text. S. Bermudo, Total domination on tree operators, Mediterr. J. Math. 20 (2023) 42. proved that for every tree Formula: see text of order Formula: see text with Formula: see text leaves, Formula: see text. In this note, we provide a constructive characterization of extremal trees attaining these two bounds.
Vadivel et al. (Fri,) studied this question.