Double Total Domination in Harary Graphs
  Let $G$ be a graph with minimum degree at least 2. A set $D\subseteq V$ is a double total dominating set of $G$ if each vertex is adjacent to at least two vertices in $D$. The double total domination number $\gamma _{\times 2,t}(G)$ of $G$ is the minimum cardinality of a double total dominating set of $G$. In this paper, we will find double total domination number of Harary graphs. 
Adel P. Kazemi, Behnaz Pahlavsay
math.CO