Counting the minimum number of arcs in an oriented graph having weak diameter 2

An oriented graph has weak diameter at most $d$ if every non-adjacent pair of vertices are connected by a directed $d$-path. The function $f_d(n)$ denotes the minimum number of arcs in an oriented graph on $n$ vertices having weak diameter $d$. It turns out that finding the exact value of $f_d(n)$ is a challenging problem even for $d = 2$. This function was introduced by Katona and Szeme\'redi [Studia Scientiarum Mathematicarum Hungarica, 1967], and after that several attempts were made to find its exact value by Znam [Acta Fac. Rerum Natur. Univ. Comenian. Math. Publ, 1970], Dawes and Meijer [J. Combin. Math. and Combin. Comput, 1987], F\"uredi, Horak, Pareek and Zhu [Graphs and Combinatorics, 1998], and Kostochka, Luczak, Simonyi and Sopena [Discrete Mathematics and Theoretical Computer Science, 1999] through improving its best known upper bounds. In that process, they also proved that this function is asymptotically equal to $n\log_2 n$ and hence, is an asymptotically increasing function. In this article, we prove that $f(n)$ is a strictly increasing function. Furthermore, we improve the best known upper bound of $f(n)$ and conjecture that it is tight.