Twin signed domination numbers in directed graphs

Let $D=(V,A)$ be a finite simple directed graph (shortly digraph). A function $f\colon V\to \{-1,1\}$ is called a twin signed dominating function (TSDF) if $f(N^-[v])\ge 1$ and $f(N^+[v])\ge 1$ for each vertex $v\in V$. The twin signed domination number of $D$ is $\gamma_{s}^*(D)=\min\{\omega(f)\mid f \text{ is a TSDF of } D\}$. In this paper, we initiate the study of twin signed domination in digraphs and we present sharp lower bounds for $\gamma_{s}^*(D)$ in terms of the order, size and maximum and minimum indegrees and outdegrees. Some of our results are extensions of well-known lower bounds of the classical signed domination numbers of graphs.