Sharp Upper Bounds on the Signless Laplacian Spectral Radius of Strongly Connected Digraphs

Let \( G = (V (G),E(G)) \) be a simple strongly connected digraph and \( q(G) \) be the signless Laplacian spectral radius of \( G \). For any vertex \( v_i \in V (G) \), let \( d+i \) denote the outdegree of \( v_i \), \( m_i^+ \) denote the average 2-outdegree of \( v_i \), and \( N_i^+ \) denote the set of out-neighbors of \( v_i \). In this paper, we prove that: (1) \( q(G) = d_1^+ + d_2^+, (d_1^+ \ne d_2^+ ) \) if and only if \( G \) is a star digraph \( \overleftrightarrow{K}_{1,n-1} \), where \( d_1^+ \), \( d_2^+ \) are the maximum and the second maximum outdegree, respectively (\( \overleftrightarrow{K}_{1,n-1} \) is the digraph on \( n \) vertices obtained from a star graph \( K_{1,n−1} \) by replacing each edge with a pair of oppositely directed arcs). (2) \( q(G) \le \text{max} \bigg\{ \frac{1}{2} \left( d_i^+ + \sqrt{ { d_i^+ }^2 + 8d_i^+ m_i^+ } \right) : v_i \in V(G) \bigg\} \) with equality if and only if \( G \) is a regular digraph. (3) \( q(G) \le \text{max} \bigg\{ \frac{1}{2} \left( d_i^+ + \sqrt{ {d_i^+}^2 + \frac{4}{d_i^+} \sum_{v_j \in N_i^+ } d_j^+ ( d_j^+ + m_j^+ ) } \right) : v_i \in V(G) \bigg\} \). Moreover, the equality holds if and only if \( G \) is a regular digraph or a bipartite semiregular digraph. (4) \( q(G) \le \text{max} \big\{ \frac{1}{2} \left( d_i^+ + 2d_j^+ - 1 + \sqrt{ ( d_i^+ - 2d_j^+ + 1 )^2 + 4d_i^+ } \right) : ( v_j, v_i ) \in E(G) \big\} \). If the equality holds, then \( G \) is a regular digraph or \( G \in \Omega \), where \( \Omega \) is a class of digraphs defined in this paper.