Improved Lower Bounds on the General Reduced Second Zagreb Index of Trees and Unicyclic Graphs

Abstract

For a simple graph $Γ$ and a real number $λ$, the general reduced second Zagreb index is defined by the formula $$GRM_λ(Γ)=\sum_{ab\in E(Γ)}[(°_Γ(a)+λ)(°_Γ(b)+λ)]\,.$$ A sharp lower bound for $GRM_λ$ over all trees of given order and maximum degree under the condition that $λ\ge -\frac{1}{2}$ is established. A parallel result is proved for unicyclic graphs under the condition $λ\ge -\frac{1}{2}$. The corresponding minimal trees and unicyclic graphs are identified. These findings improve upon the lower bounds previously established by Buyantogtokh, Horoldagva, and Das concerning $GRM_λ$ of trees and unicyclic graphs of given order.