Vertex-degree-based topological indices of catacondensed hexagonal systems

A number of vertex-degree-based topological indices (TIs) can be expressed as a linear combination of the parameters m ij , the number of edges with end vertices of degree i and j. We study the TIs of catacondensed hexagonal systems. Specifically, we introduce two operations, linearizing and unbranching, and show that TI is monotone with respect to these. As a consequence, we express TI in terms of the number of angular and branched hexagons. The catacondensed hexagonal systems for which TI assumes extremal values are characterized, as well as their TI-values derived.