On the Quasi-Ordering of Catacondensed Hexagonal Systems with Respective to their Clar Covering Polynomials

In this paper, we discuss the quasi-ordering of hexagonal systems with respective to the coefficients of their Clar covering polynomials (also known as Zhang-Zhang polynomials). The last six minimal catacondensed hexagonal systems and the hexagonal chains with the maximum Clar covering polynomial are determined. Furthermore, the smallest pair of incomparable catacondensed hexagonal systems is given.