Latent row of hexagons

John (1990) has supplied the necessary and sufficient condition for the possibility to circumscribe a benzenoid B an unlimited number of times. When this is possible, B is called a reproducible benzenoid. The mentioned condition was re-formulated in terms of the latent row of hexagons, a possible formation on the perimeter of a benzenoid (Cyvin SJ, Cyvin, Brunvoll, Gutman and John 1993). Below we give the adaptations of this approach to coronoids. 

Definition 5.4 A latent row (of hexagons) in a coronoid C, where all the hexagons are in contact with, the outer perimeter of C, is called a proper latent row. Otherwise the latent row is an improper latent row (of hexagons). 

Observation 5.2 A coronoid C is reproducible if and only if C does not possess any latent row of hexagons. 

If a coronoid (or benzenoid) has only one latent row of hexagons it must obviously be a proper latent row, as is the case in Fig. 4. If there are exactly two latent rows of hexagons. 

It is clear that an improper latent row of hexagons in a coronoid (or a benzenoid) cannot occur without the presence of proper latent rows in the same system. Hence the above observation can be given a slightly sharper formulation as follows. 

It is understood that the hexagons of the latent row are not occupied by hexagons of C. Hence the latent row is a "gap on the outer perimeter. 

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