Drcumextremal benzenoid

Definition 5.7 A single coronoid (P) is circumextremal if its associated benzenoid, P, is a drcumextremal benzenoid, and exds-P is the assodated benzenoid to a (smaller) extremal single coronoid. 

A higher member (of benzenoids) is a drcumextremal benzenoid (and itself an extremal benzenoid). A ground form is an extremal benzenoid which is not drcumextremal. Assume that (Nq 5q) is a formula for a ground form benzenoid G. Then the formulas Nq 5q), (N 5 ),., N 5, . are said to form a constant-isomer series when (N 5 pertains to k-iold drcumscribed G. All these formulas are known to have the same number of benzenoid isomers. 

The value of x gives the following information about B. (i) x > 0 B is a nonextremal benzenoid. If it can be circumscribed x times, then x-circum—B is a ground form benzenoid (which is extremal), and there always exists (at least) one isomer Be N 5 which can be circumscribed arbitrarily many times, (ii) x = 0 B is a ground form benzenoid. (iii) x < 0 B is a higher member (drcumextremal benzenoid), and specifically an x —fold circumscribed ground form. 

---