See Table 4 column 6 of Brinkmann et al. paper.
G. Brinkmann, G. Caporossi and P. Hansen, "A survey and new results on computer enumeration of polyhex and fusene hydrocarbons", J. Chem. Inf. Comput. Sci., vol. 43 (2003), pp. 842-851.
Table of n, a(n) for n=30..42.
If n=30 then the number of benzenoids with 21 hexagons with C_(2h) symmetry is 1 which is the first term in the sequence.
Sequence in context: A048364 A254675 A141549 * A147208 A281315 A010003
Adjacent sequences: A121961 A121962 A121963 * A121965 A121966 A121967
Parthasarathy Nambi, Sep 10 2006