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Number of isomeric aza-benzenoids with two nitrogen atoms and n hexagons.
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%I #12 Nov 20 2023 16:12:20

%S 3,10,48,243,1326,7349,41030,228694,1270663,7031737

%N Number of isomeric aza-benzenoids with two nitrogen atoms and n hexagons.

%H N. Trinajstić, Ž. Jeričević, J. V. Knop, W. R. Müller and K. Szymanski, <a href="https://doi.org/10.1351/pac198855020379">Computer generation of isomeric structures</a>, Pure and Appl. Chem., vol. 55 (1983) pp. 379-390. See Table 6 on p. 387.

%e In a single hexagon (benzene), any 2 of the 6 carbon atoms can be chosen to be replaced by nitrogen, and it can be done in 3 ways, obtaining pyridazine, pyrimidine, or pyrazine, so a(1) = 3. For a(2) = 10, see the corresponding column of Table 7 from Trinajstić et al.

%Y Cf. A018190, A121949, A121951, A121952, A121972, A121973, A121975, A121974.

%K nonn,more

%O 1,1

%A _Parthasarathy Nambi_, Sep 04 2006

%E a(1) corrected by _Andrey Zabolotskiy_, Nov 19 2023