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S. J. Cyvin, Zhang Fuji, B. N. Cyvin, Guo Xiaofeng, and J. Brunvoll, <a href="https://pubs.acs.org/doi/pdfplus/10.1021/ci00009a021">Enumeration and classification of benzenoid systems. 32. Normal perifusenes with two internal vertices, </a>, J. Chem. Inform. Comput. Sci., 32 (1992), 532-540.
Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Polyhex.html">Polyhex</a>.
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Vincenzo Librandi, <a href="/A039658/b039658.txt">Table of n, a(n) for n = 1..1000</a>
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In Cyvin et al. (1992), it is defined through eq. (22), p. 535. We have a(n) = Sum_{1 <= i <= 1..n-1} M(i)*M(n-i), where M(2*n) = M(2*n-1) = A007317(n) for n >= 1.
G.f.: (1+x)[*(1 -3x 3*x^2 - sqrt(1 -6x 6*x^2 +5x 5*x^4)])/[2x(2*x^2*(1-x)] ) (eq. (9), p. 1175, in Cyvin et al. (1994)).
For n >= 1, a(n) = Sum_{1 <= i <= 1..n-1} A007317(floor((i+1)/2)) * A007317(floor((n-i+1)/2)). - Petros Hadjicostas, Jan 13 2019
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S. J. Cyvin et al., Enumeration and classification of benzenoid systems. 32. Normal perifusenes with two internal vertices, J. Chem. Inform. Comput. Sci., 32 (1992), 532-540.
S. J. Cyvin et al., Enumeration and classification of certain polygonal systems...: annelated catafusenes, J. Chem. Inform. Comput. Sci., 34 (1994), 1174-1180.
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