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 A121113 Edge-rooted tree-like octagonal systems (see the Cyvin et al. reference for precise definition). 5
 0, 0, 6, 60, 522, 4452, 38130, 329832, 2884056, 25476936, 227145654, 2041930920, 18490834362, 168537705300, 1545096559812, 14238592913328, 131826509242650, 1225645805016864, 11438847800351118, 107128560124123524, 1006475582377759578, 9483340466106708180, 89593844489912910294 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS From Petros Hadjicostas, Jul 30 2019: (Start) Quoting from p. 59 in Cyvin et al. (1997): "When an octagon is rooted at an edge ... then either (a) one branch can be attached in five directions at a time, (b) two branches can be attached in six ways, or (c) three branches in one way. Let the numbers of these kinds of systems be denoted by (a) U_r^*, (b) U_r^{**}, and (c) U_r^{***}, respectively." Here r is "the number of octagons or eight-membered rings" in an edge-rooted catapolygon (here, catapolyoctagon). A catapolyoctagon is a "catacondensed polygonal system consisting of octagons" (where "catacondensed" means it has no internal vertices). On p. 59 in Cyvin et al. (1997), the total number of edge-rooted catapolyoctagons (each with r octagons) is denoted by U_r, and we have U_r = U_r^* + U_r^{**} + U_r^{***} for r >= 2. We have U_r = A036758(r), U_r^* = A121112(r), U_r^{**} = a(r) (current sequence), and U_r^{***} = A121114(r) for r >= 1. For the current sequence, we have a(r) = U_r^{**} = 6*Sum_{i = 1.. r-2} U(i) * U(r-1-i) for r >= 3, where U(r) = A036758(r), with a(1) = a(2) = 0. See Eq. (12) in Cyvin et al. (1997). The ultimate purpose of these calculations (in the paper by Cyvin et al. (1997)) is the calculation of I_r = A036760(r), which is the "number of nonisomorphic free (unrooted) catapolyoctagons when r is given." These catapolyoctagons "represent a class of polycyclic conjugated hydrocarbons, C_{6r+2} H_{4r+4}" (see p. 57 in Cyvin et al. (1997)). The g.f.'s of the sequences U, U^*, U^{**}, and U^{***} appear also in Eqs. (2) and (3) on p. 194 in Brunvoll et al. (1997). (End) REFERENCES S. J. Cyvin, B. N. Cyvin, and J. Brunvoll. Enumeration of tree-like octagonal systems: catapolyoctagons, ACH Models in Chem. 134 (1997), 55-70; see pp. 59-61. LINKS J. Brunvoll, S. J. Cyvin, and B. N. Cyvin, Enumeration of tree-like octagonal systems, J. Math. Chem., 21 (1997), 193-196; see Eqs. (2) and (3) on p. 194. FORMULA a(r) = 6*Sum_{i = 1.. r-2} U(i) * U(r-1-i) for r >= 3, where U(r) = A036758(r), with a(1) = a(2) = 0. - Petros Hadjicostas, Jul 30 2019 MAPLE # Modification of N. J. A. Sloane's Maple program from A036758: Order := 30; S := solve(series(G/(G^3 + 6*G^2 + 5*G + 1), G) = x, G); series(6*S^2*x, x = 0, 30); # Petros Hadjicostas, Jul 30 2019 CROSSREFS Cf. A036758, A036759, A036760, A121112, A121114. Sequence in context: A220411 A248217 A102232 * A213269 A091710 A054880 Adjacent sequences:  A121110 A121111 A121112 * A121114 A121115 A121116 KEYWORD nonn AUTHOR N. J. A. Sloane, Aug 13 2006 EXTENSIONS More terms from Petros Hadjicostas, Jul 30 2019 STATUS approved

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Last modified January 22 22:16 EST 2020. Contains 331166 sequences. (Running on oeis4.)