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A026118 Number of polyhexes of class PF2 (with two catafusenes annealated to pyrene). 10
5, 20, 100, 431, 1937, 8548, 38199, 171001, 770934, 3492251, 15905897, 72785480, 334571647, 1544203452, 7154247842, 33260560977, 155126129968, 725639264293, 3403612632885, 16004969728270, 75437244856898, 356337397010035, 1686618801843050 (list; graph; refs; listen; history; text; internal format)
OFFSET

6,1

COMMENTS

See reference for precise definition.

From Petros Hadjicostas, Jan 13 2019: (Start)

This sequence is defined by eq. (34), p. 536, in Cyvin et al. (1992). It is denoted by 2^Q_{4+n} (for n >= 2). Thus, a(n+4) = 2^Q_{4+n} for n >= 2 (and that is why the offset here is 6).

For n >= 2, we have a(n+4) = (3/4)*(1 + (-1)^n)*N(floor(n/2)) + (1/4)*(L(n) + 13*Sum_{1 <= i <= n-1} N(i)*N(n-i)), where N(n) = A002212(n) and L(n) = A039658(n).

The sequence (N(n): n >= 1) = (A002212(n): n >= 1) is given by eq. (1), p. 533, in Cyvin et al. (1992), while its g.f. is given by eqs. (2)-(4), p. 1174, in Cyvin et al. (1994). (The g.f. of N(n) = A002212(n) appears also in Harary and Read (1970) as eq. (9) on p. 4.)

The sequence (L(n): n >= 1) = (A039658(n): n >= 1) is given by eq. (22), p. 535, in Cyvin et al (1992), while its g.f. is given by eq. (9), p. 1175, in Cyvin et al. (1994).

The g.f. of the current sequence (a(m): m >= 6) (see below) is given in eq. (A2), p. 1180, in Cyvin et al. (1994), but it can be derived by the above formulae using standard techniques for the calculation of g.f.'s.

For the number of polyhexes of class PF2, we have 1^Q_h = A026106(h) (h >= 5, one catafusene annealated to pyrene), 3^Q_h = A026298(h) (h >= 7, three catafusenes annealated to pyrene), and 4^Q_h = A030519(h) (h >= 8, four catafusenes annealated to pyrene).

(Apparently, the word "annealated" in Cyvin et al. (1992) is spelled "annelated" in Cyvin et al. (1994).)

(End)

LINKS

Table of n, a(n) for n=6..28.

S. J. Cyvin, Zhang Fuji, B. N. Cyvin, Guo Xiaofeng, and J. Brunvoll, 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, B. N. Cyvin, J. Brunvoll, and E. Brendsdal, Enumeration and classification of certain polygonal systems representing polycyclic conjugated hydrocarbons: annelated catafusenes, J. Chem. Inform. Comput. Sci., 34 (1994), 1174-1180.

F. Harary and R. C. Read, The enumeration of tree-like polyhexes, Proc. Edinburgh Math. Soc. (2) 17 (1970), 1-13.

Eric Weisstein's World of Mathematics, Fusenes.

Eric Weisstein's World of Mathematics, Polyhex.

FORMULA

From Petros Hadjicostas, Jan 14 2019: (Start)

a(n+4) = (3/4)*(1 + (-1)^n)*N(floor(n/2)) + (1/4)*(L(n) + 13*Sum_{1 <= i <= n-1} N(i)*N(n-i)) for n >= 2, where N(n) = A002212(n) and L(n) = A039658(n).

G.f.: (x^2/4)*(1-x)^(-1)*(10 - 48*x + 74*x^2 - 38*x^3) - (x^2/8)*[13*(1 - 3*x)*(1 - x)^(1/2)*(1 - 5*x)^(1/2) + (1 - x)^(-1)*(7 - 5*x)*(1 - x^2)^(1/2)*(1 - 5*x^2)^(1/2)] (see eq. (A2), p. 1180, in Cyvin et al. (1994)).

(End)

CROSSREFS

Cf. A002212, A026106, A026118, A026298, A030519, A030520, A030525, A030529, A030532, A030534, A039658.

Sequence in context: A020046 A319731 A272218 * A108509 A110595 A092640

Adjacent sequences:  A026115 A026116 A026117 * A026119 A026120 A026121

KEYWORD

nonn

AUTHOR

N. J. A. Sloane

EXTENSIONS

Name edited by Petros Hadjicostas, Jan 13 2019

Terms a(17)-a(28) computed by Petros Hadjicostas, Jan 13 2019 using a g.f. in Cyvin et al. (1994)

STATUS

approved

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Last modified October 16 13:51 EDT 2019. Contains 328093 sequences. (Running on oeis4.)