OFFSET
0,1
COMMENTS
Denoting by P[n] the path on n vertices, a(n) is the number of vertices of the tree obtained by identifying the roots of 3 identical rooted trees g[n], where g[n] is obtained recursively in the following manner: g[0]=P[2] and g[n] (n>=1) is obtained by identifying the roots of 2 copies of g[n-1] and one of the extremities of P[n+1]; the root of g[n] is defined to be the other extremity of P[n+1]. Most references contain pictures of these trees; however, the small circles have to be viewed as vertices rather than hexagons.
LINKS
R. Kopelman, M. Shortreed, Z. Y. Shi, W. Tan, Z. F. Xu, J. S. Moore, A. Bar-Haim, J. Klafter, Spectroscopic evidence for excitonic localization in fractal antenna supermolecules, Phys. Rev. Letters, 78, 1997, 1239-1242.
M. A. MartÃn-Delgado, J. Rodriguez-Laguna, G. Sierra, A density matrix renormalization group study of excitons in dendrimers, Phys. Rev. B 65, 2002, 155116(1-11).
S. Raychaudhuri, Y. Shapir, S. Mukamel, Disorder and funneling effect on exciton migration in treelike dendrimers, Phys. Rev. E, 65, 2002, 021803(1-12).
S. Tretiak, V. Chernyak, S. Mukamel, Localized electronic excitations in phenylacetylene dendrimers, J. Phys. Chem. B, 102, 1998, 3310-3315.
Index entries for linear recurrences with constant coefficients, signature (4,-5,2).
FORMULA
G.f.: (4-6*x+5*x^2)/((1-2*x)*(1-x)^2).
a(0)=4, a(1)=10, a(2)=25, a(n) = 4*a(n-1)-5*a(n-2)+2*a(n-3). - Harvey P. Dale, Apr 15 2015
a(n)= 3*A079583(n) + 1. - Emeric Deutsch, Feb 18 2016
EXAMPLE
a(1) = 10 because g[1] is the rooted tree in the shape of Y (4 vertices) and a "bouquet" of three Y's has 3*4 - 2 = 10 vertices.
MAPLE
a := proc (n) options operator, arrow: 9*2^n-3*n-5 end proc: seq(a(n), n = 0 .. 35);
MATHEMATICA
Table[9*2^n-3n-5, {n, 0, 40}] (* or *) LinearRecurrence[{4, -5, 2}, {4, 10, 25}, 40] (* Harvey P. Dale, Apr 15 2015 *)
PROG
(PARI) Vec((4-6*x+5*x^2)/((1-2*x)*(1-x)^2) + O(x^100)) \\ Altug Alkan, Oct 17 2015
(Magma) [9*2^n-3*n-5: n in [0..40]]; // Vincenzo Librandi, Feb 19 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Emeric Deutsch, Aug 06 2013
STATUS
approved