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 A054886 Layer counting sequence for hyperbolic tessellation by cuspidal triangles of angles (Pi/3,Pi/3,0) (this is the classical modular tessellation). 34
 1, 3, 6, 10, 16, 26, 42, 68, 110, 178, 288, 466, 754, 1220, 1974, 3194, 5168, 8362, 13530, 21892, 35422, 57314, 92736, 150050, 242786, 392836, 635622, 1028458, 1664080, 2692538, 4356618, 7049156, 11405774, 18454930, 29860704, 48315634, 78176338, 126491972 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The layer sequence is the sequence of the cardinalities of the layers accumulating around a ( finite-sided ) polygon of the tessellation under successive side-reflections; see the illustration accompanying A054888. Equivalently, coordination sequence for (3,3,infinity) tiling of hyperbolic plane. - N. J. A. Sloane, Dec 29 2015 Equivalently, spherical growth series for modular group. Also, number of sequences of length n with terms 1, 2, and 3, with no adjacent terms equal, and no three consecutive terms (1, 2, 3) or (3, 2, 1). - Pontus von Brömssen, Jan 03 2022 REFERENCES P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 156. LINKS G. C. Greubel, Table of n, a(n) for n = 0..999 [Offset changed to 0 by Georg Fischer, Mar 01 2022] J. W. Cannon and P. Wagreich, Growth functions of surface groups, Mathematische Annalen, 1992, Volume 293, pp. 239-257. See Prop. 3.1. Index entries for Coordination Sequences [A layer sequence is a kind of coordination sequence. - N. J. A. Sloane, Nov 20 2022] Index entries for linear recurrences with constant coefficients, signature (1,1). FORMULA G.f.: (1+2*x+2*x^2+x^3)/(1-x-x^2) = (x^2+x+1)*(1+x)/(1-x-x^2). a(n) = 2*F(n+2) for n >= 2, with F(n) the n-th Fibonacci number (cf. A000045). E.g.f.: 2*exp(x/2)*(5*cosh(sqrt(5)*x/2) + 3*sqrt(5)*sinh(sqrt(5)*x/2))/5 - 1 - x. - Stefano Spezia, Apr 18 2022 MATHEMATICA Join[{1, 3}, 2Fibonacci[Range[4, 40]]] (* Harvey P. Dale, Jan 06 2012 *) PROG (PARI) my(x='x+O('x^50)); Vec((1+2*x+2*x^2+x^3)/(1-x-x^2)) \\ G. C. Greubel, Aug 06 2017 CROSSREFS Coordination sequences for triangular tilings of hyperbolic space: A001630, A007283, A054886, A078042, A096231, A163876, A179070, A265057, A265058, A265059, A265060, A265061, A265062, A265063, A265064, A265065, A265066, A265067, A265068, A265069, A265070, A265071, A265072, A265073, A265074, A265075, A265076, A265077. Essentially the same as A006355. Cf. A000045, A054888. Sequence in context: A114324 A265073 A265074 * A130578 A107068 A033541 Adjacent sequences: A054883 A054884 A054885 * A054887 A054888 A054889 KEYWORD nonn,easy,nice AUTHOR Paolo Dominici (pl.dm(AT)libero.it), May 23 2000 EXTENSIONS Offset changed to 0 by N. J. A. Sloane, Jan 03 2022 at the suggestion of Pontus von Brömssen STATUS approved

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Last modified December 5 20:36 EST 2022. Contains 358589 sequences. (Running on oeis4.)