%I A054886
%S A054886 1,3,6,10,16,26,42,68,110,178,288,466,754,1220,1974,3194,5168,8362,
%T A054886 13530,21892,35422,57314,92736,150050,242786,392836,635622,1028458,
%U A054886 1664080,2692538,4356618,7049156,11405774,18454930,29860704,48315634
%N A054886 Layer counting sequence for hyperbolic tessellation by cuspidal triangles 
               of angles (Pi/3,Pi/3,0) (this is the classical modular tessellation).
%C A054886 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.
%C A054886 Also spherical growth series for modular group.
%D A054886 P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 
               2000, p. 156.
%H A054886 <a href="Sindx_Gre.html#groups_modular">Index entries for sequences related 
               to modular groups</a>
%F A054886 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) 
               for n>2, with F(n) the n-th Fibonacci number (cf. A000045 )
%Y A054886 Essentially the same as A006355.
%Y A054886 Sequence in context: A145131 A152009 A114324 this_sequence A130578 A107068 
               A033541
%Y A054886 Adjacent sequences: A054883 A054884 A054885 this_sequence A054887 A054888 
               A054889
%K A054886 nonn,easy,nice
%O A054886 1,2
%A A054886 Paolo Dominici (pl.dm(AT)libero.it), May 23 2000