%I A080254
%S A080254 1,1,9,75,865,12483,216113,4364979,100757313,2616517443,75496735057,
%T A080254 2396212835283,82968104980961,3112139513814243,125716310807844081,
%U A080254 5441108944839913587,251195548533025953409,12321551453507301079683
%N A080254 For n>3, a(n) is the number of elements in the Coxeter complex of type 
               D_n (although the sequence starts at n=0. See comments below for 
               precise explanation).
%C A080254 The sequence makes most sense when n>3. The values for a(2) and a(3) 
               make sense if we regard D_2=A_1 x A_1 and D_3=A_3. The values for 
               a(0) and a(1) have to be regarded as conventions and were included 
               to give a nice recursive description. The corresponding sequence 
               for type B is A080253. There one can find a worked example as well 
               as a geometric interpretation.
%C A080254 Also, Eulerian D-polynomials (A066094) evaluated at 2. - Ralf Stephan, 
               Apr 23 2004
%D A080254 Kenneth S. Brown, Buildings, Springer-Verlag, 1988
%F A080254 a(0)=a(1)=1. For n>1, a(n)=1 + sum('2^r*binomial(n, r)*a(n-r)', 'r'=1..n)
%F A080254 E.g.f: (2*x-exp(x))/(exp(2*x)-2) - Antonio G. Astudillo (afg_astudillo(AT)hotmail.com), 
               Feb 14 2003
%Y A080254 Cf. A000670, A080253.
%Y A080254 Sequence in context: A136659 A126965 A066222 this_sequence A161736 A056339 
               A056329
%Y A080254 Adjacent sequences: A080251 A080252 A080253 this_sequence A080255 A080256 
               A080257
%K A080254 easy,nonn
%O A080254 0,3
%A A080254 Paul Boddington & Tim Honeywill (psb(AT)maths.warwick.ac.uk), Feb 10 
               2003
%E A080254 More terms from Antonio G. Astudillo (afg_astudillo(AT)hotmail.com), 
               Feb 14 2003