|
| |
|
|
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).
|
|
4
|
|
|
|
1, 1, 9, 75, 865, 12483, 216113, 4364979, 100757313, 2616517443, 75496735057, 2396212835283, 82968104980961, 3112139513814243, 125716310807844081, 5441108944839913587, 251195548533025953409, 12321551453507301079683
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
|
OFFSET
|
0,3
|
|
|
COMMENTS
|
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.
Also, Eulerian D-polynomials (A066094) evaluated at 2. - Ralf Stephan, Apr 23 2004
|
|
|
REFERENCES
|
Kenneth S. Brown, Buildings, Springer-Verlag, 1988
|
|
|
LINKS
|
Table of n, a(n) for n=0..17.
|
|
|
FORMULA
|
a(0)=a(1)=1. For n>1, a(n)=1 + sum('2^r*binomial(n, r)*a(n-r)', 'r'=1..n)
E.g.f: (2*x-exp(x))/(exp(2*x)-2) - Antonio G. Astudillo (afg_astudillo(AT)hotmail.com), Feb 14 2003
|
|
|
CROSSREFS
|
Cf. A000670, A080253.
Sequence in context: A136659 A126965 A066222 * A190916 A161736 A226180
Adjacent sequences: A080251 A080252 A080253 * A080255 A080256 A080257
|
|
|
KEYWORD
|
easy,nonn
|
|
|
AUTHOR
|
Paul Boddington & Tim Honeywill, Feb 10 2003
|
|
|
EXTENSIONS
|
More terms from Antonio G. Astudillo (afg_astudillo(AT)hotmail.com), Feb 14 2003
|
|
|
STATUS
|
approved
|
| |
|
|
