OFFSET
0,2
LINKS
R. Kehinde, S. O. Makanjuola and A. Umar, On the semigroup of order-decreasing partial isometries of a finite chain, arXiv:1101.2558
Index entries for linear recurrences with constant coefficients, signature (5,-7,-3,16,-14,4).
FORMULA
a(n) = 3*a(n-1)-2*a(n-2)-2^floor(n/2)+n+1.
G.f.: ( -1+3*x-2*x^2-5*x^3-4*x^5+10*x^4 ) / ( (2*x-1)*(2*x^2-1)*(x-1)^3 ). - R. J. Mathar, Jul 03 2011
EXAMPLE
a(2) = 5 because there are exactly 5 order-decreasing partial isometries (on a 2-chain) namely: empty map; 1-->1; 2-->1; 2-->2; (1,2)-->(1,2) - the mappings are coordinate-wise
CROSSREFS
KEYWORD
nonn
AUTHOR
Abdullahi Umar, Jan 12 2011
STATUS
approved