OFFSET
1,2
COMMENTS
The number with 2n+1 orderings is the same as the number with 2n orderings (cf. A213332).
LINKS
N. J. A. Sloane, Table of n, a(n) for n = 1..400
Thomas C. Craven, An application of Pólya's theory of counting to an enumeration problem arising in quadratic form theory, J. Combin. Theory Ser. A 29 (1980), no. 2, 174--181. MR0583956 (81j:10027).
MAPLE
read transforms;
w:=proc(n) option remember; global did; local v; # did(n, d)=1 if d|n otherwise 0
if n=1 then 1 elif (n mod 2) = 1 then w(n-1);
else v:=n/2;
(1/n)* ( add(2*i*w(i)*did(v, i), i=1..v) +
add( add(2*i*w(i)*w(n-2*k)*did(k, i), i=1..k), k=1..v-1));
fi; end;
[seq(w(2*n), n=1..50)];
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jun 08 2012
STATUS
approved