OFFSET
0,3
COMMENTS
Also number of ways to partition the multiset consisting of 2 copies each of 1, 2, ..., n into 2 multisets of size n.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000
FORMULA
EXAMPLE
a(4) = 10: (2*3*5*7)^2 = 44100 = 210*210 = 225*196 = 294*150 = 315*140 = 350*126 = 441*100 = 490*90 = 525*84 = 735*60 = 1225*36.
MAPLE
a:= proc(n) option remember; `if`(n<3, [1, 1, 2][n+1],
((3*n^2-7*n+3)*a(n-1) +(n-1)*(n-3)*a(n-2)
-3*(n-1)*(n-2)*a(n-3)) / (n*(n-2)))
end:
seq(a(n), n=0..40);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Apr 27 2015
STATUS
approved