OFFSET
7,2
COMMENTS
Also number of self-inverse permutations in S_n with longest increasing subsequence of length >= 7. a(7)=1: 1234567; a(8)=8: 12345678, 12345687, 12345768, 12346578, 12354678, 12435678, 13245678, 21345678.
LINKS
Alois P. Heinz, Table of n, a(n) for n = 7..300
Wikipedia, Involution (mathematics)
Wikipedia, Young tableau
MAPLE
b:= proc(n) b(n):= `if`(n<2, 1, b(n-1) +(n-1)*b(n-2)) end:
g:= proc(n) option remember;
`if`(n<4, [1, 1, 2, 4][n+1], ((20*n^2+184*n+336)*g(n-1)
+4*(n-1)*(10*n^2+58*n+33)*g(n-2) -144*(n-1)*(n-2)*g(n-3)
-144*(n-1)*(n-2)*(n-3)*g(n-4)) / ((n+5)*(n+8)*(n+9)))
end:
a:= n-> b(n) -g(n):
seq(a(n), n=7..30);
CROSSREFS
KEYWORD
nonn
AUTHOR
Alois P. Heinz, Oct 24 2012
STATUS
approved