OFFSET
0,10
COMMENTS
The reverse of a permutation is the reverse in one line notation. For example the reverse of 43521 is 12534.
T(n,k) is the number of size k bases of S_n which remain unchanged by reverse.
FORMULA
T(n,k) = C(n!/2, k/2) if k is even and T(n,k) = 0 if k is odd.
EXAMPLE
For n = 4 and k = 2 the subsets that remain unchanged by reverse are {4321, 1234}, {1243, 3421}, {4231, 1324}, {1342, 2431}, {1423, 3241}, {1432, 2341}, {2134, 4312}, {3412, 2143}, {2314, 4132}, {3142, 2413}, {4213, 3124} and {4123, 3214} so T(4,2) = 12.
For n = 3 and k = 4 the subsets that remain unchanged by reverse are {231, 321, 132, 123}, {321, 213, 312, 123} and {231, 132, 312, 213} so T(3,4) = 3.
The triangle starts:
1, 1;
1, 1;
1, 0, 1;
1, 0, 3, 0, 3, 0, 1;
PROG
(Sage) def T(n, k):
if k % 2 == 1:
return 0
return binomial( factorial(n)/2, k/2 )
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
Christian Bean, Sep 28 2016
STATUS
approved