OFFSET
0,16
COMMENTS
A permutation, p, can be thought of as a set of points (i, p(i)). If you plot all the points and rotate the picture by 90 degrees then you get a permutation back.
T(n,k) is the number of size k subsets that remain unchanged by a rotation of 90 degrees.
FORMULA
EXAMPLE
For n = 4 and k = 2, the subsets unchanged by a 90-degree rotation are {4321,1234}, {4231,1324}, {3412,2143} and {3142,2413}. Hence T(4,2) = 4.
Triangle starts:
1, 1;
1, 1;
1, 0, 1;
1, 0, 1, 0, 1, 0, 1;
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
Christian Bean, Sep 28 2016
STATUS
approved