OFFSET
0,2
COMMENTS
The Y function of order n is a self-dual monotone Boolean function of (n+1)(n+2)/2 points arranged in a triangular grid, with n+1 points on each side. Suppose we place black or white stones on that grid.
Then, as apparently first pointed out by John Milnor about 1950, either the white or black stones form a "Y" - that is, they touch all three sides of the board. (The corner points each touch two of the sides.)
The Y function tells us whether the white stones or the black stones have the Y.
REFERENCES
D. E. Knuth, The Art of Computer Programming, Vol. 4A, Exercise 7.1.1-67.
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Don Knuth, Jan 26 2008
STATUS
approved