A321531 a(n) is the maximum number of distinct directions between n non-attacking rooks on an n X n chessboard.

0, 1, 2, 4, 6, 8, 11, 14, 18, 23

Offset: 1

Author

Peter Kagey, Nov 12 2018

Keywords

• nonn
• more

Comments

Directions are determined up to scaling and dihedral action of the square, as in (m,k)-riders. For example, the moves (1,2) and (2,-4) are considered to have the same direction.

Examples

			For n = 5, a 5 X 5 board with a(5) = 6 distinct directions is
   +---+---+---+---+---+
  5| X |   |   |   |   |
   +---+---+---+---+---+
  4|   |   | X |   |   |
   +---+---+---+---+---+
  3|   |   |   | X |   |
   +---+---+---+---+---+
  2|   |   |   |   | X |
   +---+---+---+---+---+
  1|   | x |   |   |   |
   +---+---+---+---+---+
     A   B   C   D   E
Where the six distinct directions are:
1) 1:4 by (A5,B1).
2) 1:2 by (A5,C4).
3) 2:3 by (A5,D3).
4) 3:4 by (A5,E2).
5) 1:3 by (B1,C4) and (B1,E2).
6) 1:1 by (B1,D3), (C4,D3), (C4,E2), and (D3,E2).
		

Crossrefs

Cf. A320448 (analogous with distance instead of direction).