MathDB
k-knight visits cells on a grid

Source: APMO 2024 P2

July 29, 2024
combinatoricsAPMO 2024

Problem Statement

Consider a 100×100100 \times 100 table, and identify the cell in row aa and column bb, 1a,b1001 \leq a, b \leq 100, with the ordered pair (a,b)(a, b). Let kk be an integer such that 51k9951 \leq k \leq 99. A kk-knight is a piece that moves one cell vertically or horizontally and kk cells to the other direction; that is, it moves from (a,b)(a, b) to (c,d)(c, d) such that (ac,bd)(|a-c|, |b - d|) is either (1,k)(1, k) or (k,1)(k, 1). The kk-knight starts at cell (1,1)(1, 1), and performs several moves. A sequence of moves is a sequence of cells (x0,y0)=(1,1)(x_0, y_0)= (1, 1), (x1,y1),(x2,y2)(x_1, y_1), (x_2, y_2), ,(xn,yn)\ldots, (x_n, y_n) such that, for all i=1,2,,ni = 1, 2, \ldots, n, 1xi,yi1001 \leq x_i , y_i \leq 100 and the kk-knight can move from (xi1,yi1)(x_{i-1}, y_{i-1}) to (xi,yi)(x_i, y_i). In this case, each cell (xi,yi)(x_i, y_i) is said to be reachable. For each kk, find L(k)L(k), the number of reachable cells.