Practice problem
The Algebra and Combinatorics of PIE
Justin the robot is on a mission to rescue abandoned treasure from a minefield. To do this, he must travel from the point to in three-dimensional space, only taking one-unit steps in the positive or -directions. However, the evil David anticipated Justin's arrival, and so he has surreptitiously placed a mine at the point If at any point Justin is at most one unit away from this mine (in any direction), the mine detects his presence and explodes, thwarting Justin. How many paths can Justin take to reach his destination safely? (NIMO)