The return of Brilli

Brilli the Ant is at the origin of the four-dimensional Euclidean space R 4 \mathbb{R}^4 . For each step she moves to another lattice point exactly 2 2 units away from the point she is currently on. How many ways can she return to the origin for the first time after exactly 6 6 steps?


The answer is 725568.

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1 solution

Patrick Corn
Nov 22, 2017

I don't know how to do this without a computer. Brilli has 24 24 different points to choose from at each step.

My approach was to write down all the three-step paths that Brilli could take which started at the origin and didn't touch the origin again. I found 13 , 056 13\text{,}056 such paths (there were a total of 2 4 3 24^3 paths, of which 768 768 touched the origin too early--at least I can verify that part without a computer), and there were 624 624 endpoints. Then the answer was i = 1 624 n i 2 , \sum\limits_{i=1}^{624} n_i^2, where n i n_i is the number of such paths ending in a point p i . p_i.

So I got 725 , 568. 725\text{,}568.

I'd be interested to hear of other solutions.

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