Brilli the Ant is at the origin of the four-dimensional Euclidean space . For each step she moves to another lattice point exactly units away from the point she is currently on. How many ways can she return to the origin for the first time after exactly steps?
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I don't know how to do this without a computer. Brilli has 2 4 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 1 3 , 0 5 6 such paths (there were a total of 2 4 3 paths, of which 7 6 8 touched the origin too early--at least I can verify that part without a computer), and there were 6 2 4 endpoints. Then the answer was i = 1 ∑ 6 2 4 n i 2 , where n i is the number of such paths ending in a point p i .
So I got 7 2 5 , 5 6 8 .
I'd be interested to hear of other solutions.