Solution o'clock! Hopefully you have all solved out the balls in boxes problem from last time. Given that you probably have, writing a solution is a totally pointless act, done simply to amuse myself. Here we go.

First of all, lets find some N for which we cannot always win. Specifically, consider that N=1 and there are 1000 balls. Box 999 might have 500 of them and box 1000 has the other 500 of them. In this case we have no legal moves and cannot win. What is the largest N can be so that we still might have no legal moves? To put it another way, when is N big enough to guarantee that we always have a legal move?

The most balls we can have with no legal move is 1 ball in box 2, 2 balls in box 3, 3 balls in box 4,..... k-1 balls in box k. If we have any more balls than that it will be guaranteed that we always have at least one legal move. That number of balls is 1+2+3+4+..+999 = 999*1000/2. So if N is larger than 999/2, so 500 or more, then we always have a legal move. If N is less than 500 we might have no legal moves, thus we may have started in a position with no legal moves.

Now, if N is 500 or larger, we know we always have a legal move, is this enough to guarantee that we can eventually wander our way to a solution? First note that if all the balls were in box 1, we would basically be done, we can move balls from box 1 to any target box simply by moving them 1 at a time. In fact, if we want to move balls from box i to box j, it costs us nothing to move from box i to box 1 then from box 1 to box j, so we might as well only consider moves to and from box 1.

So, assume N is 500 or larger, we have some boxes that have enough balls in them such that we can perform a move. Make all the moves you can from those boxes to box 1, such that when we are done the only legal moves we have left are from box 1. When we are done this box 1 must be nonempty, for if box 1 was empty we would have no moves, and that is impossible for N at least 500. Now, either all the balls are in box 1 or they are not, if all the balls are in box 1 we are done, we can solve the problem as mentioned earlier. If not all the balls are in box 1, there exists some k and x where box k has x balls in it, with k>1, k>x, and x>0. One can see that box 1 has at least k-x balls in it, this is apparent because if it did not I could move all the balls from box 1 into box k and leave myself with no legal moves (which is impossible). So, we can move k-x balls from box 1 into box k, bringing box k up to k balls, and we may then move all the balls from box k to box 1. We repeat this process for each nonempty box k with k>1. In the end all the balls are in box 1 and the game is trivially won.

I'm not sure why I liked this problem so much, I guess I was just pleasantly surprised to see something so simple on a Putnam. I cannot say that their questions lack creativity, but this particular problem is just the kind of creativity I like.

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