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EXAMPLE
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a(2): We can have either 2 balls in a box and the other box empty, or a ball in each box. We may place both the boxes into one container, or one in each.
So we have [(2)()][], [(1)(1)][], [(2)][()] and [(1)][(1)], hence a(2)=4.
For a(3) we have: (0 is an empty box, / is a new container, - is an empty container)
300/-/-, 30/0/-, 3/00/-, 3/0/0
210/-/-, 21/0/-, 20/1/-, 2/10/-, 2/1/0
111/-/-, 11/1/-, 1/1/1
total 12
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