1. If there are n individuals A1, A2,.…, An, in how many ways can they be put into groups? For example, if there are three individuals A, B, C, they may be grouped as
that is, in 5 ways, the respective subgroups, 1, 3, 1 in number, corresponding to the partitions 1 + 1 + 1, 1 + 2, 3 of the integer 3. Hence P (3), say, is 5.
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