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In combinatorial mathematics, the Bell numbers count the number of partitions of a set. These numbers have been studied by mathematicians since the 19th century, and their roots go back to medieval Japan, but they are named after Eric Temple Bell, who wrote about them in the 1930s. Starting with ''B''0 = ''B''1 = 1, the first few Bell numbers are: :1, 1, 2, 5, 15, 52, 203, 877, 4140, 21147, 115975, 678570, 4213597, 27644437, 190899322, 1382958545, 10480142147, 82864869804, 682076806159, 5832742205057, ... . The ''n''th of these numbers, ''Bn'', counts the number of different ways to partition a set that has exactly ''n'' elements, or equivalently, the number of equivalence relations on it. Outside of mathematics, the same number also counts the number of different rhyme schemes for ''n''-line poems.〔.〕 As well as appearing in counting problems, these numbers have a different interpretation, as moments of probability distributions. In particular, ''Bn'' is the ''n''th moment of a Poisson distribution with mean 1. ==What these numbers count== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bell number」の詳細全文を読む スポンサード リンク
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