Some independent families of one-letter languages
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摘要
A set of languages is independent if no language in the set can be obtained from other languages in the set by a sequence of full AFL operations. It is proved that the set {Pk|k≥2} is independent, where Pk={ank|n≥1}. For J⊆{2, 3, 4,…}, it is proved that {tEk|k∈J} is independent if and only if no two numbers in J are powers of the same integer, where Ek={akn|n≥1}.
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论文评审过程：Received 1 August 1973, Available online 27 December 2007.
论文官网地址：https://doi.org/10.1016/S0022-0000(75)80006-7
