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Published online by Cambridge University Press: 20 January 2009
A basic set (formerly basic sequence) ℬ is a set of pairs (a, b) of positive integers satisfying
(1) if (a, b) ∈ ℬ, then (b, a) ∈ ℬ,
(2) (a, bc) ∈ ℬ if and only if (a, b) ∈ ℬ and (a, c) ∈ ℬ,
(3) (1, k ∈ ℬ, k = 1, 2, ….
Some familiar examples of basic sets are
, where Sk = {(1, k), (k, 1)},
= {(a, b)| a and b are relatively prime positive integers},
ℒ = {(a, b)| a and b are any positive integers}.