In this paper, we consider a pair (A, B) of closed operators on a Banach space X with domain D(A) and D(B). The pair (A, B) is called regular if for every f ∈ X, the problem Au + Bu = f possesses one and only one solution.
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机译：在本文中，我们考虑在具有域D（A）和D（B）的Banach空间X上的一对（A，B）闭合算子。如果对每个f∈X，问题Au + Bu = f都只有一个解，则该对（A，B）被称为正则对。
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