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Published online by Cambridge University Press: 20 November 2018
Let  be an arbitrary family of continuous complex-valued functions defined on a compact Hausdorff space X. A closed subset B ⊆ X is called a boundary for
 be an arbitrary family of continuous complex-valued functions defined on a compact Hausdorff space X. A closed subset B ⊆ X is called a boundary for  if every
 if every  attains its maximum modulus at some point of B. A boundary, B, is said to be minimal if there exists no boundary for
 attains its maximum modulus at some point of B. A boundary, B, is said to be minimal if there exists no boundary for  properly contained in B. It can be shown that minimal boundaries exist regardless of the algebraic structure which
 properly contained in B. It can be shown that minimal boundaries exist regardless of the algebraic structure which  may possess. Under certain conditions on the family
 may possess. Under certain conditions on the family  , it can be shown that a unique minimal boundary for
, it can be shown that a unique minimal boundary for  exists. In particular, this is the case if
 exists. In particular, this is the case if  is a subalgebra or subspace of C(X) where X is compact and Hausdorff (see for example [2]). This unique minimal boundary for an algebra
 is a subalgebra or subspace of C(X) where X is compact and Hausdorff (see for example [2]). This unique minimal boundary for an algebra  of functions is called the Silov boundary of
 of functions is called the Silov boundary of  .
.