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The unital algebra generated by a separating complex family and its conjugates is dense
Statement
Let be a compact Hausdorff space and let separate points. The smallest unital complex function algebra containing is uniformly dense in .
Facts & Assumptions
Given: A compact Hausdorff space , a point-separating family , and the unital complex function algebra generated by and all pointwise conjugates of members of .
Every unital point-separating self-adjoint complex function algebra on a compact Hausdorff space is uniformly dense in (Complex Stone–Weierstrass dichotomy for separating self-adjoint algebras; the unital case is dense).
Complex conjugation respects sums and products and is involutive: , , and (Conjugation is an involutive real-field automorphism, , and modulus is definite, multiplicative, and subadditive).
A complex function algebra is self-adjoint when it contains with every , unital when it contains all constants, and point-separating when it distinguishes every distinct pair (Self-adjoint complex function algebras, unitality, and point separation).
Proof
By construction, is unital and contains the point-separating family , so it is unital and point-separating in the sense of [L3].
Conjugation maps every generator to another generator, fixes the real constants and conjugates complex constants, and respects sums and products by [L2]; therefore the conjugate of every finite algebraic expression in the generators belongs to , so is self-adjoint.
Steps 1.1 and 1.2 make a unital point-separating self-adjoint complex function algebra, so [L1] gives exactly the asserted conclusion that is uniformly dense in .
Depends on
- Complex Stone–Weierstrass dichotomy for separating self-adjoint algebras; the unital case is dense
- Self-adjoint complex function algebras, unitality, and point separation
- Conjugation is an involutive real-field automorphism, $z\overline z=|z|^2$, and modulus is definite, multiplicative, and subadditive
Used by
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Sources
- J. M. Erdman, A Companion to Real Analysis, Theorem 21.2.14 (standard reference, not scraped)
- E. Carlen, Notes on Topology and the Stone-Weierstrass Theorem, Theorem 1.29 (standard reference, not scraped)