nLab dense subalgebra




topology (point-set topology, point-free topology)

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A dense subalgebra of a topological algebra is a subalgebra? which is also a dense subspace.

For instance, the algebra of smooth functions on a given smooth manifold MM is a dense subalgebra of the C-star-algebra of continuous functions on XX. Conversely, if we start with a commutative C-star-algebra AA and view it as the algebra of continuous maps on its spectrum SS, then picking a dense subalgebra of AA may specify a smooth structure on SS (which may be any compact Hausdorff space). For noncommutative geometry, we can let AA be an arbitrary C *C^*-algebra. One also speaks of smooth C-star algebras if a C *C^\ast-algebra is equipped with an inverse system of inclusions of dense subalgebra (a noncommutative version of Frechet spaces).

Hence equipping a C-star-algebra with a choice of dense subalgebra serves may be thought of as refining from noncommutative topology to noncommutative geometry. This is for instance what happens in the definition of spectral triples and in smooth refinements of KK-theory.


  • E. Kissin, V. S. Shulman, Differential properties of some dense subalgebras of C *C^\ast-algebras, Proceedings of The Edinburgh Mathematical Society, vol. 37, no. 03 (1994)

  • Larry Schweitzer, Dense nuclear Fréchet ideals in C *C^\ast-algebras (arXiv:1205.0089)

Last revised on May 31, 2022 at 16:00:55. See the history of this page for a list of all contributions to it.