In abstract algebra, a quasi-free algebra is an associative algebra that satisfies the lifting property similar to that of a formally smooth algebra in commutative algebra. The notion was introduced by Cuntz and Quillen for the applications to cyclic homology.[1] A quasi-free algebra generalizes a free algebra, as well as the coordinate ring of a smooth affine complex curve. Because of the latter generalization, a quasi-free algebra can be thought of as signifying smoothness on a noncommutative space.[2]

Definition

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Let A be an associative algebra over the complex numbers. Then A is said to be quasi-free if the following equivalent conditions are met:[3][4][5]

  • Given a square-zero extension  , each homomorphism   lifts to  .
  • The cohomological dimension of A with respect to Hochschild cohomology is at most one.

Let   denotes the differential envelope of A; i.e., the universal differential-graded algebra generated by A.[6][7] Then A is quasi-free if and only if   is projective as a bimodule over A.[3]

There is also a characterization in terms of a connection. Given an A-bimodule E, a right connection on E is a linear map

 

that satisfies   and  .[8] A left connection is defined in the similar way. Then A is quasi-free if and only if   admits a right connection.[9]

Properties and examples

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One of basic properties of a quasi-free algebra is that the algebra is left and right hereditary (i.e., a submodule of a projective left or right module is projective or equivalently the left or right global dimension is at most one).[10] This puts a strong restriction for algebras to be quasi-free. For example, a hereditary (commutative) integral domain is precisely a Dedekind domain. In particular, a polynomial ring over a field is quasi-free if and only if the number of variables is at most one.

An analog of the tubular neighborhood theorem, called the formal tubular neighborhood theorem, holds for quasi-free algebras.[11]

References

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  1. ^ Cuntz & Quillen 1995
  2. ^ Cuntz 2013, Introduction
  3. ^ a b Cuntz & Quillen 1995, Proposition 3.3.
  4. ^ Vale 2009, Proposotion 7.7.
  5. ^ Kontsevich & Rosenberg 2000, 1.1.
  6. ^ Cuntz & Quillen 1995, Proposition 1.1.
  7. ^ Kontsevich & Rosenberg 2000, 1.1.2.
  8. ^ Vale 2009, Definition 8.4.
  9. ^ Vale 2009, Remark 7.12.
  10. ^ Cuntz & Quillen 1995, Proposition 5.1.
  11. ^ Cuntz & Quillen 1995, § 6.

Bibliography

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  • Cuntz, Joachim (June 2013). "Quillen's work on the foundations of cyclic cohomology". Journal of K-Theory. 11 (3): 559–574. arXiv:1202.5958. doi:10.1017/is012011006jkt201. ISSN 1865-2433.
  • Cuntz, Joachim; Quillen, Daniel (1995). "Algebra Extensions and Nonsingularity". Journal of the American Mathematical Society. 8 (2): 251–289. doi:10.2307/2152819. ISSN 0894-0347.
  • Kontsevich, Maxim; Rosenberg, Alexander L. (2000). "Noncommutative Smooth Spaces". The Gelfand Mathematical Seminars, 1996–1999. Birkhäuser: 85–108. arXiv:math/9812158. doi:10.1007/978-1-4612-1340-6_5.
  • Maxim Kontsevich, Alexander Rosenberg, Noncommutative spaces, preprint MPI-2004-35
  • Vale, R. (2009). "notes on quasi-free algebras" (PDF).

Further reading

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