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Bass–Quillen conjecture

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Would relate vector bundles over a regular Noetherian ring and over a polynomial ring

In mathematics, the Bass–Quillen conjecture relates vector bundles over a regular Noetherian ring A and over the polynomial ring A [ t 1 , … , t n ] {\displaystyle A} . The conjecture is named for Hyman Bass and Daniel Quillen, who formulated the conjecture.

Statement of the conjecture

The conjecture is a statement about finitely generated projective modules. Such modules are also referred to as vector bundles. For a ring A, the set of isomorphism classes of vector bundles over A of rank r is denoted by Vect r ⁡ A {\displaystyle \operatorname {Vect} _{r}A} .

The conjecture asserts that for a regular Noetherian ring A the assignment

M ↦ M ⊗ A A [ t 1 , … , t n ] {\displaystyle M\mapsto M\otimes _{A}A}

yields a bijection

Vect r ⁡ A → ∼ Vect r ⁡ ( A [ t 1 , … , t n ] ) . {\displaystyle \operatorname {Vect} _{r}A\,{\stackrel {\sim }{\to }}\operatorname {Vect} _{r}(A).}

Known cases

If A = k is a field, the Bass–Quillen conjecture asserts that any projective module over k [ t 1 , … , t n ] {\displaystyle k} is free. This question was raised by Jean-Pierre Serre and was later proved by Quillen and Suslin; see Quillen–Suslin theorem. More generally, the conjecture was shown by Lindel (1981) in the case that A is a smooth algebra over a field k. Further known cases are reviewed in Lam (2006).

Extensions

The set of isomorphism classes of vector bundles of rank r over A can also be identified with the nonabelian cohomology group

H N i s 1 ( S p e c ( A ) , G L r ) . {\displaystyle H_{Nis}^{1}(Spec(A),GL_{r}).}

Positive results about the homotopy invariance of

H N i s 1 ( U , G ) {\displaystyle H_{Nis}^{1}(U,G)}

of isotropic reductive groups G have been obtained by Asok, Hoyois & Wendt (2018) by means of A homotopy theory.

References

  1. Bass, H. (1973), Some problems in 'classical' algebraic K-theory. Algebraic K-Theory II, Berlin-Heidelberg-New York: Springer-Verlag, Section 4.1
  2. Quillen, D. (1976), "Projective modules over polynomial rings", Invent. Math., 36: 167–171, Bibcode:1976InMat..36..167Q, doi:10.1007/bf01390008, S2CID 119678534
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