Bass–Quillen conjecture

In mathematics, the Bass–Quillen conjecture relates vector bundles over a regular Noetherian ring A and over the polynomial ring $A[t_{1},\dots ,t_{n}]$ . The conjecture is named for Hyman Bass and Daniel Quillen, who formulated the conjecture.^[1]^[2]

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 $\operatorname {Vect} _{r}(A)$ .

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

M\mapsto M\otimes _{A}A[t_{1},\dots ,t_{n}]

yields a bijection

\operatorname {Vect} _{r}(A){\stackrel {\sim }{\to }}\operatorname {Vect} _{r}(A[t_{1},\dots ,t_{n}]).

Known cases

If A = k is a field, the Bass–Quillen conjecture asserts that any projective module over $k[t_1, \dots, t_n]$ 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 in the case that A is a smooth algebra over a field k.^[3] 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_{Nis}^{1}(Spec(A),GL_{r}).

Positive results about the homotopy invariance of

H_{Nis}^{1}(U,G)

of isotropic reductive groups G have been obtained by means of A¹ homotopy theory.^[4]

References

↑ Bass, H. (1973), Some problems in ‘classical’ algebraic K-theory. Algebraic K-Theory II, Berlin-Heidelberg-New York: Springer-Verlag , Section 4.1
↑ Quillen, D. (1976), "Projective modules over polynomial rings", Invent. Math., 36: 167–171, doi:10.1007/bf01390008
↑ Lindel, H. (1981), "On the Bass–Quillen conjecture concerning projective modules over polynomial rings", Invent. Math., 65 (2): 319–323, doi:10.1007/bf01389017
↑ Asok, Aravind; Hoyois, Marc; Wendt, Matthias (2015). "Affine representability results in A^1-homotopy theory II: principal bundles and homogeneous spaces". arXiv:1507.08020 .

Lam, T. Y. (2006), Serre’s problem on projective modules, Berlin: Springer, ISBN 3-540-23317-2, Zbl 1101.13001

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