Comparing \(\mathbb{A}^1\)-\(h\)-cobordism and \(\mathbb{A}^1\)-weak equivalence. (English) Zbl 1401.14058

Asok, Aravind; Kebekus, Stefan; Wendt, Matthias

Ann. Sc. Norm. Super. Pisa, Cl. Sci. (5) 17, No. 2, 531-572 (2017).

Summary: We study the problem of classifying projectivizations of rank-two vector bundles over \(\mathbb{P}^2\) up to two notions of equivalence that arise naturally in \(\mathbb{A}^1\)-homotopy theory, namely \(\mathbb{A}^1\)-weak equivalence and \(\mathbb{A}^1\)-\(h\)-cobordism.
First, we classify such varieties up to \(\mathbb{A}^1\)-weak equivalence: over algebraically closed fields having characteristic unequal to two the classification can be given in terms of characteristic classes of the underlying vector bundle. When the base field is \(\mathbb{C}\), this classification result can be compared to a corresponding topological result and we find that the algebraic and topological homotopy classifications agree.
Second, we study the problem of classifying such varieties up to \(\mathbb{A}^1\)-\(h\)-cobordism using techniques of deformation theory. To this end, we establish a deformation rigidity result for \(\mathbb{P}^1\)-bundles over \(\mathbb{P}^2\) which links \(\mathbb{A}^1\)-\(h\)-cobordism to deformations of the underlying vector bundles. Using results from the deformation theory of vector bundles we show that if \(X\) is a \(\mathbb{P}^1\)-bundles over \(\mathbb{P}^2\) and \(Y\) is the projectivization of a direct sum of line bundles on \(\mathbb{P}^2\), then if \(X\) is \(\mathbb{A}^1\)-weakly equivalent to \(Y\), \(X\) is also \(\mathbb{A}^1\)-\(h\)-cobordant to \(Y\).
Finally, we discuss some subtleties inherent in the definition of \(\mathbb{A}^1\)-\(h\)-cobordism. We show, for instance, that direct \(\mathbb{A}^1\)-\(h\)-cobordism fails to be an equivalence relation.

MSC:

14D20	Algebraic moduli problems, moduli of vector bundles
14F42	Motivic cohomology; motivic homotopy theory
57R22	Topology of vector bundles and fiber bundles