On triviality of $\mathbb{A}^2$-forms admitting a nontrivial $\mathbb{G}_a$-action

Abstract

T. Kambayashi had shown that $\mathbb{A}^2$-forms over separable field extensions are necessarily polynomial rings. However, there exist inseparable $\mathbb{A}^2$-forms which are not necessarily polynomial rings. In this paper, we give a structure theorem for $\mathbb{A}^2$-forms over arbitrary field extensions admitting a nontrivial $\mathbb{G}_a$-action. From this structure theorem we derive some conditions under which an $\mathbb{A}^2$-form becomes trivial. In particular, we prove that over a field $k$, a factorial $\mathbb{A}^2$-form having a $k$-rational point and a non-trivial $\mathbb{G}_a$-action is trivial and we also give examples demonstrating that none of these hypotheses can be discarded. As a consequence of the structure theorem, we obtain a generalization of the Zariski Cancellation Theorem for the affine plane over an arbitrary field.

Theorems & Definitions (25)

• Theorem 1.1
• Theorem 1.2: Zariski Cancellation Theorem
• Definition 2.1
• Lemma 2.2
• Theorem 2.3
• Theorem 2.4
• Theorem 2.5
• Lemma 2.6
• Theorem 3.1
• proof