# Affine Extensions of Principal Additive Bundles over a Punctured Surface

Hedén, Isac. (2016) Affine Extensions of Principal Additive Bundles over a Punctured Surface. Transformation groups, 21 (2). pp. 427-449.

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Official URL: http://edoc.unibas.ch/42980/

The aim of this article is to make a first step towards the classification of complex normal affine $G_a$-threefolds $X$. We consider the case where the restriction of the quotient morphism $picolon Xto S$ to $pi^{-1}(S_*)$, where $S_*$ denotes the complement of some regular closed point in $S$, is a principal $G_a$-bundle. The variety $SL_2$ will be of special interest and a source of many examples. It has a natural right $G_a$-action such that the quotient morphism $SL_2toA^2$ restricts to a principal $G_a$-bundle over the punctured plane $A^2_*$.