Let (X={text {Spec}};B) be a factorial affine variety defined over an algebraically closed field k of characteristic zero with a nontrivial action of the additive group (G_a) associated to a locally nilpotent derivation (delta ) on B. In this article, we study X of dimension (ge 3) under the assumption that the plinth ideal (text {pl}(delta )=delta (B)cap A) is contained in an ideal (alpha A) generated by a prime element (alpha in A={text {Ker}},delta ). Suppose that (A={text {Ker}},delta ) is an affine k-domain. The quotient morphism (pi : X rightarrow Y={text {Spec}};A) splits to a composite (textrm{pr} circ p) of the projection (textrm{pr}: Ytimes mathbb A^1 rightarrow Y) and a (G_a)-equivariant birational morphism (p: X rightarrow Ytimes mathbb A^1) where (G_a) acts on (mathbb A^1) by translation. By decomposing (p: X rightarrow Ytimes mathbb A^1) to a sequence of (G_a)-equivariant affine modifications, we investigate the structure of X. We also show that the general closed fiber of (pi ) over the closed set (V(alpha )={text {Spec}};A/alpha A) consists of a disjoint union of m affine lines where (mge 2).
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