This article describes the geometry of isomorphisms between complements of geometrically irreducible closed curves in the affine plane A 2 , over an arbitrary field, which do not extend to an automorphism of A 2 .We show that such isomorphisms are quite exceptional. In particular, they occur only when both curves are isomorphic to open subsets of the affine line A 1 , with the same number of complement points, over any field extension of the ground field. Moreover, the isomorphism is uniquely determined by one of the curves, up to left composition with an automorphism of A 2 , except in the case where the curve is isomorphic to the affine line A 1 or to the punctured line A 1 \ {0}. If one curve is isomorphic to A 1 , then both curves are equivalent to lines. In addition, for any positive integer n, we construct a sequence of n pairwise non-equivalent closed embeddings of A 1 \ {0} with isomorphic complements. In characteristic 0 we even construct infinite sequences with this property.Finally, we give a geometric construction that produces a large family of examples of non-isomorphic geometrically irreducible closed curves in A 2 that have isomorphic complements, answering negatively the Complement Problem posed by Hanspeter Kraft [Kra96]. This also gives a negative answer to the holomorphic version of this problem in any dimension n ≥ 2. The question had been raised by Pierre-Marie Poloni in [Pol16].
In this article, we study isomorphisms between complements of irreducible curves in the projective plane $\mathbb{P}^2$, over an arbitrary algebraically closed field. Of particular interest are rational unicuspidal curves. We prove that if there exists a line that intersects a unicuspidal curve $C \subset \mathbb{P}^2$ only in its singular point, then any other curve whose complement is isomorphic to $\mathbb{P}^2 \setminus C$ must be projectively equivalent to $C$. This generalizes a result of H. Yoshihara who proved this result over the complex numbers. Moreover, we study properties of multiplicity sequences of irreducible curves that imply that any isomorphism between the complements of these curves extends to an automorphism of $\mathbb{P}^2$. Using these results, we show that two irreducible curves of degree $\leq 7$ have isomorphic complements if and only if they are projectively equivalent. Finally, we describe new examples of irreducible projectively non-equivalent curves of degree $8$ that have isomorphic complements.
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