# Basic Algebraic Geometry 1: Varieties in Projective Space by Igor R. Shafarevich, Miles Reid

By Igor R. Shafarevich, Miles Reid

Shafarevich's simple Algebraic Geometry has been a vintage and universally used creation to the topic on the grounds that its first visual appeal over forty years in the past. because the translator writes in a prefatory notice, ``For all [advanced undergraduate and starting graduate] scholars, and for the numerous experts in different branches of math who want a liberal schooling in algebraic geometry, Shafarevich’s publication is a must.'' The 3rd variation, as well as a few minor corrections, now bargains a brand new therapy of the Riemann--Roch theorem for curves, together with an explanation from first principles.

Shafarevich's e-book is an enticing and obtainable advent to algebraic geometry, compatible for starting scholars and nonspecialists, and the recent variation is decided to stay a well-liked creation to the field.

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Passing to homogeneous coordinates, we can write its equation in the form η2 ζ = ξ 3 + pξ ζ 2 + qζ 3 . Hence it has a unique point on the line at infinity ζ = 0, namely the point o = (0 : 1 : 0). Dividing through by η3 we write the equation of the curve in the form v = u3 + puv 2 + qv 3 , in coordinates u, v, where u = ξ/η and v = ζ /η. The point o = (0, 0) in these coordinates is also nonsingular. Hence our curve is nonsingular. The map (x, y) → (x, −y) is obviously a birational map of the curve to itself.

Xn )) is an autoi morphism of An then the Jacobian J (f ) = det | ∂P ∂xj | ∈ k. Prove that f → J (f ) is a homomorphism from the group of automorphisms of An into the multiplicative group of nonzero elements of k. 11 Suppose that X consists of two points. Prove that the coordinate ring k[X] is isomorphic to the direct sum of two copies of k. 12 Let f : X → Y be a regular map. The subset Γf ⊂ X × Y consisting of all points of the form (x, f (x)) is called the graph of f . Prove that (a) Γf ⊂ X × Y is a closed subset, and (b) Γf is isomorphic to X.

Find the image f (A2 ); is it open in A2 ? Is it dense? Is it closed? 7 The same question as in Exercise 6 for the map f : A3 → A3 defined by f (x, y, z) = (x, xy, xyz). 8 An isomorphism f : X → X of a closed set X to itself is called an automorphism. Prove that all automorphisms of the line A1 are of the form f (x) = ax + b with a = 0. 9 Prove that the map f (x, y) = (αx, βy + P (x)) is an automorphism of A2 , where α, β ∈ k are nonzero elements, and P (x) is a polynomial. Prove that maps of this type form a group B.