Reference no: EM131084004
Math 176: Algebraic Geometry, Fall 2014- Assignment 5
1. (a) Let V be a variety and P ∈ V. Show that there is a one-to-one corespondence between prime ideals in OP(V) and the subvarieties of V that pass through P. (Hint: If I is a prime ideal in OP(V), show that I ∩ Γ(V) is a prime ideal in Γ(V), and I is generated by I ∩ Γ(V).)
(b) Why is the previous problem important?
2. Let V ⊂ An and W ⊂ Am be varieties. Let φ: V → W be an isomorphism of varieties. Prove Oφ(P) (W) ≅ OP(V).
3. (a) Show that the map φ: V(x2 + y2 - 1) → A1 given by φ(x, y) = x/1 - y is a dominant rational map.
(b) Show that ψ: A1 → V(x2 + y2 - 1) given by ψ(t) = ((2t/t2 + 1), (t2 - 1/t2 + 1)), is a dominant rational map.
(c) Deduce that V(x2 + y2 - 1) ∼ A1. Draw a picture to explain what's going on.
4. A conic is a variety of the form
V(ax2 + by2 + cxy + dx + ey + f)
where a, b, c, d, e, f ∈ k. It is well known that any irreducible conic can be transformed into either the conic V(y - x2), V(xy - 1) or the conic V(x2 + y2 - 1) via an affine transformation (that is, via a linear transformation followed by a translation). Deduce that any irreducible conic is birationally equivalent to A1.
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