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Show that φ2 : E(k

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MATH-GA 2420.006 : Homework 1, due by Februry 8 morning (before 10am); send the solutions to pirutka@cims.nyu.edu

1. Show that the cubic curveY2Z =X3+AXZ2+BZ3is smooth iff4A3+27B2 6=

0.

2. Let k be an algebraically closed field.

(a) LetE be an elliptic curve overkgiven by the equationy2 =x3+Ax+B.

Show that φ1 :E(k)→E(k),(x, y)7→(x,−y) is is a group morphism.

(b) LetE be an elliptic curve overkgiven by the equationy2 =x3+B. Show that φ2 : E(k) → E(k),(x, y) → (ζx,−y), where ζ3 = 1 is a primitive root of unity is a group morphism.

(c) Let E be an elliptic curve over k given by the equation y2 = x3+Ax.

Show that φ3 :E(k)→E(k),(x, y)7→(−x, iy) is a group morphism.

3. [j-invariant] Letk be an algebraically closed field of characteristic different from2or3. LetE be an elliptic curve given by an equationy2 =x3+Ax+B.

Define thej-invariant ofE by the formula

j =j(E) = 1728 4A3 4A3+ 27B2.

(a) Let Ei, i = 1,2 be two elliptic curves given by the equations y2 =x3 + Aix+Bi. Show that if j(E1) = j(E2), then there exists µ∈ K, µ 6= 0 such that A24A1 etB26B1.

(b) Deduce that the map x2 = µ2x1, y2 = µ3y1 is a group isomorphism between E1(k)and E2(k).

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