Normal form for Cubic


by Newtime
Tags: cubic, form, normal
Newtime
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Aug8-11, 05:32 PM
P: 353
Is there an algorithmic way to put a given elliptic curve into Weierstrass normal form? If not, what's the general procedure?
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Aug8-11, 08:15 PM
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Is there an algorithmic way to put a given elliptic curve into Weierstrass normal form? If not, what's the general procedure?
Yes. Say that C is a cubic. We start by picking a rational point O on the cubic. Then we take the tangent line of C at the rational point O. This will intersect the cubic in another rational point. We take the X-axis to be the tangent line at that other rational point. And we let the Y-axis to be any line through O.

By changing coordinates, you get an equation of the form

[itex]xy^2+(ax+b)y=cx^2+d+e[/itex]

Multiply by x and change into the variable u=xy. This will get you something of the form

[tex]u^2+(ax+b)u=cx^3+dx^2+ex[/tex]

Change the variable again by setting [itex]v^2=u^2+(ax+b)[/itex].

Now change the variables once more to obtain that c=1.


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