Twists of curves


In the mathematical field of algebraic geometry, an elliptic curve E over a field K has an associated quadratic twist, that is another elliptic curve which is isomorphic to E over an algebraic closure of K. In particular, an isomorphism between elliptic curves is an isogeny of degree 1, that is an invertible isogeny. Some curves have higher order twists such as cubic
and quartic twists. The curve and its twists have the same j-invariant.

Quadratic twist

First assume K is a field of characteristic different from 2.
Let E be an elliptic curve over K of the form:
Given not quadratic residue, the quadratic twist of is the curve, defined by the equation:
or equivalently
The two elliptic curves and are not isomorphic over, but rather over the field extension.
Now assume K is of characteristic 2. Let E be an elliptic curve over K of the form:
Given such that is an irreducible polynomial over K, the quadratic twist of E is the curve Ed, defined by the equation:
The two elliptic curves and are not isomorphic over, but over the field extension.

Quadratic twist over finite fields

If is a finite field with elements, then for all there exist a such that the point belongs to either or.
In fact, if is on just one of the curves, there is exactly one other on that same curve.
As a consequence, or equivalently
where is the trace of the Frobenius endomorphism of the curve.

Quartic twist

It is possible to "twist" elliptic curves with j-invariant equal to 1728 by quartic characters; twisting a curve E by a quartic twist, one obtains precisely four curves: one is isomorphic to E, one is its quadratic twist, and only the other two are really new.
Also in this case, twisted curves are isomorphic over the field extension given by the twist degree.

Cubic twist

Analogously to the quartic twist case, an elliptic curve over with j-invariant equal to zero can be twisted by cubic characters. The curves obtained are isomorphic to the starting curve over the field extension given by the twist degree.

Examples