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An elliptic curve is an algebraic curve of genus one with some additional properties. Questions with this tag will often have the top-level tags nt.number-theory or ag.algebraic-geometry. Note also the tag arithmetic-geometry as well as some related tags such as rational-points, abelian-varieties, heights. Please do not use this tag for questions related to ellipses; instead use conic-sections.

2 votes

Homomorphism of Legendre curve

Pete's is certainly the right way to look at this problem, but in this example one can argue naively using explicit calculations. One loses no generality by assuming $c=0$ (by replacing $x$ by $x+c$). …
Robin Chapman's user avatar
7 votes

Repeated digits of squares in different bases

Your equation $n^2=(b+1)(b^2+1)$ defines an elliptic curve. By Siegel's theorem http://en.wikipedia.org/wiki/Siegel%27s_theorem_on_integral_points the set of integer solutions will be finite. Mordell' …
Robin Chapman's user avatar
23 votes

Why is an elliptic curve a group?

A proof I like is that the group of points on the curve is the classgroup of the ring $R=k[x,\sqrt{x^3+Ax+B}]$ where $k$ is the field you're working over. Set $y=\sqrt{x^3+Ax+B}\in R$ and let $K$ deno …
Robin Chapman's user avatar
12 votes
Accepted

isogeny of elliptic curves

Yes. In zero characteristic the image of an isogeny of elliptic curves is determined up to isomorphism by its kernel. Your isogeny has the same kernel as the doubling map from $E$ to itself.
Robin Chapman's user avatar
7 votes

Additive reduction of elliptic curves

If $p\ge5$ then $E$ has equation $y^2=x^3+Ax+B$ with $p\mid A$ and $p\mid B$. A quadratic twist alters the discriminant, essentially $4A^3+27B^2$, by a sixth power, so for it to have good reduction $v …
Robin Chapman's user avatar
5 votes

How to generate the n-torsion group in an elliptic curve

Under the assumption that $E[n]\subseteq E(\mathbb{F}_q)$, to compute $E[n]$ I'd avoid using division polynomials, as they rapidly become cumbersome. Rather I would generate random elements of $E[n]$ …
Robin Chapman's user avatar
10 votes

Algorithms for finding rational points on an elliptic curve?

There is a whole industry devoted to this. The basic method is by descent, which is a formalized version of the infinite descent proofs of Fermat and Euler. It helps if there are rational 2-torsion po …
Robin Chapman's user avatar
5 votes
Accepted

Analysis of a quadratic diophantine equation

One thing to do is to try to express these in terms of squares. Note that $$12x(3x-1)=36x^2-12x=(6x-1)^2-1$$ so that your equations become $$a_1^2+b_1^2=c_1^2+1$$ and $$a_1^2-b_1^2=d_1^2-1$$ where $a_ …
Robin Chapman's user avatar
9 votes
Accepted

Generalizations of Belyi's theorem

The compactification is the usual one coming up in the theory of modular forms, with the cusps being orbits of $\Gamma$ on $\mathbb{Q}\cup\{\infty\}$. As for the proof, I like this paper by Bernhard …
Robin Chapman's user avatar