Elliptic Curves - General structure of the group - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T16:20:47Z http://mathoverflow.net/feeds/question/33191 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/33191/elliptic-curves-general-structure-of-the-group Elliptic Curves - General structure of the group Kikwai 2010-07-24T12:25:19Z 2010-07-24T13:24:41Z <p>Let $K$ be a field and $E$ be an elliptic curve defined over $K$. It well understood the points on $E$ forms an abelian group. What is the structure of this group?(depending on char($K$)?) Is it a direct sum of some well known abelian groups such as $\mathbb{Z}/m\mathbb{Z}$?</p> http://mathoverflow.net/questions/33191/elliptic-curves-general-structure-of-the-group/33194#33194 Answer by Anweshi for Elliptic Curves - General structure of the group Anweshi 2010-07-24T12:51:19Z 2010-07-24T13:01:55Z <p>First case: Complex number. Over $\mathbb C$ the structure as an abstract group is $\mathbb S^1 \oplus \mathbb S^1$ where $\mathbb S^1$ is the circle, i.e., $\mathbb R/\mathbb Z$. This follows as Robin Chapman denotes below, i.e., it is a complex torus in the form $\mathbb C/\Lambda$ where $\Lambda$ is a lattice in $\mathbb C$. </p> <p>Let $K$ be an algebraically closed field of char $p$. If $n$ is prime to $p$, the the $n$-torsion is $\mathbb{Z}/n\mathbb{Z} \oplus \mathbb{Z}/n\mathbb{Z}$. The $p^e$-torsion could be either trivial for all $e$, or $\mathbb{Z}/p^e\mathbb{Z}$ for all $e$.</p> <p>Over a non-algebraically closed-field, this is going to be much more complicated. I will try to give just an introduction. </p> <p>Over $\mathbb Q$ and number fields: Over $\mathbb Q$ or a number field, it is finitely generated by the Mordell-Weil theorem. So it has a torsion part and free part. The free part could be arbitrarily large. </p> <p>The Mordell-Weil theorem is in fact true for arbitrary finitely generated fields. This is due to Néron.</p> <p>Over $\mathbb Q$, The torsion part has exactly $15$ possibilities according to the <a href="http://planetmath.org/encyclopedia/MazursTheoremOnTorsionOfEllipticCurves.html" rel="nofollow">theorem of Mazur</a>. Over number fields, this had been generalized that the torsion part is uniformly bounded.</p> <p>Over finite fields, the torsion group would be $\mathbb{Z}/n\mathbb{Z} \oplus \mathbb{Z}/m\mathbb{Z}$ where $n |m$. There is no free part.</p> <p>And it could go on like this. Please have a look at Silverman's "Advanced topics in the Arithmetic of Elliptic Curves" for elliptic curves over real numbers, $p$-adic numbers, function fields, etc..</p> http://mathoverflow.net/questions/33191/elliptic-curves-general-structure-of-the-group/33196#33196 Answer by S. Carnahan for Elliptic Curves - General structure of the group S. Carnahan 2010-07-24T13:24:41Z 2010-07-24T13:24:41Z <p>If you don't specify more about the structure of the field $K$, then we can't say much about the structure of the group $E(K)$. There are special cases (described in <a href="http://en.wikipedia.org/wiki/Elliptic_curve" rel="nofollow">the Wikipedia article</a>):</p> <ol> <li>If $K$ is a number field, then the <a href="http://en.wikipedia.org/wiki/Mordell-Weil_theorem" rel="nofollow">Mordell-Weil theorem</a> implies the group is finitely generated (and this has been generalized, as Anweshi mentioned). In fact, for each number field, there is a global bound on the size of the torsion of any elliptic curve over that field. In particular, if $K = \mathbf{Q}$, then it is a direct sum of a free abelian group of finite rank with a torsion group that is one of 15 types.</li> <li>If $K$ is finite of order $q$, then (by a theorem of Hasse) the group is finite of order about $q+1$ with error bounded by $2\sqrt{q}$. It is a sum of two cyclic groups.</li> </ol> <p>If $K$ is larger than that, then $E(K)$ can be quite large. For example, if $K$ is separably closed, then $E(K)$ is divisible. In this case, if $K$ has characteristic zero, then <code>$E(K) \cong (\mathbf{Q}/\mathbf{Z})^2 \oplus \bigoplus \mathbf{Q}$</code>. If $K$ has characteristic $p>0$, then <code>$E(K) \cong \bigoplus_{\ell \neq p} (\mathbf{Q}_\ell/\mathbf{Z}_\ell)^2 \oplus (\mathbf{Q}_p/\mathbf{Z}_p)^h \oplus \bigoplus \mathbf{Q}$</code>. Here, $h$ is zero or one depending on whether the curve is supersingular or ordinary, and the $\bigoplus \mathbf{Q}$ is a vector space whose dimension is:</p> <ol> <li>zero if K is an algebraic closure of a finite field.</li> <li>countably infinite if $K$ is countable and not an algebraic closure of a finite field.</li> <li>equal to the cardinality of $K$ otherwise.</li> </ol> <p>Away from the separably closed case, you get a subgroup of one of these groups, but you can have very complicated subgroups of $\mathbf{Q}$ as summands, and very complicated torsion subgroups.</p>