Derive Group Law on Elliptic Curve with Riemann Roch

The idea is to prove that the map that you defined above (called the Abel-Jacobi map) $$\begin{align*} J \colon E(k) &\to \mathrm{Pic}^0(E)\\ P &\mapsto (P)-(O) \end{align*}$$

is a bijection, where $\mathrm{Pic}^0(E)$ is the subgroup of $\mathrm{Pic}(E)$ of elements of degree $0$. Then the group law of $E(k)$ will be the one that makes the map above an isomorphism of groups.

Remarks: In this context $\mathrm{Pic}(E)$ is just another notation for $\mathrm{Cl}(E)$. As $k$ may not be algebraically closed we have to recall that the degree of $D=\sum_{p\in E}n_p(P)$ is given by $\deg(D)=\sum_{p\in E}n_p[k(p):k]$ so we have $P\in E(k)$ if and only if $deg(P)=1$.

• Injectivity of the map came from the fact that if $(P)-(Q)=\text{div}(f)$ then by replacing $D=(Q)$ on Riemann-Roch we get $h^0(Q)=2-g=1$ and hence $f\in H^0(E,Q)$ must be constant.

  Over an algebraically closed field you can also proceed without R-R as here.

• Surjectivity came from the fact that if $D\in \mathrm{Div}^0(E)$ then by R-R we have $h^0(D+(O))=1$ so if $f\in H^0(E,D+(O))$ is not constant we have $\text{div}(f)=-D-(O)+(P)$ for some $P$. As $\deg(D)=\deg(\mathrm{div}(f))=0$ we get $\deg(P)=\deg(O)=1$ hence $P\in E(k)$ and then $$P\mapsto (P)-(O)\sim D$$

Now to prove that this group law $\oplus$ coincides with the geometric group law (the one defined with lines) when $E$ is given by a Weierstrass equation $$E:Y^2Z=4X^3-aX^2-bZ^3$$ it's enough to notice that $P\oplus Q\oplus R = O$ $\iff$ $P,Q,R$ are coolinear $\iff$ there is a degree one homogeneous polynomial $F(X,Y,Z)$ with $V(F)=\{P,Q,R\}$ $\iff$ $\mathrm{div}(\frac{F}{Z})=(P)+(Q)+(R)-3(O)$ (notice that the intersection multiplicity between $V(Z)$ and $E$ is $3$, hence the $3(O)$ term) $\iff$ $P+Q+R=O$ with the addition descrived above. Notice that all the above its true when the set $\{P,Q,R\}$ degenerates with tangencies.

Do you know that for an elliptic curve $K$ is the zero divisor? That is, $K=\mathcal{O}_E$? If you knew this, RR implies $l(D)\geq 3$ first. Thus $D$ is an effective divisor of degree 3 and thus $l(K-D)=l(-D)=0$, since negative degree divisor can not be effective. Then, you have $l(D)=3$, which is what you want.

I do not understand your statement `so it suffices to show $l(D)-l(K-D)=3$', but isn't that what RR says, since $\deg D=3$?