Velu's famous article explains how to get Weierstrass coordinates for the quotient of an elliptic curve by a finite subgroup. There's a part of this article I don't understand however. Here's a print screen:
The $X$ and $Y$, taken together, constitute a morphism to the quotient curve, but they're defined by summing over a set $S$ whose definition isn't clear to me (I don't speak French). It appears that Velu defines $S$ as the union $F_2 \cup R$, but I'm not sure what $R$ is. Does anyone know?