 Formal-group morphism induced by an isogeny

An isogeny $\varphi :E\to E^{\prime }$ sends the identity to the identity, so substituting the formal coordinates into $t^{\prime }=-x^{\prime }/y^{\prime }$ gives a series $f(t)\in tK[[t]]$. Compatibility with the elliptic-curve group laws givesmaking $f$ a morphism of formal group laws.