Diophantine Geometry: An Introduction
《Diophantine Geometry: An Introduction》 · 第 95 页 · Chapter A，Section 7「Isogenies」，Proposition A.7.2
Let $\alpha:A\to B$ be an isogeny of degree $d$. There is a unique isogeny $\widehat\alpha:B\to A$ satisfying $\widehat\alpha\circ\alpha=[d]_A$ and $\alpha\circ\widehat\alpha=[d]_B$. The isogeny $\widehat\alpha$ is called the dual isogeny of $\alpha$.