Diophantine Geometry: An Introduction
《Diophantine Geometry: An Introduction》 · 第 130–131 页 · Section A.7.3「Dual Abelian Varieties and Poincaré Divisors」，定义段
The map $\Phi_c:A\to\widehat A$ is an isogeny provided that its kernel $K(c)$ is finite, or equivalently, $c$ is ample. Such an isogeny is called a polarization.