= Bilinear cross-effect of a line bundle on an abelian variety
{title2=$D_L(f,g)=(f+g)^*L\otimes(f^*L)^{-1}\otimes(g^*L)^{-1}$}
For <group homomorphisms> $f,g:A\to B$, this <Picard group> class is symmetric and additive in both arguments. The <Theorem of the Cube> says precisely that the third additive difference of $h\mapsto[h^*L]$ vanishes, proving additivity of the cross-effect. The <Poincaré line bundle> expresses it as $(f,\phi_Lg)^*P_B$, so it depends only on the <Néron-Severi group> class of $L$. Its associated homomorphism is $\widehat f\phi_Lg+\widehat g\phi_Lf$.
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