Non-simple principally polarised abelian varieties

The paper investigates the locus of non-simple principally polarised abelian $g$-folds. We show that the irreducible components of this locus are ${\text{Is}}_D^g$, defined as the locus of principally polarised $g$-folds having an abelian subvariety with induced polarisation of type $D=(d_1,\dots ,d_k)$, where $k\le \frac{g}{2}$. The main theorem produces Humbert-like equations for irreducible components of ${\text{Is}}_D^g$ for any $g$ and $D$. Moreover, there are theorems which characterise the Jacobians of curves that are 'etale double covers or double covers branched in two or four points.