We prove a conjecture from [BK2] that the multi-dimensional vector addition formula
for Baker-Akhiezer functions obtained there characterizes Jacobians among principally polarized
abelian varieties. We also show that this addition formula is equivalent to Gunning's multisecant
formula for the Kummer variety obtained in [Gu2].
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We then use this addition formula to obtain cubic relations among theta functions that characterize
the locus of hyperelliptic Jacobians among irreducible abelian varieties. In genus 3 our equations
are equivalent to the vanishing of one theta-null, and thus are classical (see [M], [P]), but already for
genus 4 they appear to be new.