Defective dual varieties for real spectra.

We introduce an invariant of a finite point configuration A⊂R1+n which we denote the cuspidal form of A. We use this invariant to extend Esterov's characterization of dual-defective point configurations to exponential sums; the dual variety associated with A has codimension at least 2 if and only if A does not contain any iterated circuit. [ABSTRACT FROM AUTHOR]