  We study phase retrieval from magnitude measurements of an unknown signal as
an algebraic estimation problem. Indeed, phase retrieval from rank-one and more
general linear measurements can be treated in an algebraic way. It is verified
that a certain number of generic rank-one or generic linear measurements are
sufficient to enable signal reconstruction for generic signals, and slightly
more generic measurements yield reconstructability for all signals. Our results
solve a few open problems stated in the recent literature. Furthermore, we show
how the algebraic estimation problem can be solved by a closed-form algebraic
estimation technique, termed ideal regression, providing non-asymptotic success
guarantees.
