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We give sufficient conditions for the exponential system to be a Riesz basisin $L^2(E)$, where $E$ is a union of two intervals. We show that theseconditions are close to be necessary. In addition, we demonstrate ``extra pointeffect'' for such systems, i.e. it may happen that the Riesz basis in $L^2(E)$differs by one point from the Riesz basis on an interval.

Exponential Riesz bases in $L^2$ on two interval