In mathematical analysis, Krein's condition provides a necessary and sufficient condition for exponential sums { ∑ k = 1 n a k exp ⁡, a k ∈ C, λ k ≥ 0 }, {\displaystyle \left\{\sum _{k=1}^{n}a_{k}\exp,\quad a_{k}\in \mathbb {C},\,\lambda _{k}\geq 0\right\},} to be dense in a weighted L2 space on the real line. Wikipedia