Geodesic
The stability of the completeness and minimality in $L^2$ of a system of exponential functions
Matematičeskie zametki, Tome 15 (1974) no. 2, pp. 213-219 
A. M. Sedletskii. The stability of the completeness and minimality in $L^2$ of a system of exponential functions. Matematičeskie zametki, Tome 15 (1974) no. 2, pp. 213-219. http://geodesic.mathdoc.fr/item/MZM_1974_15_2_a4/
@article{MZM_1974_15_2_a4,
     author = {A. M. Sedletskii},
     title = {The stability of the completeness and minimality in $L^2$ of a~system of exponential functions},
     journal = {Matemati\v{c}eskie zametki},
     pages = {213--219},
     year = {1974},
     volume = {15},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1974_15_2_a4/}
}
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Voir la notice de l'article provenant de la source Math-Net.Ru

Let the sequences $\{\lambda_n\}$ and $\{\alpha_n\}$ of complex numbers satisfy the conditions: 1) $\sup|\operatorname{Im}\lambda_n|=h<\infty$; 2) the number of points $\lambda_n$ in the rectangle $|t-\operatorname{Re}z|\le1$, $|\operatorname{Im}z|\le h$ is uniformly bounded with respect to $t\in(-\infty,\infty)$; 3) $\{\alpha_n\}\in l^p$ for some $p<\infty$. Then the systems $\{\exp(i\lambda_nx)\}$ and $\{\exp(ix(\lambda_n+\alpha_n))\}$ are simultaneously complete or noncomplete (minimal or nonminimal) in $L^2(-a,a)$ ($a<\infty$).