И. Г. Петровский, О диффузии волн и лакунах для систем гиперболических уравнений, Изв. АН СССР. Сер. матем., 8:3 (1944), 101–106 pdf with French summary and title Sur la diffusion des ondes et les lacunes pour les systèmes d’équations hyperboliques, MR11880
I. G. Petrovsky, Dependence of solutions of Cauchy’s problem on initial conditions, C. R. (Doklady) Acad. Sci. URSS (N.S.) 38, (1943). 151–153 MR8450; On the diffusion of waves and the lacunas for hyperbolic equations, Recueil Mathématique (Matematicheskii Sbornik) 17 (59)1945 (3): 289–368, MR16861
M. F. Atiyah, R. Bott, L. Gårding, Lacunas for hyperbolic differential operators with constant coefficients. I, Acta Mathematica 124 (1970) 109–189, doi, [MR0470499]; Lacunas for hyperbolic differential operators with constant coefficients. II, Acta Mathematica 131 (1973) 145–206, doi, MR0470500
Yuri Berest, The problem of lacunas and analysis on root systems, Trans. Amer. Math. Soc. 352 (2000), 3743–3776 doipdf
Yuri Yu. Berest, The theory of lacunas and quantum integrable systems, Calogero-Moser-Sutherland models (Montréal, QC, 1997), 53–64, CRM Ser. Math. Phys., Springer 2000.