1. G. D. Birkhoff, “On the asymptotic character of the solutions of certain linear differential equations containing a parameter,” Trans. Amer. Math. Soc., 9, 219–231 (1908).
2. G. D. Birkhoff, “Boundary value and expansion problems of ordinary linear differential equations,” Trans. Amer. Math. Soc., 9, 373–395 (1908).
3. H. K. Cheng, “A variational method for the differential equation arising from aerolastic problems,” Thesis, Graduate School of Aeronautical Engineering Cornell University (1950).
4. G. V. Djudjaeva, “On generalized mean-periodic elements,” Differ. Uravn. Vychisl. Mat., 5, No. 1, 84–120 (1975).
5. O. Yu. Dmitriev, “Eigenfunction expansion of an nth-order differential operator with irregular boundary-value conditions,” Mathematics and Its Applications, Saratov State Univ., Saratov, 70–72 (1991).