On continuity of the spectrum of a singular quasi-differential operator with respect to a parameter

We obtain sufficient conditions for continuity of the eigenvalues of semibounded quasi-differential operators of order $2n$ on the half-axis with respect to the parameters that appear in the corresponding differential expression. In addition we obtain a generalization of the well-known result of M. G. Krein [9] concerning description of the quadratic form of a regular quasi-differential operator in the singular case, when the deficiency indices of the minimal operator are equal to $(n,n)$.