# Nonlocal multipoint problem for a differential equation of $2n$-th order with operator coefficients

## Keywords:

multipoint problem, antiperiodic boundary condition, root function, method of transmutation operators, Riesz basis### Abstract

In the article, the spectral properties of a multipoint problem for a differential operator equation of order $2n$ are studied. The operator of the problem has an infinite number of multiple eigenvalues. Each multiple eigenvalue corresponds to a finite set of root functions. A commutative group of transmutation operators is constructed. Each element of the group corresponds to the isospectral perturbation of the problem operator with antiperiodic conditions. The conditions for the existence and uniqueness of the solution are established for the selected family of multipoint problems, and this solution is constructed too.

*Carpathian Math. Publ.*

**2021**,

*13*, 501-514.