An approach to calculate the natural frequencies of an elastic parallelepiped with different boundary conditions is proposed. The approach rationally combines the inverse-iteration method of successive approximations and the advanced Kantorovich-Vlasov method. The efficiency of the approach (the accuracy of the results and the number of approximating functions) is demonstrated against the Ritz method with different basis systems, including B-splines. The dependence of the lower frequencies of a three-dimensional cantilever beam on its cross-sectional dimensions is examined Keywords: isotropic elastic parallelepiped, different boundary conditions, natural frequency, advanced Kantorovich-Vlasov methodIntroduction. The present study was inspired by the paper [15] where the Ritz method with B-splines as basis functions was tested by calculating the natural frequencies of a rectangular parallelepiped. The vibrations of elastic bodies of such shape are addressed in a great number of Ukrainian and foreign publications reviewed in [15] and [3] with reference to isotropic and anisotropic materials, respectively.The spectrum of natural frequencies of a parallelepiped with four sides hinged was analyzed in detail in [1, 11, 16] using trigonometric Fourier series expansion. Other types of boundary conditions can be examined by using analytical and numerical methods. Many relevant publications employ the Ritz method with emphasis on the selection of systems of basis functions. As such, power functions, simple and orthogonal polynomials, Chebyshev polynomials, etc. are used [10,[12][13][14]17].The methods that use finitely supported functions as basis systems occupy a special place in the class of Ritz methods. This approach gave rise to numerous modifications of the finite-element method having many applications. Among such methods are B-splines which have become popular recently. The results obtained with the Ritz method for various basis systems, including B-splines, were compared in [15] by solving three-dimensional eigenvalue problems for an isotropic parallelepiped.B-splines in combination with the Kantorovich-Vlasov method were employed in [6-9] to reduce two-dimensional boundary-value and eigenvalue problems to ordinary differential equations. Unlike the Ritz method where approximating functions with respect to all variables of the domain are chosen beforehand, this approach makes it possible to find the functional coefficients of B-splines from the solution of the original problem. The efficiency of this method was tested against a wide class of two-dimensional problems for shells and plates.To determine the natural frequencies of an isotropic parallelepiped, we will use the advanced Kantorovich-Vlasov method (AKM) for three-dimensional problems of elasticity. Such an approach to two-dimensional stationary problems for shallow shells was outlined in [4]. The AKM was used in [5] to solve the problem of twisting of a rectangular anisotropic prism.