The article develops the apparatus of derived numbers, the use of which allows one to study the behavior of functions of several variables without requiring their differentiability. In addition, the application of this apparatus to the problem of integrability of the field of planes tangent to a differential manifold allows one to generalize the Frobenius theorem and expand its scope by weakening the restrictions on the degree of smoothness of the differential manifolds under consideration. Conditions and criteria for using the apparatus of partial and external derivatives of numbers are proposed.