The main issues of modeling a linear digital filter are considered in the approximation of discrete integer mathematics corresponding to the real-time filter operation on digital platforms with integer calculation arithmetic. An integer model of recursive and non-recursive filters in the form of a connection of cascade second-order sections is given. The performance and noise characteristics of recursive integer filters are estimated. The formulation of the problem of multifunctional filter synthesis is considered by methods of integer nonlinear programming taking into account the given radius of the transfer function poles. The efficiency of this approach is illustrated by typical examples of solving problems of synthesis of cascade recursive and non-recursive filters with complex selective requirements and with minimum width of integer coefficients, as well as polynomial and minimum-phase digital filters. The theoretical and experimental characteristics of the synthesized digital filters are analyzed.