An error estimate for approximate solutions to elliptic equations with non-coercive bilinear form

An error estimation algorithm for approximate solutions to elliptic equations is proposed. This algorithm is based on the Nakao method and is also suitable in the case when the bilinear form of the problem under study is not coercive. For Helmholtz-type equations, another method is developed on the basis of the Nakao method to obtain a more accurate estimate. Some numerical results are given to illustrate the error estimates calculated by these methods.