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We consider residual-based stabilised finite element methods for the generalised Oseen problem. The unique solvability based on a modified stability condition and an error estimate are proved for inf-sup stable discretisations of velocity and pressure. The analysis highlights the role of an additional stabilisation of the incompressibility constraint. It turns out that the stabilisation terms of the streamline-diffusion (SUPG) type play a less important role. In particular, there exists a conditional stability problem of the SUPG stabilisation if two relevant problem parameters tend to zero. The analysis extends a recent result to general shape-regular meshes and to discontinuous pressure interpolation. Some numerical observations support the theoretical results.

Dedicated to Raytcho Lazarov on the occasion of his 60th birthday.Abstract -A boundary-value problem with a nonlocal integral condition is considered for a two-dimensional elliptic equation with constant coefficients and a mixed derivative. The existence and uniqueness of a weak solution of this problem are proved in a weighted Sobolev space. A difference scheme is constructed using the Steklov averaging operators. It is proved that the difference scheme converges in discrete W

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-The elements of the stability theory of nonselfadoint difference schemes for nonstationary problems of mathematical physics are discussed. Difference schemes for the heat conduction equation with nonlocal boundary conditions are considered in detail from the viewpoint of the general stability theory of two-layer operator-difference schemes. The necessary and sufficient stability conditions in the sense of the initial data in special energy norm have been found. The equivalence of the energy norm to the grid L 2 -norm has been proved. A priori estimates expressing the difference schemes stability in the sense of the right-hand side have been constructed.2000 Mathematics Subject Classification: 65M06; 65M12.

The paper deals with the stability, with respect to initial data, of difference schemes that approximate the heat-conduction equation with constant coefficients and nonlocal boundary conditions. Some difference schemes are considered for the one-dimensional heat-conduction equation, the energy norm is constructed, and the necessary and sufficient stability conditions in this norm are established for explicit and weighted difference schemes.2000 Mathematics Subject Classification: 65M06, 65M12.

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