Cache optimization for structured and unstructured grid multigrid.
It is proved that checking positive definiteness, stability or nonsingularity of all [symmetric] matrices contained in a symmetric interval matrix is NP-hard.
The preconditioned conjugate gradient method for solving the system of linear algebraic equations with a positive definite matrix is investigated. The initial approximation for conjugate gradient is constructed as a result of a matrix iteration method after steps. The behaviour of the error vector for such a combined method is studied and special numerical tests and conclusions are made.