Hakopian, Y. R.; Hovhannisyan, H. A. ALGEBRAIC MULTIGRID PRECONDITIONER FOR SECOND ORDER FINITE ELEMENT APPROXIMATIONS IN RECTANGULAR DOMAINS I. TWO-LEVEL PRECONDITIONER. Proc. YSU A: Phys. Math. Sci. 2003, 37 (1 (200), 3-13. https://doi.org/10.46991/PYSUA.2003.37.1.003.