Y. R. Hakopian and H. A. Hovhannisyan, “ALGEBRAIC MULTIGRID PRECONDITIONER FOR SECOND ORDER FINITE ELEMENT APPROXIMATIONS IN RECTANGULAR DOMAINS I. TWO-LEVEL PRECONDITIONER”, Proc. YSU A: Phys. Math. Sci., vol. 37, no. 1 (200), pp. 3–13, Mar. 2003, doi: 10.46991/PYSUA.2003.37.1.003.