1. Numerical Methods for Grid Equations. V. II. Iterative Methods;Samarskii,1989
2. Multigrid Methods or Finite Elements;Shaidurov,1995
3. Linear and Quasilinear Equations of Elliptic Type;Ladyz̆enskaja,1973
4. The Finite Element Method for Elliptic Problems;Ciarlet,1978
5. Cascadic conjugate gradient methods for elliptic partial differential equations I. Algorithm and numerical results;Deuflhard,1993