Two kinds of Stencil elimination schemes with preserved symmetry are presented.Correlative symmetric positive definite difference equations are obtained.Condition number of coefficient matrix decreases over 7/9 folding ratio than that of five point difference Jacobi's.Their eigenvalues have a good clustered spectrum.Theoretic analysis and numerical experiments show that they are better than un-symmetric ones,and are more useful.

A reordering method,named alternate hyperplane ordering,is proposed to solve linear systems from two-dimension three-temperature nonlinear energy equations.Numerical experiments are performed with Krylov subspace iterative associated ILU(k) preconditioning.It is showed that with nearly same preconditioning cost,the proposed ordering method is better than red-black ordering and hyperplane ordering etc.