function [P1, P2 ] = Precondition(A)
  D = diag(diag(A));
  L = tril(A) - D;
  U = triu(A) - D;
  
  du = inv(D+U);
  dl = inv(D+L);
  
  P1 = du * L * dl * U;
  P2 = du - du * L * dl;