Before revision:  With a unified view to the SSE method and other existing ones through the proposed variational principle, we analyze the convergence behavior of the SSE method and explain the reason for  improved performance compared to others. Numerical experiments that support our theoretical results are presented .
After revision:  With a unified view to the SSE method and other existing ones through the proposed variational principle, we analyze the convergence behavior of the SSE method and explain the reason for improved performance compared to other methods. We also present numerical experiments that support our theoretical results are presented .