“…The simple problem can be solved by many different numerical methods, such as immersed boundary method (IBM) [27], immersed interface method (IIM) [18], and variants of finite element methods [5,6,13,15]. It is known that IBM for the problem only yields first-order accuracy in the maximum norm [21,26] and IIM generates second-order accuracy in the maximum norm for both the numerical solution and its gradient [2,22,23].…”