Before revision:  The proposed FD method has 2-\alpha order of accuracy with respect to time, where \alpha \in(0, 1) is the subdiffusion parameter, and 2 with respect to space .
After revision:  The proposed FD method has 2-\alpha order of accuracy with respect to time, where \alpha \in(0,1) is the subdiffusion parameter, and 2 with respect to space .