非常に疲れた。 Not only P2221 but also P212121 is a subgroup of C2221 P212121 has the following operators: x, y, z: Identity 1/2 + x, 1/2 - y, -z: 2_1 screw at (x, 1/4, 0) -x, 1/2 + y, 1/2 - z: 2_1 screw at (0, y, 1/4) 1/2 - x, -y, 1/2 + z: 2_1 screw at (1/4, 0, z) Addition of a C centering produces the fo…