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(d1−d2)k/(2M)(d_1 - d_2)k/(2M)(d1−d2)k/(2M)
(d1−d2)k/M(d_1 - d_2)k/M(d1−d2)k/M
2kM/(d1−d2)2kM/(d_1 - d_2)2kM/(d1−d2)