The tide of reckoning on systemic racism and police brutality that has been sweeping through institutions — including scientific ones — has reached universities’ normally reclusive mathematics departments. A group of mathematicians in the United States has written a letter calling for their colleagues to stop collaborating with police because of the widely documented disparities in how US law-enforcement agencies treat people of different races and ethnicities. They concentrate their criticism on predictive policing, a maths-based technique aimed at stopping crime before it occurs.