Ziegler's Multi-Reflection Arrangements are free
  In 1989, Ziegler introduced the concept of a multi-arrangement. One natural example is the reflection arrangement of a unitary reflection group with multiplicity given by the number of reflections associated with each hyperplane. For all but three irreducible groups, Ziegler showed that each such multi-reflection arrangement is free. We complete Ziegler's example by confirming these outstanding cases. 
Torsten Hoge, Gerhard Roehrle
math.GR math.AC math.CO