Let A be an irreducible Coxeter arrangement and W be its Coxeter group. Then W naturally acts on A. A multiplicity m:A→Z is said to be equivariant when m is constant on each W-orbit of A. In this article, we prove that the multi-derivation module D(A,m) is a free module whenever m is equivariant by explicitly constructing a basis, which generalizes the main theorem of Terao (2002). . The main tool is a primitive derivation and its covariant derivative. Moreover, we show that the W-invariant part D(A,m)W for any multiplicity m is a free module over the W-invariant subring.
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