There is an error in Proposition 3.2. This arose because the symbols considered in this section of the paper are not exactly the same as those of section 1 (one must replace $f_j$ in the displayed equation after 3.1 with its transform by multiplication by -1).
In the formula of 3.2, one should consequently read $(\nu+\mu)$ in place of $(\nu-\mu)$.
Other results in the paper are not affected.