The q, t-Macdonald polynomials are conjectured by Garsia and Haiman to have a representation theoretic interpretation in terms of the S_n-module M_#mu# spanned by the derivatives of a certain polynomial #DELTA#_#mu#(x_1, x_2, ..., x_n; y_1, y_2, ..., y_n). The diagonal action of a permutation #sigma# implied by S_n on a polynomial P = P(x_1, x_2, ..., x_n; y_1, y_2, ..., y_n) is defined by setting #sigma#P = P(x_(#sigma#_1), x_(#sigma#_2), ..., x_(#sigma#_n); y_(#sigma#_1), y_(#sigma#_2), ..., y_(#sigma#_n)). Since the polynomial #DELTA#_#mu# alternates under the diagonal action, M_#mu# is S_n-invariant. We analyze here the diagonal action of S_n on M_#mu# and show that its decomposition into irreducibles is equivalent to a certain isotypic expansion for the translate #DELTA#_#mu#(x_1 + x'_1, x_2 + x'_2, ..., x_n + x'_n; y_1 + y'_1, y_2 + y'_2, ..., y_n + y'_n) of the polynomial #DELTA#_#mu#.
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