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In this paper, we are concerned with the fractional order equations (1) with Hartree typeḢ α 2-critical nonlinearity and its equivalent integral equations (3). We first prove a regularity result which indicates that weak solutions are smooth (Theorem 1.2). Then, by applying the method of moving planes in integral forms, we prove that positive solutions u to (1) and (3) are radially symmetric about some point x 0 ∈ R d and derive the explicit forms for u (Theorem 1.3 and Corollary 1). As a consequence, we also derive the best constants and extremal functions in the corresponding Hardy-Littlewood-Sobolev inequalities (Corollary 2).

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