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MR1683473 (2000a:90048) Avella, Pasquale; Sforza, Antonio Logical reduction tests for the $p$-median problem. Advances in combinatorial optimization (London, 1996). Ann. Oper. Res. 86 (1999), 105--115. (Reviewer: Sandra L. Arlinghaus) 90C27 (90B80)