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Multiplication table for the new product for n=11 and n=12

Source: IMO Longlist 1989, Problem 1

September 18, 2008
algebra unsolvedalgebra

Problem Statement

In the set S_n \equal{} \{1, 2,\ldots ,n\} a new multiplication ab a*b is defined with the following properties: (i) c \equal{} a * b is in Sn S_n for any aSn,bSn. a \in S_n, b \in S_n. (ii) If the ordinary product ab a \cdot b is less than or equal to n, n, then a*b \equal{} a \cdot b. (iii) The ordinary rules of multiplication hold for , *, i.e.: (1) a * b \equal{} b * a (commutativity) (2) (a * b) * c \equal{} a * (b * c) (associativity) (3) If a * b \equal{} a * c then b \equal{} c (cancellation law). Find a suitable multiplication table for the new product for n \equal{} 11 and n \equal{} 12.