MathDB
Vietnam TST 2003 set of all permutations

Source: Vietnam TST 2003 for the 44th IMO, problem 5

June 26, 2005
inequalitiestriangle inequalitycombinatorics unsolvedcombinatorics

Problem Statement

Let AA be the set of all permutations a=(a1,a2,,a2003)a = (a_1, a_2, \ldots, a_{2003}) of the 2003 first positive integers such that each permutation satisfies the condition: there is no proper subset SS of the set {1,2,,2003}\{1, 2, \ldots, 2003\} such that {akkS}=S.\{a_k | k \in S\} = S. For each a=(a1,a2,,a2003)Aa = (a_1, a_2, \ldots, a_{2003}) \in A, let d(a)=k=12003(akk)2.d(a) = \sum^{2003}_{k=1} \left(a_k - k \right)^2. I. Find the least value of d(a)d(a). Denote this least value by d0d_0. II. Find all permutations aAa \in A such that d(a)=d0d(a) = d_0.