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Number theory Difficulty 6.0 AIME, harder Prove it Romania

Fix an integer n3n \ge 3 and let a0=na_0 = n. Does there exist a permutation a1,a2,,an1a_1, a_2, \dots, a_{n-1} of the first n1n-1 positive integers such that j=0k1aj\sum_{j=0}^{k-1} a_j is divisible by aka_k for all indices k<nk < n?

Solution

The answer is in the affirmative. If nn is odd, set a1=1a_1 = 1 and a2=2a_2 = 2, and if n>3n > 3 define the other aka_k recursively by akak1=a0+a1++ak1a_k a_{k-1} = a_0 + a_1 + \dots + a_{k-1}, k=3,,nk = 3, \dots, n. It is easily seen that ak+1=ak1+1a_{k+1} = a_{k-1} + 1, k=3,,n1k = 3, \dots, n-1, so a2k=k+1a_{2k} = k+1 and a2k+1=k+(n+1)/2a_{2k+1} = k + (n+1)/2, k=1,,(n1)/2k = 1, \dots, (n-1)/2. By construction, the integers aka_k satisfy the divisibility condition (even at k=nk = n). The a2ka_{2k} and a2k+1a_{2k+1}, k=1,,(n1)/2k = 1, \dots, (n-1)/2, form strictly increasing sequences of integers greater than 1, and no a2ia_{2i} equals an a2j+1a_{2j+1}, i,j=1,,(n1)/2i, j = 1, \dots, (n-1)/2, so the aka_k, k=1,,n1k = 1, \dots, n-1, form an injective sequence of positive integers. Since they all lie in the range 1,2,,n11, 2, \dots, n-1, they form indeed a permutation of the latter.

Similarly, if nn is even, set a1=2a_1 = 2 and define aka_k, k=2,,n2k = 2, \dots, n-2, by the same recursive relation, to get a2k1=k+1a_{2k-1} = k+1 and a2k=k+n/2a_{2k} = k + n/2, k=1,,n/21k = 1, \dots, n/2 - 1. Setting an1=1a_{n-1} = 1 settles the case as above and completes the proof.

Remark. For an odd n>3n > 3, we may equally well start by setting a1=1a_1 = 1 and a2=(n+1)/2a_2 = (n+1)/2, and define the remaining aka_k by the recursive relation in the solution, to obtain another sequence of positive integers satisfying the divisibility condition. Explicitly, this time a2k=k+(n1)/2a_{2k} = k + (n-1)/2, k=1,,(n1)/2k = 1, \dots, (n-1)/2, and a2k+1=k+2a_{2k+1} = k + 2,

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