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Number theory Difficulty 6.3 National Olympiad Prove it Canada

The sequence a1,a2,,ana_1, a_2, \dots, a_n consists of the numbers 1,2,,n1, 2, \dots, n in some order. For which positive integers nn is it possible that the n+1n+1 numbers 0,a1,a1+a2,a1+a2+a3,,a1+a2++an0, a_1, a_1+a_2, a_1+a_2+a_3, \dots, a_1+a_2+\dots+a_n all have different remainders when divided by n+1n+1?

Solution

Solution: It is possible if and only if nn is odd.
If nn is even, then a1+a2++an=1+2++n=n2(n+1)a_1 + a_2 + \dots + a_n = 1 + 2 + \dots + n = \frac{n}{2} \cdot (n + 1), which is congruent to 00 mod n+1n + 1. Therefore, the task is impossible.

Now suppose nn is odd. We will show that we can construct a1,a2,,ana_1, a_2, \dots, a_n that satisfy the conditions given in the problem. Then let n=2k+1n = 2k + 1 for some non-negative integer kk. Consider the sequence: 1,2k,3,2k2,5,2k3,,2,2k+11, 2k, 3, 2k-2, 5, 2k-3, \dots, 2, 2k+1, i.e. for each 1i2k+11 \le i \le 2k+1, ai=ia_i = i if ii is odd and ai=2k+2ia_i = 2k+2-i if ii is even.

We first show that each term 1,2,,2k+11, 2, \dots, 2k+1 appears exactly once. Clearly, there are 2k+12k+1 terms. For each odd number mm in {1,2,,2k+1}\{1, 2, \dots, 2k+1\}, am=ma_m = m. For each even number mm in this set, a2k+2m=2k+2(2k+2m)=ma_{2k+2-m} = 2k+2-(2k+2-m)=m. Hence, every number appears in a1,,a2k+1a_1, \dots, a_{2k+1}. Hence, a1,,a2k+1a_1, \dots, a_{2k+1} does consist of the numbers 1,2,,2k+11, 2, \dots, 2k+1 in some order.

We now determine a1+a2++am(mod2k+2)a_1 + a_2 + \dots + a_m \pmod{2k+2}. We will consider the cases when mm is odd and when mm is even separately. Let bm=a1+a2++amb_m = a_1 + a_2 + \dots + a_m.

If mm is odd, note that a11(mod2k+2)a_1 \equiv 1 \pmod{2k+2}, a2+a3=a4+a5==a2k+a2k+1=2k+31(mod2k+2)a_2 + a_3 = a_4 + a_5 = \dots = a_{2k} + a_{2k+1} = 2k+3 \equiv 1 \pmod{2k+2}. Therefore, {b1,b3,,b2k+1}={1,2,3,,k+1}(mod2k+2)\{b_1, b_3, \dots, b_{2k+1}\} = \{1, 2, 3, \dots, k+1\} \pmod{2k+2}.

If mm is even, note that a1+a2=a3+a4==a2k1+a2k=2k+11(mod2k+2)a_1 + a_2 = a_3 + a_4 = \dots = a_{2k-1} + a_{2k} = 2k+1 \equiv -1 \pmod{2k+2}. Therefore, {b2,b4,,b2k}={1,2,,k}(mod2k+2){2k+1,2k,,k+2}(mod2k+2)\{b_2, b_4, \dots, b_{2k}\} = \{-1, -2, \dots, -k\} \pmod{2k+2} \equiv \{2k+1, 2k, \dots, k+2\} \pmod{2k+2}.

Therefore, b1,b2,,b2k+1b_1, b_2, \dots, b_{2k+1} do indeed have different remainders when divided by 2k+22k+2. This completes the problem. \square

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.