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Nineteenth IMAR Mathematical Competition

Number theory Difficulty 5.6 AIME, harder Prove it Romania

Let pp be an odd prime. Does there exist a permutation a1,a2,…,apa_1, a_2, \dots, a_p of 1,2,…,p1, 2, \dots, p satisfying
(i−j)ak+(j−k)ai+(k−i)aj≠0, (i-j)a_k + (j-k)a_i + (k-i)a_j \neq 0,
for all pairwise distinct i,j,ki, j, k?

Solution

The answer is in the affirmative. To define the desired permutation, let aa be a quadratic non-residue modulo pp, let iai≡a(modp)ia_i \equiv a \pmod p, i=1,2,…,p−1i = 1, 2, \dots, p-1, and let ap=pa_p = p. Clearly, the aia_i form a permutation of 1,2,…,p1, 2, \dots, p; moreover, ap−i=p−aia_{p-i} = p-a_i, i=1,2,…,p−1i = 1, 2, \dots, p-1, and, since aa is a quadratic non-residue modulo pp, ai≠ia_i \neq i, i=1,2,…,p−1i = 1, 2, \dots, p-1.

To prove (∗)(*), let first i,j,ki, j, k be all (strictly) less than pp. For convenience, write ≡\equiv for congruence modulo pp. Then
ijk((i−j)ak+(j−k)ai+(k−i)aj)≡ij(i−j)a+jk(j−k)a+ki(k−i)a=−a(i−j)(j−k)(k−i)≠0. \begin{aligned} ijk((i-j)a_k + (j-k)a_i + (k-i)a_j) &\equiv ij(i-j)a + jk(j-k)a + ki(k-i)a \\ &= -a(i-j)(j-k)(k-i) \neq 0. \end{aligned}
Let now one of i,j,ki, j, k be equal to pp. Since the left-hand member of (∗)(*) is antisymmetric in i,j,ki, j, k, we may and will assume that k=pk = p, so ak=ap=pa_k = a_p = p. Then
ij((i−j)ak+(j−k)ai+(k−i)aj)≡j2a−i2a=a(j−i)(j+i). ij((i-j)a_k + (j-k)a_i + (k-i)a_j) \equiv j^2a - i^2a = a(j-i)(j+i).
The latter is non-zero modulo pp, and (∗)(*) follows, unless j=p−ij = p-i, in which case aj=ap−i=p−aia_j = a_{p-i} = p-a_i, and
(i−j)ak+(j−k)ai+(k−i)aj=(2i−p)p−iai+(p−i)(p−ai)=p(i−ai)≠0, (i-j)a_k + (j-k)a_i + (k-i)a_j = (2i-p)p - ia_i + (p-i)(p-a_i) = p(i-a_i) \neq 0,
since ai≠ia_i \neq i. This completes the argument and concludes the proof.

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