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Number theory Difficulty 6.2 National olympiad Prove it Ukraine

Prime number p>2p > 2 and the polynomial QQ with integer coefficients are such that there do not exist two positive integers i,ji, j, such that i<j<pi < j < p and the number (Q(j)Q(i))(jQ(j)iQ(i))(Q(j) - Q(i))(jQ(j) - iQ(i)) is divisible by pp. What's the smallest possible degree of QQ?

Solution

Example: Q(x)=xp21Q(x) = x^{p-2} - 1. For any i<j<pi < j < p:
Q(j)Q(i)=yp2xp21y1x(modp), Q(j) - Q(i) = y^{p-2} - x^{p-2} \equiv \frac{1}{y} - \frac{1}{x} \pmod{p},
which clearly isn't divisible by pp, and
jQ(j)iQ(i)=(yp1xp1)(yx)(xy)(modp) jQ(j) - iQ(i) = (y^{p-1} - x^{p-1}) - (y - x) \equiv (x - y) \pmod{p}
also isn't divisible by pp.

Suppose that there exists such a polynomial of smaller degree. It follows from the statement that all of Q(1),Q(2),,Q(p1)Q(1), Q(2), \dots, Q(p-1) give different remainders modulo pp, and also 1Q(1),2Q(2),,(p1)Q(p1)1Q(1), 2Q(2), \dots, (p-1)Q(p-1) give different remainders modulo pp. Let's use the following fact: 1k+2k++(p1)k0(modp)1^k + 2^k + \dots + (p-1)^k \equiv 0 \pmod{p} for any kk from 11 to p2p-2. Then consider 1Q(1)+2Q(2)++(p1)Q(p1)1Q(1) + 2Q(2) + \dots + (p-1)Q(p-1). From the fact above, this number is 00 modulo pp (as each degree in the polynomial xQ(x)xQ(x) is in the range [1,p2][1, p-2]). Suppose that among the numbers 1P(1),,(p1)P(p1)1P(1), \dots, (p-1)P(p-1) there are all remainders modulo pp, except for xx. Then:
00+1++(p1)1Q(1)+2Q(2)++(p1)Q(p1)+xx(modp), 0 \equiv 0 + 1 + \dots + (p-1) \equiv 1Q(1) + 2Q(2) + \dots + (p-1)Q(p-1) + x \equiv x \pmod{p},
so 1Q(1),2Q(2),,(p1)Q(p1)1Q(1), 2Q(2), \dots, (p-1)Q(p-1) modulo pp give remainders 1,2,,p11, 2, \dots, p-1 in some order. Then none of Q(1),Q(2),,Q(p1)Q(1), Q(2), \dots, Q(p-1) is divisible by pp, and they also give remainders 1,2,,p11, 2, \dots, p-1 in some order.
But then:
(p1)!1Q(1)×2Q(2)××(p1)Q(p1)(p1)!2(modp), (p-1)! \equiv 1Q(1) \times 2Q(2) \times \dots \times (p-1)Q(p-1) \equiv (p-1)!^2 \pmod{p},
which, by the Wilson theorem, gives, 11(modp)-1 \equiv 1 \pmod{p}, this contradiction completes the proof.

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