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Number theory Difficulty 6.9 National olympiad Prove it

Let p=ab+bc+acp=ab+bc+ac be a prime number where a,b,ca,b,c are different two by two, show that a3,b3,c3a^3,b^3,c^3 gives different residues modulo pp

Solution

To show that a3,b3,c3a^3, b^3, c^3 give different residues modulo pp, where p=ab+bc+acp = ab + bc + ac is a prime number and a,b,ca, b, c are distinct integers, we can proceed as follows:

1. Given Conditions and Polynomial Formation:
Since a,b,ca, b, c are distinct and p=ab+bc+acp = ab + bc + ac is a prime number, we know that a,b,ca, b, c are roots of the polynomial:
P(x)=(xa)(xb)(xc)=x3(a+b+c)x2+(ab+bc+ac)xabc P(x) = (x - a)(x - b)(x - c) = x^3 - (a+b+c)x^2 + (ab+bc+ac)x - abc
In the field Fp\mathbb{F}_p, this polynomial can be written as:
P(x)=x3(a+b+c)x2+pxabc P(x) = x^3 - (a+b+c)x^2 + px - abc
Since p=ab+bc+acp = ab + bc + ac, the polynomial simplifies to:
P(x)=x3(a+b+c)x2+0xabc=x3(a+b+c)x2abc P(x) = x^3 - (a+b+c)x^2 + 0 \cdot x - abc = x^3 - (a+b+c)x^2 - abc

2. **Non-zero Sum Modulo pp:**
Since a,b,ca, b, c are distinct, a+b+ca + b + c is non-zero modulo pp. This is because if a+b+c0(modp)a + b + c \equiv 0 \pmod{p}, then a,b,ca, b, c would not be distinct roots of the polynomial x3abcx^3 - abc modulo pp.

3. **Distinct Cubes Modulo pp:**
Consider the function f(x)=x3f(x) = x^3 in Fp\mathbb{F}_p. We need to show that a3,b3,c3a^3, b^3, c^3 are distinct modulo pp. Suppose for contradiction that a3b3(modp)a^3 \equiv b^3 \pmod{p}. Then:
a3b30(modp) a^3 - b^3 \equiv 0 \pmod{p}
This implies:
(ab)(a2+ab+b2)0(modp) (a - b)(a^2 + ab + b^2) \equiv 0 \pmod{p}
Since pp is a prime and aba \neq b, it must be that:
a2+ab+b20(modp) a^2 + ab + b^2 \equiv 0 \pmod{p}
Similarly, if b3c3(modp)b^3 \equiv c^3 \pmod{p} or a3c3(modp)a^3 \equiv c^3 \pmod{p}, we would get:
b2+bc+c20(modp)ora2+ac+c20(modp) b^2 + bc + c^2 \equiv 0 \pmod{p} \quad \text{or} \quad a^2 + ac + c^2 \equiv 0 \pmod{p}
However, since a,b,ca, b, c are distinct and a+b+c≢0(modp)a + b + c \not\equiv 0 \pmod{p}, these equations cannot hold simultaneously. Therefore, a3,b3,c3a^3, b^3, c^3 must be distinct modulo pp.

\blacksquare

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.