To show that a3,b3,c3 give different residues modulo p, where p=ab+bc+ac is a prime number and a,b,c are distinct integers, we can proceed as follows:
1. Given Conditions and Polynomial Formation:
Since a,b,c are distinct and p=ab+bc+ac is a prime number, we know that a,b,c are roots of the polynomial:
P(x)=(x−a)(x−b)(x−c)=x3−(a+b+c)x2+(ab+bc+ac)x−abc
In the field Fp, this polynomial can be written as:
P(x)=x3−(a+b+c)x2+px−abc
Since p=ab+bc+ac, the polynomial simplifies to:
P(x)=x3−(a+b+c)x2+0⋅x−abc=x3−(a+b+c)x2−abc
2. **Non-zero Sum Modulo p:**
Since a,b,c are distinct, a+b+c is non-zero modulo p. This is because if a+b+c≡0(modp), then a,b,c would not be distinct roots of the polynomial x3−abc modulo p.
3. **Distinct Cubes Modulo p:**
Consider the function f(x)=x3 in Fp. We need to show that a3,b3,c3 are distinct modulo p. Suppose for contradiction that a3≡b3(modp). Then:
a3−b3≡0(modp)
This implies:
(a−b)(a2+ab+b2)≡0(modp)
Since p is a prime and a=b, it must be that:
a2+ab+b2≡0(modp)
Similarly, if b3≡c3(modp) or a3≡c3(modp), we would get:
b2+bc+c2≡0(modp)ora2+ac+c2≡0(modp)
However, since a,b,c are distinct and a+b+c≡0(modp), these equations cannot hold simultaneously. Therefore, a3,b3,c3 must be distinct modulo p.
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