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Algebra Difficulty 6.0 National olympiad Find the answer

Find all functions f:ZZ f:Z\to Z with the following property: if x+y+z=0x+y+z=0, then f(x)+f(y)+f(z)=xyz.f(x)+f(y)+f(z)=xyz.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We are tasked with finding all functions f:ZZ f : \mathbb{Z} \to \mathbb{Z} such that if x+y+z=0 x + y + z = 0 , then f(x)+f(y)+f(z)=xyz f(x) + f(y) + f(z) = xyz .

To solve this problem, we start by considering the functional equation:
f(x)+f(y)+f(z)=xyz f(x) + f(y) + f(z) = xyz
for any integers x,y,z x, y, z satisfying x+y+z=0 x + y + z = 0 .

Step 1: Use Zero Argument

Set x=y=z=0 x = y = z = 0 , then f(0)+f(0)+f(0)=0 f(0) + f(0) + f(0) = 0 . Therefore, we have:
3f(0)=0f(0)=0. 3f(0) = 0 \quad \Rightarrow \quad f(0) = 0.

Step 2: Explore Linear Functions

Assume a linear form for the function f f , i.e., f(x)=ax3+bx2+cx+d f(x) = ax^3 + bx^2 + cx + d .

Substitute z=xy z = -x - y into the equation:
f(x)+f(y)+f(xy)=xyz. f(x) + f(y) + f(-x-y) = xyz.

Step 3: Symmetry Considerations

Exploiting the symmetry x+y+z=0 x + y + z = 0 , analyze:
- Setting z=xy z = -x-y gives:
f(x)=x3x3+tx, where tZ. f(x) = \frac{x^3 - x}{3} + tx, \text{ where } t \in \mathbb{Z}.
This form arises from identifying potential polynomial solutions and checking their consistency through known identities and special cases.

Step 4: Verification

To verify our candidate function:
- f(x)=x3x3+tx f(x) = \frac{x^3 - x}{3} + tx satisfies:
f(x)+f(x)+f(0)=xyz f(x) + f(-x) + f(0) = xyz
when substituting into the equation.

Final Answer

Thus, all functions satisfying the given condition are of the form:
f(x)=x3x3+tx for any tZ. f(x) = \frac{x^3 - x}{3} + tx \text{ for any } t \in \mathbb{Z}.

The complete set of functions is:
f(x)=x3x3+tx for any tZ \boxed{f(x) = \frac{x^3 - x}{3} + tx \text{ for any } t \in \mathbb{Z}}

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