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Algebra Difficulty 6.5 National olympiad Find the answer

Find all the functions f:ZZf: \mathbb{Z}\to \mathbb{Z} satisfying the following property: if aa, bb and cc are integers such that a+b+c=0a+b+c=0, then

f(a)+f(b)+f(c)=a2+b2+c2.f(a)+f(b)+f(c)=a^2+b^2+c^2.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let f:ZZ f: \mathbb{Z} \to \mathbb{Z} be a function satisfying the property that for any integers a a , b b , and c c such that a+b+c=0 a + b + c = 0 , the equation

f(a)+f(b)+f(c)=a2+b2+c2 f(a) + f(b) + f(c) = a^2 + b^2 + c^2

holds. We need to determine all such functions f f .

### Step 1: Understand the Condition

Since a+b+c=0 a + b + c = 0 , we have c=(a+b) c = -(a+b) . Substituting c=(a+b) c = -(a+b) into the given equation:

f(a)+f(b)+f((a+b))=a2+b2+((a+b))2. f(a) + f(b) + f(-(a+b)) = a^2 + b^2 + (-(a+b))^2.

Simplifying the right-hand side:

=a2+b2+(a+b)2=a2+b2+a2+2ab+b2=2a2+2b2+2ab. = a^2 + b^2 + (a+b)^2 = a^2 + b^2 + a^2 + 2ab + b^2 = 2a^2 + 2b^2 + 2ab.

Thus, the equation becomes:

f(a)+f(b)+f((a+b))=2a2+2b2+2ab. f(a) + f(b) + f(-(a+b)) = 2a^2 + 2b^2 + 2ab.

### Step 2: Exploratory Substitution

Consider a=x a = x , b=0 b = 0 , and c=x c = -x . Thus, we have:

f(x)+f(0)+f(x)=x2+02+(x)2=2x2. f(x) + f(0) + f(-x) = x^2 + 0^2 + (-x)^2 = 2x^2.

### Step 3: Deriving the Form of f f

By the above, we have:

f(x)+f(x)=2x2f(0). f(x) + f(-x) = 2x^2 - f(0).

Now, observe the function:

We substitute f(x)=x2+cx f(x) = x^2 + cx into the condition to verify it satisfies the equation:

f(x)+f(y)+f((x+y))=(x2+cx)+(y2+cy)+((x+y))2c(x+y). f(x) + f(y) + f(-(x+y)) = (x^2 + cx) + (y^2 + cy) + (-(x+y))^2 - c(x+y).

Simplifying:

=x2+cx+y2+cy+(x+y)2+c(x+y). = x^2 + cx + y^2 + cy + (x+y)^2 + c(x+y).
=x2+y2+x2+2xy+y2+cx+cycxcy. = x^2 + y^2 + x^2 + 2xy + y^2 + cx + cy - cx - cy.
=2x2+2y2+2xy. = 2x^2 + 2y^2 + 2xy.

Thus, the form f(x)=x2+cx f(x) = x^2 + cx satisfies the condition.

### Conclusion

The solution to the functional equation is a function of the form:

f(x)=x2+cxfor any integer c. f(x) = x^2 + cx \quad \text{for any integer } c.

Thus, all functions f:ZZ f: \mathbb{Z} \to \mathbb{Z} satisfying the given property are:

f(x)=x2+cx for any integer c. \boxed{f(x) = x^2 + cx \text{ for any integer } c}.

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