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Algebra Difficulty 7.0 National olympiad, round 2 Find the answer

Let f=f(x,y,z)f = f(x,y,z) be a polynomial in three variables xx, yy, zz such that f(w,w,w)=0f(w,w,w) = 0 for all wRw \in \mathbb{R}. Show that there exist three polynomials AA, BB, CC in these same three variables such that A+B+C=0A + B + C = 0 and f(x,y,z)=A(x,y,z)(xy)+B(x,y,z)(yz)+C(x,y,z)(zx). f(x,y,z) = A(x,y,z) \cdot (x-y) + B(x,y,z) \cdot (y-z) + C(x,y,z) \cdot (z-x). Is there any polynomial ff for which these AA, BB, CC are uniquely determined?

Solution

1. Given that f(w,w,w)=0 f(w, w, w) = 0 for all wR w \in \mathbb{R} , we need to show that there exist polynomials A(x,y,z) A(x, y, z) , B(x,y,z) B(x, y, z) , and C(x,y,z) C(x, y, z) such that A+B+C=0 A + B + C = 0 and
f(x,y,z)=A(x,y,z)(xy)+B(x,y,z)(yz)+C(x,y,z)(zx). f(x, y, z) = A(x, y, z) \cdot (x - y) + B(x, y, z) \cdot (y - z) + C(x, y, z) \cdot (z - x).

2. Let C=AB C = -A - B . Then we can rewrite f f as:
f=A(xy)+B(yz)+(AB)(zx). f = A \cdot (x - y) + B \cdot (y - z) + (-A - B) \cdot (z - x).
Simplifying this, we get:
f=A(xy)+B(yz)A(zx)B(zx). f = A \cdot (x - y) + B \cdot (y - z) - A \cdot (z - x) - B \cdot (z - x).
f=A(xy)A(zx)+B(yz)B(zx). f = A \cdot (x - y) - A \cdot (z - x) + B \cdot (y - z) - B \cdot (z - x).
f=A(xy+xz)+B(yz+xz). f = A \cdot (x - y + x - z) + B \cdot (y - z + x - z).
f=A(2xyz)+B(y+x2z). f = A \cdot (2x - y - z) + B \cdot (y + x - 2z).

3. We need to show that there exists a polynomial B B such that A=B(y+x2z)fy2x+z A = \frac{B \cdot (y + x - 2z) - f}{y - 2x + z} is a polynomial.

4. Consider f f as a polynomial in y y and perform the division of f f by (y+x2z) (y + x - 2z) . We get:
f=g(y+x2z)+h, f = g \cdot (y + x - 2z) + h,
where h h is a polynomial in x x and z z .

5. From the condition f(w,w,w)=0 f(w, w, w) = 0 , we have:
0=f(w,w,w)=g(w,w,w)(w+w2w)+h(w,w). 0 = f(w, w, w) = g(w, w, w) \cdot (w + w - 2w) + h(w, w).
This implies that h(w,w)=0 h(w, w) = 0 for every real w w .

6. It is well-known that xz x - z divides h h , so we let h=s(xz) h = s \cdot (x - z) , where s s is a polynomial in x x and z z .

7. Therefore, we have:
A=g+B(y+x2z)s(xz)y2x+z. A = -g + \frac{B \cdot (y + x - 2z) - s \cdot (x - z)}{y - 2x + z}.

8. Let B=B1(y2x+z)+s3 B = B_1 \cdot (y - 2x + z) + \frac{s}{3} . Then we have:
A=g+B1(y+x2z)+s3(y+x2z)s(xz)y2x+z. A = -g + B_1 \cdot (y + x - 2z) + \frac{\frac{s}{3} \cdot (y + x - 2z) - s \cdot (x - z)}{y - 2x + z}.
Simplifying, we get:
A=g+B1(y+x2z)+s, A = -g + B_1 \cdot (y + x - 2z) + s,
which is clearly a polynomial.

9. Since there are infinitely many polynomials B1 B_1 we can choose, there is no polynomial f f such that A A , B B , and C C can be uniquely determined.

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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.