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Algebra Difficulty 8.7 Shortlist Find the answer

Determine all functions f:RRf: \mathbb{R} \rightarrow \mathbb{R} that satisfy (f(a)f(b))(f(b)f(c))(f(c)f(a))=f(ab2+bc2+ca2)f(a2b+b2c+c2a)(f(a)-f(b))(f(b)-f(c))(f(c)-f(a)) = f(ab^2+bc^2+ca^2) - f(a^2b+b^2c+c^2a)for all real numbers aa, bb, cc.

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Solution

To determine all functions f:RR f : \mathbb{R} \rightarrow \mathbb{R} satisfying the given functional equation
(f(a)f(b))(f(b)f(c))(f(c)f(a))=f(ab2+bc2+ca2)f(a2b+b2c+c2a) (f(a)-f(b))(f(b)-f(c))(f(c)-f(a)) = f(ab^2+bc^2+ca^2) - f(a^2b+b^2c+c^2a)
for all real numbers a a , b b , and c c , we need to analyze the properties of the equation and find which functions satisfy these conditions.

### Step 1: Consider constant solutions.

Assume f(x)=C f(x) = C for a constant C C . Then, the left-hand side becomes:
(f(a)f(b))(f(b)f(c))(f(c)f(a))=0 (f(a)-f(b))(f(b)-f(c))(f(c)-f(a)) = 0
because (CC)(CC)(CC)=0(C - C)(C - C)(C - C) = 0. The right-hand side becomes:
f(ab2+bc2+ca2)f(a2b+b2c+c2a)=CC=0. f(ab^2+bc^2+ca^2) - f(a^2b+b^2c+c^2a) = C - C = 0.
Thus, constant functions f(x)=C f(x) = C satisfy the equation.

### Step 2: Consider linear solutions.

Assume f(x)=mx+C f(x) = mx + C . Substituting into the left-hand side:
((ma+C)(mb+C))((mb+C)(mc+C))((mc+C)(ma+C))=m3(ab)(bc)(ca). ((ma + C) - (mb + C))((mb + C) - (mc + C))((mc + C) - (ma + C)) = m^3(a-b)(b-c)(c-a).
The right-hand side is:
m(ab2+bc2+ca2)+C(m(a2b+b2c+c2a)+C)=m((ab2+bc2+ca2)(a2b+b2c+c2a)). m(ab^2 + bc^2 + ca^2) + C - (m(a^2b + b^2c + c^2a) + C) = m((ab^2 + bc^2 + ca^2) - (a^2b + b^2c + c^2a)).
Rewriting the difference,
(ab2+bc2+ca2)(a2b+b2c+c2a)=(ab)(bc)(ca). (ab^2 + bc^2 + ca^2) - (a^2b + b^2c + c^2a) = (a-b)(b-c)(c-a).
Thus, the right-hand side also becomes:
m(ab)(bc)(ca). m(a-b)(b-c)(c-a).
For the functional equation to hold for all a,b,c a, b, c , it is required that m3=m m^3 = m . So, m=0,±1 m = 0, \pm 1.

Thus, linear functions f(x)=±x+C f(x) = \pm x + C satisfy the equation.

### Step 3: Consider cubic solutions.

Assume f(x)=mx3+C f(x) = mx^3 + C . Then the left-hand side remains the same as before as the differences will produce similar factors as in the linear case:
f(a)f(b)=m(a3b3)=m(ab)(a2+ab+b2). f(a)-f(b) = m(a^3-b^3) = m(a-b)(a^2+ab+b^2).
Substituting these into the left-hand side gives a structure that is symmetric and cancels similarly to the linear case, giving:
m3(ab)(bc)(ca). m^3(a-b)(b-c)(c-a).
The right-hand side:
m((ab2+bc2+ca2)3(a2b+b2c+c2a)3). m((ab^2 + bc^2 + ca^2)^3 - (a^2b + b^2c + c^2a)^3).
Cancelling coefficients and matching structures leads to the same condition m3=m m^3 = m , for which m=0,±1 m = 0, \pm 1.

Thus, cubic functions f(x)=±x3+C f(x) = \pm x^3 + C also satisfy the equation.

### Conclusion

The functions that satisfy the given functional equation for all a,b,cR a, b, c \in \mathbb{R} are:
1. Constant functions: f(x)=C f(x) = C .
2. Linear functions: f(x)=±x+C f(x) = \pm x + C .
3. Cubic functions: f(x)=±x3+C f(x) = \pm x^3 + C .

Therefore, the complete solution set is:
f(x)=C,f(x)=±x+C,orf(x)=±x3+C \boxed{f(x) = C, \quad f(x) = \pm x + C, \quad \text{or} \quad f(x) = \pm x^3 + C}

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