To determine all functions f:R→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)
for all real numbers a, b, and 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 for a constant C. Then, the left-hand side becomes:
(f(a)−f(b))(f(b)−f(c))(f(c)−f(a))=0
because (C−C)(C−C)(C−C)=0. The right-hand side becomes:
f(ab2+bc2+ca2)−f(a2b+b2c+c2a)=C−C=0.
Thus, constant functions f(x)=C satisfy the equation.
### Step 2: Consider linear solutions.
Assume f(x)=mx+C. Substituting into the left-hand side:
((ma+C)−(mb+C))((mb+C)−(mc+C))((mc+C)−(ma+C))=m3(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)).
Rewriting the difference,
(ab2+bc2+ca2)−(a2b+b2c+c2a)=(a−b)(b−c)(c−a).
Thus, the right-hand side also becomes:
m(a−b)(b−c)(c−a).
For the functional equation to hold for all a,b,c, it is required that m3=m. So, m=0,±1.
Thus, linear functions f(x)=±x+C satisfy the equation.
### Step 3: Consider cubic solutions.
Assume f(x)=mx3+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(a3−b3)=m(a−b)(a2+ab+b2).
Substituting these into the left-hand side gives a structure that is symmetric and cancels similarly to the linear case, giving:
m3(a−b)(b−c)(c−a).
The right-hand side:
m((ab2+bc2+ca2)3−(a2b+b2c+c2a)3).
Cancelling coefficients and matching structures leads to the same condition m3=m, for which m=0,±1.
Thus, cubic functions f(x)=±x3+C also satisfy the equation.
### Conclusion
The functions that satisfy the given functional equation for all a,b,c∈R are:
1. Constant functions: f(x)=C.
2. Linear functions: f(x)=±x+C.
3. Cubic functions: f(x)=±x3+C.
Therefore, the complete solution set is:
f(x)=C,f(x)=±x+C,orf(x)=±x3+C