We need to find all functions f:R→R satisfying the given functional equation for all x,y,z∈R:
f(f(x)+f(y)+f(z))=f(f(x)−f(y))+f(2xy+f(z))+2f(xz−yz).
To solve this, we'll explore potential forms of f(x) and check if they satisfy the equation.
### Step 1: Check Constant Solutions
First, consider the possibility that f(x) is a constant function, say f(x)=c for all x∈R.
Substituting f(x)=c into the equation, we have:
f(f(x)+f(y)+f(z))=f(3c)=c,
and
f(f(x)−f(y))+f(2xy+f(z))+2f(xz−yz)=c+c+2c=4c.
For the equality to hold, we must have 4c=c, which implies c=0. So, f(x)=0 for all x is a solution.
### Step 2: Check Non-constant Polynomial Solutions
Assume that f(x) is a non-constant polynomial. Checking degree 2 polynomials, let f(x)=x2.
Substitute f(x)=x2 into the functional equation:
Left-hand side:
f(f(x)+f(y)+f(z))=f(x2+y2+z2)=(x2+y2+z2)2.
Right-hand side:
f(f(x)−f(y))+f(2xy+f(z))+2f(xz−yz)=f(x2−y2)+f(2xy+z2)+2f(xz−yz).
Calculate each term:
- f(x2−y2)=(x2−y2)2,
- f(2xy+z2)=(2xy+z2)2,
- 2f(xz−yz)=2((xz−yz)2)=2(xz−yz)2.
Upon substitution, we need:
(x2+y2+z2)2=(x2−y2)2+(2xy+z2)2+2(xz−yz)2.
This identity holds, showing that f(x)=x2 is a solution.
### Conclusion
The following functions satisfy the given identity for all x,y,z∈R:
f(x)=0andf(x)=x2.
Thus, the functions are:
f(x)=0 and f(x)=x2
These are the solutions to the functional equation.