Maths Olympiad Prep

Library / /17 of 48

Algebra Difficulty 7.2 National olympiad, round 2 Find the answer

Find all functions ff from the set R\mathbb{R} of real numbers into R\mathbb{R} which satisfy for all x,y,zRx, y, z \in \mathbb{R} the identity f(f(x)+f(y)+f(z))=f(f(x)f(y))+f(2xy+f(z))+2f(xzyz).f(f(x)+f(y)+f(z))=f(f(x)-f(y))+f(2xy+f(z))+2f(xz-yz).

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

Solution

We need to find all functions f:RR f: \mathbb{R} \rightarrow \mathbb{R} satisfying the given functional equation for all x,y,zR x, y, z \in \mathbb{R} :

f(f(x)+f(y)+f(z))=f(f(x)f(y))+f(2xy+f(z))+2f(xzyz). 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) f(x) and check if they satisfy the equation.

### Step 1: Check Constant Solutions

First, consider the possibility that f(x) f(x) is a constant function, say f(x)=c f(x) = c for all xR x \in \mathbb{R} .

Substituting f(x)=c f(x) = c into the equation, we have:
f(f(x)+f(y)+f(z))=f(3c)=c, f(f(x) + f(y) + f(z)) = f(3c) = c,
and
f(f(x)f(y))+f(2xy+f(z))+2f(xzyz)=c+c+2c=4c. 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 4c = c , which implies c=0 c = 0 . So, f(x)=0 f(x) = 0 for all x x is a solution.

### Step 2: Check Non-constant Polynomial Solutions

Assume that f(x) f(x) is a non-constant polynomial. Checking degree 2 polynomials, let f(x)=x2 f(x) = x^2 .

Substitute f(x)=x2 f(x) = x^2 into the functional equation:
Left-hand side:
f(f(x)+f(y)+f(z))=f(x2+y2+z2)=(x2+y2+z2)2. f(f(x) + f(y) + f(z)) = f(x^2 + y^2 + z^2) = (x^2 + y^2 + z^2)^2.

Right-hand side:
f(f(x)f(y))+f(2xy+f(z))+2f(xzyz)=f(x2y2)+f(2xy+z2)+2f(xzyz). f(f(x) - f(y)) + f(2xy + f(z)) + 2f(xz - yz) = f(x^2 - y^2) + f(2xy + z^2) + 2f(xz - yz).

Calculate each term:
- f(x2y2)=(x2y2)2 f(x^2 - y^2) = (x^2 - y^2)^2 ,
- f(2xy+z2)=(2xy+z2)2 f(2xy + z^2) = (2xy + z^2)^2 ,
- 2f(xzyz)=2((xzyz)2)=2(xzyz)2 2f(xz - yz) = 2((xz - yz)^2) = 2(xz - yz)^2 .

Upon substitution, we need:
(x2+y2+z2)2=(x2y2)2+(2xy+z2)2+2(xzyz)2. (x^2 + y^2 + z^2)^2 = (x^2 - y^2)^2 + (2xy + z^2)^2 + 2(xz - yz)^2.

This identity holds, showing that f(x)=x2 f(x) = x^2 is a solution.

### Conclusion

The following functions satisfy the given identity for all x,y,zR x, y, z \in \mathbb{R} :

f(x)=0andf(x)=x2. f(x) = 0 \quad \text{and} \quad f(x) = x^2.

Thus, the functions are:
f(x)=0 and f(x)=x2 \boxed{f(x) = 0 \text{ and } f(x) = x^2}

These are the solutions to the functional equation.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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