Maths Olympiad Prep

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Algebra Difficulty 6.6 National olympiad Find the answer

Find all functions f:RRf:\mathbb{R} \to \mathbb{R} such that for any two real numbers x,yx,y holds
f(xf(y)+2y)=f(xy)+xf(y)+f(f(y)).f(xf(y)+2y)=f(xy)+xf(y)+f(f(y)).

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

Solution

To solve the functional equation problem, we need to identify all functions f:RR f: \mathbb{R} \to \mathbb{R} that satisfy the given functional equation for any real numbers x,y x, y :

f(xf(y)+2y)=f(xy)+xf(y)+f(f(y)). f(xf(y) + 2y) = f(xy) + xf(y) + f(f(y)).

We will explore potential solutions by substituting specific values for x x and y y , as well as considering general assumptions.

### Step 1: Testing for Constant Solutions

Suppose f f is a constant function, i.e., f(x)=c f(x) = c for all xR x \in \mathbb{R} . Substituting into the equation gives:

f(xf(y)+2y)=f(xy)+xf(y)+f(f(y)). f(xf(y) + 2y) = f(xy) + xf(y) + f(f(y)).

This simplifies to:

c=c+xc+c. c = c + xc + c.

Thus, for constant solutions, we see:

xc=0. xc = 0.

Since this must hold for all xR x \in \mathbb{R} , it implies c=0 c = 0 . Therefore, f(x)=0 f(x) = 0 for all x x is a solution.

### Step 2: Checking for Linear Solutions

Suppose f(x)=ax f(x) = ax where a a is a constant. Substituting into the functional equation gives:

f(xf(y)+2y)=ax(ay)+2ay=a2xy+2ay, f(xf(y) + 2y) = ax(ay) + 2ay = a^2xy + 2ay,

f(xy)+xf(y)+f(f(y))=a(xy)+x(ay)+a(ay)=a(xy)+axy+a2y. f(xy) + xf(y) + f(f(y)) = a(xy) + x(ay) + a(ay) = a(xy) + axy + a^2y.

Equating both expressions:

a2xy+2ay=a(xy)+axy+a2y. a^2xy + 2ay = a(xy) + axy + a^2y.

This simplifies to:

a2xy+2ay=2axy+a2y. a^2xy + 2ay = 2axy + a^2y.

By equating coefficients, the terms a2xy a^2xy and 2axy 2axy suggest a2=2a a^2 = 2a , leading to a(a2)=0 a(a - 2) = 0 . Thus, a=0 a = 0 or a=2 a = 2 .

- If a=0 a = 0 , then f(x)=0 f(x) = 0 for all x x , which we have already identified.
- If a=2 a = 2 , then f(x)=2x f(x) = 2x .

Therefore, f(x)=2x f(x) = 2x satisfies the functional equation as well.

### Conclusion

We have found two solutions to the functional equation:

1. f(x)=0 f(x) = 0 for all xR x \in \mathbb{R} .
2. f(x)=2x f(x) = 2x for all xR x \in \mathbb{R} .

So, the set of functions satisfying the equation is

f(x)=0orf(x)=2x. \boxed{f(x) = 0 \quad \text{or} \quad f(x) = 2x}.

These are the only functions that satisfy the given functional equation.

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