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Algebra Difficulty 8.3 Shortlist Find the answer

Let R\mathbb R be the set of real numbers. Determine all functions f:RRf:\mathbb R\to\mathbb R that satisfy the equationf(x+f(x+y))+f(xy)=x+f(x+y)+yf(x)f(x+f(x+y))+f(xy)=x+f(x+y)+yf(x)for all real numbers xx and yy.

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A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

To solve the functional equation:

f(x+f(x+y))+f(xy)=x+f(x+y)+yf(x) f(x + f(x+y)) + f(xy) = x + f(x+y) + yf(x)

for all x,yR x, y \in \mathbb{R} , we start by considering particular values for x x and y y to simplify the equation and gain insight into the form of the function f f .

### Step 1: Substitute y=0 y = 0

Let y=0 y = 0 . The equation becomes:
f(x+f(x))+f(0)=x+f(x) f(x + f(x)) + f(0) = x + f(x)

### Step 2: Substitute x=0 x = 0

Let x=0 x = 0 . The equation becomes:
f(f(y))+f(0)=f(y) f(f(y)) + f(0) = f(y)

### Step 3: Simplifying with substitutions

From the equation in Step 2, we can rearrange it as:
f(f(y))=f(y)f(0) f(f(y)) = f(y) - f(0)

Now, let's analyze the behavior of f(x) f(x) given different hypothetical forms:

#### Case 1: Assume f f is linear of the form f(x)=ax+b f(x) = ax + b .

Substitute f(x)=ax+b f(x) = ax + b into the original equation:
a(x+ax+ay+b)+b+axy+b=x+ay+b+(a+b)y a(x+ax+ay+b)+b + axy + b = x + ay + b + (a+b)y

Equating both sides for all x,y x, y , we get:
- Coefficient of xy xy gives: a2=a a^2 = a implying a=0 a = 0 or a=1 a = 1 .
- Coefficient of x x : a2+a=1 a^2 + a = 1
- Constant terms and linear terms need to match.

#### Subcase 1.1: a=1 a = 1

If f(x)=x+b f(x) = x + b , substituting back gives contradictions unless b=0 b = 0 , hence one solution is:
f(x)=x f(x) = x

#### Subcase 1.2: a=0 a = 0

If f(x)=b f(x) = b , substitute back to verify consistency. However, checking individual substitutions lead us to understand that non-variable forms would not satisfy the functional equation universally.

Checking specific values and transformations, we also observe:
If we assume f(x)=2x f(x) = 2 - x , substitute back:
2(x+(2(x+y)))+2xy=x+(2(x+y))+y(2x) 2 - (x + (2 - (x+y))) + 2 - xy = x + (2 - (x+y)) + y(2 - x)
Remarkably, operations simplify to show consistency as well:
2x(2xy)+2xy=x+2xy+2yyx 2 - x - (2 - x - y) + 2 - xy = x + 2 - x - y + 2y - yx

Both forms f(x)=x f(x) = x and f(x)=2x f(x) = 2 - x satisfy the functional equation. Thus, these are the functional solutions.

Therefore, the solutions are:
f(x)=xandf(x)=2x \boxed{f(x) = x \quad \text{and} \quad f(x) = 2 - x}

These solutions represent all functions f:RR f : \mathbb{R} \to \mathbb{R} that satisfy the given equation for all x,yR x, y \in \mathbb{R} .

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