# Functional Equation: When $f(x+y)=f(x)+f(y)-(xy-1)^2$

How does one solve the following functional equation when $f:\mathbb{R}\rightarrow\mathbb{R}$

$$f(x+y)=f(x)+f(y)-(xy-1)^2$$

When I assumed it was a polynomial equation, it can be seen through induction that $$f(nx)=nf(x)-\sum _{ i=1 }^{ n-1 }{ (ix^2-1)^2 }$$

for some natural number $n$.

This implies that $$f(n)-nf(1)+\sum _{ i=1 }^{ n-1 }{ (i-1)^2 }=0$$ is true for all natural $n$, or that for all $n$ $$f(n)-nf(1)+\frac{(n-2)(n-1)(2n-3)}{6}=0$$

Since there are infinite number of solutions to $f(n)-nf(1)+\frac{(n-2)(n-1)(2n-3)}{6}=0$, it can be said that $$f(n)=nf(1)-\frac{(n-2)(n-1)(2n-3)}{6}$$ for all real number $n$.

This implies that $f(n)$ is of the degree $3$, but this is a contradiction since if $f(n)$ had a degree of $3$, the coefficient for $x^2y^2$ would be $0$.

So there appeared to be no polynomial solutions to this function. I further thought that there would be no functions that satisfied this equation.

How does one solve this equation? Any help would be appreciated.

• You can take a look at a generalized version of your question here. :) – H. R. Jan 28 '16 at 21:18

$$f(x+y)=f(x)+f(y)-(xy-1)^2\\ f(2)=2f(1)\\ f(3)=f(2)+f(1)-1=3f(1)-1\\ f(2)+f(2)-9=f(4)=f(1)+f(3)-4\\ 4f(1)-9=4f(1)-5$$

• I get two different expressions from $4=2+2$ and $4=3+1$, not from rearranging earlier lines. – Empy2 Jan 28 '16 at 15:08
• Conceptually, first let $y=1$ and solve the equation for $f\vert_{\mathbb N}$, then restrict the original equation to $x,y\in\mathbb N$, which is strong enough to lead to a contradiction. – Yai0Phah Jan 28 '16 at 15:17

First put $x=y=0$ into the functional equation to obtain that

$$f(0)=1 \tag{1}$$

Then suppose that $y \ne 0$ so dividing the functional equation by $y$ we can write

$$\frac{f(x+y)-f(x)}{y}=\frac{f(y)-1}{y}-\frac{(xy-1)^2-1}{y} \tag{2}$$

$$\frac{f(x+y)-f(x)}{y}=\frac{f(y)-f(0)}{y-0}-\frac{x^2y^2-2xy}{y} \tag{3}$$

Next, take the limit when $y \to 0$ to obtain

$$f'(x)=f'(0)+2x \tag{4}$$

Where we have assumed that $f(x)$ is differentiable over $\mathbb{R}$. Next, integrating $(4)$ we get

$$f(x)-f(0)=f'(0)x+x^2 \tag{5}$$

Or equivalently

$$f(x)=x^2+f'(0)x+1 \tag{6}$$

But we can observe that $(6)$ will never satisfy the functional equation since $f(x)$ is a second degree polynomial and there are fourth degree terms like $x^2y^2$ in the functional equation . So the only thing that we are left with is that

There is no differentiable $f(x)$ which satisfies the functional equation!

• Are we sure there's a limit when ${y \to 1}$ ? – peter.petrov Jan 28 '16 at 14:50
• As the OP said, you need degree at least 4 to produce $x^2y^2$ – Empy2 Jan 28 '16 at 14:51
• @peter.petrov: It is a assumption in this solution. I can say that in these types of questions it is a common assumption. :) – H. R. Jan 28 '16 at 14:52
• ignore my earlier comment, looks good. This is a proof that $f$ can't be differentiable, then? – hunter Jan 28 '16 at 14:54
• This answer assumes $f(x)$ is differentiable. You should at least mention it. The original question doesn't mention it. – user236182 Jan 28 '16 at 15:01