Find all functions \( f(x) : \mathbb{R} \rightarrow \mathbb{R} \) which satisfy

\[f(x+y) = f(x) + f(y)+2xy.\]

Hint: If \(r\) is a rational number, what can we say about \( f(rk) \) for any \(k \)?

**Prove that these are the only possible ones.**

Note:

1. It is not sufficient to just find a family of solutions.

2. You may not assume that \( f(x) \) is continuous or differentiable.

3. There is more than 1 function that satisfies those conditions.

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TopNewestFor any additive function \(h(x)\) the function \(f(x)=h(x)+x^2\) satisfies the equation. So there can be infinitely many wild solutions without additional constraints. – Jubayer Nirjhor · 2 years, 1 month ago

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\[ f(x)= r_1 + 2r_2 + x ^2 \]

Such a function is neither differentiable, nor continuous. – Calvin Lin Staff · 2 years, 1 month ago

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By definition, \[f(r(k+1))=f(kr)+f(r)+2r^{2}k\quad---(1)\]

Through pattern recognition of \({f(kr)}_{k=2}^{k=5} \) in terms of \(f(r)\), it seems to follow the relation:

\[f(kr)=kf(r)+k(k-1)r^{2} \quad --- (2)\] of which is directly derived from \((1)\)

If this is the only solution, \(f(n)\) has to have only \(1\) value, where \(n\) is any real number, which is dependent on the definition of the question. For instance, in the inspiration question, \(f(1)\) is defined to be \(4\) and only \(4\).

Is this complete? – Julian Poon · 2 years, 1 month ago

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– Calvin Lin Staff · 2 years, 1 month ago

What is the value of \( f ( \pi ) \), if \( f(1) = 4 \)?Log in to reply

For a general case, if given \(f(r)\), \(f(x)\) can be found if it is continuous.

Using Chew-Seong Cheong's method,

\[f(x+r)=f(x)+f(r)+2rx\\ f(x+r)-f(x)=f(r)+2rx\]

So, \[f(x+r)=\sum _{ k=1 }^{ \frac { x }{ r } }{ (f(r)+2k{ r }^{ 2 }) } +f(r)\]

Therefore, \[f(x+r)=\frac { xf(r) }{ r } +x\left( x+r \right) +f(r)\\ ={ x }^{ 2 }+\left( r+\frac { f(r) }{ r } \right) x+f(r)\\ ={ (x+r) }^{ 2 }+\left( \frac { f(r) }{ r } -r \right) \left( x+r \right) \]

\[\boxed{f(x)={ x }^{ 2 }+\left( \frac { f(r) }{ r } -r \right) x}\] – Julian Poon · 2 years, 1 month ago

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However, since your argument never uses the condition that the function is continuous, hence it is flawed. The error is that you made the assumption that \( \frac{x}{r} \) is an integer, otherwise your summation is meaningless. It could be adjusted to the case where \( \frac{x}{r} \) is a rational number, but cannot apply to the case of irrational numbers. In particular, we don't know what \( f(\pi ) \) is.

So, how do we use the condition of continuity (but not differentiability) to prove it? – Calvin Lin Staff · 2 years, 1 month ago

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– Julian Poon · 2 years, 1 month ago

I don't know... Any clues? :DLog in to reply

Hint:What is \( f(3), f(3.1), f(3.14), f(3.141), f(3.1415), f(3.14159), ... \)? – Calvin Lin Staff · 2 years, 1 month agoLog in to reply

– Julian Poon · 2 years, 1 month ago

So we just have to approximate as we get closer and closer to \(\pi\)? Probably finding the upper and lower boundaries?Log in to reply

If a function is continuous, then \( f( \pi ) = \lim f(x_i) \) for any series of points that converge to \( \pi \). We can pick \( \( x_i = 10^{-i} \lfloor 10^i \pi \rfloor \) as I did above.

The idea of upper and lower boundaries would apply for "increasing functions", which doesn't require the assumption of continuity. In this case, we have \[ f(3) \leq f(3.1) \leq f(3.14) \ldots \leq f( \pi ) \leq \ldots f(3.15) \leq f(3.2) \leq f(4) . \]

Because the inner inequalities converge to each other, we get the result (without assuming continuity).

The take home is that for such functional equations, you have to be careful to work with exactly what you are given, instead of adding additional assumptions because it makes your working simpler.

@Abhishek Sharma See the above and it's relevance to "assume function extends to real numbers and is differentiable". – Calvin Lin Staff · 2 years, 1 month ago

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