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Combinatorics Difficulty 6.5 National olympiad Find the answer

Let f:RR f : \mathbb{R}\to \mathbb{R} be a continuous function. Suppose that for any c>0 c > 0, the graph of f f can be moved to the graph of cf cf using only a translation or a rotation. Does this imply that f(x)=ax+b f(x) = ax+b for some real numbers a a and b b?

Solution

1. Given: f:RR f : \mathbb{R} \to \mathbb{R} is a continuous function. For any c>0 c > 0 , the graph of f f can be moved to the graph of cf cf using only a translation or a rotation.
2. Objective: Determine if this implies that f(x)=ax+b f(x) = ax + b for some real numbers a a and b b .

Let's analyze the given condition in detail.

3. Translation: Consider the translation (x,y)(xh,y) (x, y) \mapsto (x - h, y) . This means shifting the graph horizontally by h h . If the graph of f f can be translated to the graph of cf cf , then there exists some h h such that:
f(xh)=cf(x) f(x - h) = cf(x)

4. Rotation: Consider the rotation around the origin by an angle θ \theta . This means transforming the graph by rotating it. If the graph of f f can be rotated to the graph of cf cf , then there exists some θ \theta such that:
f(xcosθysinθ)=cf(xsinθ+ycosθ) f(x \cos \theta - y \sin \theta) = cf(x \sin \theta + y \cos \theta)

5. Example Function: The solution suggests that f(x)=ex f(x) = e^x also works. Let's verify this:
- For f(x)=ex f(x) = e^x , consider the translation (x,y)(xlnc,y) (x, y) \mapsto (x - \ln c, y) . This means shifting the graph horizontally by lnc \ln c . Then:
f(xlnc)=exlnc=exelnc=ex1c=1cex f(x - \ln c) = e^{x - \ln c} = e^x \cdot e^{-\ln c} = e^x \cdot \frac{1}{c} = \frac{1}{c} e^x
- To match cf(x) cf(x) , we need:
cf(x)=cex cf(x) = c e^x
- Therefore, the translation (x,y)(xlnc,y) (x, y) \mapsto (x - \ln c, y) maps the graph of f f onto the graph of cf cf .

6. Conclusion: Since f(x)=ex f(x) = e^x satisfies the given condition and is not of the form ax+b ax + b , the given condition does not necessarily imply that f(x)=ax+b f(x) = ax + b .

The final answer is False.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.