The power of Hölder Continuity

Algebra Level 4

Let f : R R f : \mathbb{R} \to \mathbb{R} and f ( 1 ) = 2016 f(1) = 2016 and f ( x ) f ( y ) ( x y ) 2016 , x , y R |f(x) - f(y)| \le (x-y)^{2016}, \forall x,y \in \mathbb{R}

For the answer: Type in f ( 2 ) f(2)


The answer is 2016.

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2 solutions

Replace x x + h x \to x + h and y x y \to x

f ( x + h ) f ( x ) ( x + h x ) 2016 f ( x + h ) f ( x ) h 2016 \Rightarrow |f(x+h) - f(x)| \leq (x + h - x)^{2016} \Rightarrow |f(x+h) -f(x)| \leq h^{2016}

h 2016 f ( x + h ) f ( x ) h 2016 \Rightarrow -h^{2016} \leq f(x+h) - f(x) \leq h^{2016}

h 2015 f ( x + h ) f ( x ) h h 2015 \Rightarrow -h^{2015} \leq \dfrac{f(x+h) - f(x)}{h} \leq h^{2015}

lim h 0 h 2015 lim h 0 f ( x + h ) f ( x ) h lim h 0 h 2015 \Rightarrow \displaystyle \lim_{h\to 0} -h^{2015} \leq \lim_{h \to 0} \dfrac{f(x+h) -f(x)}{h} \leq \lim_{h\to 0} h^{2015}

0 f ( x ) 0 f ( x ) = 0 \Rightarrow 0 \leq f'(x) \leq 0 \Rightarrow f'(x) = 0

Hence f ( x ) f(x) is a constant function.

f ( 1 ) = f ( 2 ) = 2016 \Rightarrow f(1) = f(2) = \boxed{2016}

Arturo Presa
Jun 16, 2019

It is easy to see that f ( 2 ) f ( 1 ) i = 1 n f ( 1 + i n ) f ( 1 + i 1 n ) i = 1 n ( 1 n ) 2016 = 1 n 2015 , |f(2)-f(1)|\leq \sum_{i=1}^n|f(1+\frac{i}{n})-f(1+\frac{i-1}{n})|\leq \sum_{i=1}^n (\frac{1}{n})^{2016}=\frac{1}{n^{2015}}, for any natural number n . n. Then make n . n\rightarrow \infty. So, we get that f ( 2 ) f ( 1 ) 0. |f(2)-f(1)|\leq 0. Therefore, f ( 2 ) = f ( 1 ) = 2016 . f(2)=f(1)=\boxed{2016}.

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