Find the antiderivative of f Let S = <mo fence="false" stretchy="false">{ 0 <mo fen

Dashawn Clark

Dashawn Clark

Answered question

2022-05-03

Find the antiderivative of f
Let S = { 0 } { 1 n : n N } and define f : R R
a) Prove that | f ( x ) f ( y ) | | x y | for any x , y R .
b) Find the antiderivatives of f
I obtained the following form for the function:
f ( x ) = { x , x 0 x 1 n + 1 , 1 n + 1 x 1 2 ( 1 n + 1 + 1 n ) 1 n x , 1 2 ( 1 n + 1 + 1 n ) < x < 1 n x 1 , x 1
I then tried to prove the inequality from a) considering many cases for x and y, but there are many possibilities. For the antiderivatives, I tried to combine the antiderivatives for each of the 4 branches of the function, but again, the calculations are messy.

Answer & Explanation

Leia Wiggins

Leia Wiggins

Beginner2022-05-04Added 18 answers

Step 1
Let S be a subset of R .
1. For any ϵ > 0 there is s S such that | x s | f ( x ) + ϵ. We know that f ( y ) | y s | = | y x + x s | | y x | + f ( x ) + ϵ.
Since this holds for any ϵ > 0 we have f ( y ) f ( x ) + | y x | .
Step 2
2. For any δ > 0 there is t S such that | y t | f ( y ) + δ. Analogously, f ( x ) | x t | = | x y + y t | | x y | + f ( y ) + δ.
Since this holds for any δ > 0 we have f ( x ) f ( y ) + | y x | .
Step 3
3. Thus we have f ( x ) f ( y ) | x y | f ( y ) f ( x ) | y x | .
Thus | f ( y ) f ( x ) | | y x | .
No let S be defined as in your example and you are done.

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