Let f : R → R and f ( 1 ) = 2 0 1 6 and ∣ f ( x ) − f ( y ) ∣ ≤ ( x − y ) 2 0 1 6 , ∀ x , y ∈ R
For the answer: Type in f ( 2 )
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It is easy to see that ∣ f ( 2 ) − f ( 1 ) ∣ ≤ ∑ i = 1 n ∣ f ( 1 + n i ) − f ( 1 + n i − 1 ) ∣ ≤ ∑ i = 1 n ( n 1 ) 2 0 1 6 = n 2 0 1 5 1 , for any natural number n . Then make n → ∞ . So, we get that ∣ f ( 2 ) − f ( 1 ) ∣ ≤ 0 . Therefore, f ( 2 ) = f ( 1 ) = 2 0 1 6 .
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Replace x → x + h and y → x
⇒ ∣ f ( x + h ) − f ( x ) ∣ ≤ ( x + h − x ) 2 0 1 6 ⇒ ∣ f ( x + h ) − f ( x ) ∣ ≤ h 2 0 1 6
⇒ − h 2 0 1 6 ≤ f ( x + h ) − f ( x ) ≤ h 2 0 1 6
⇒ − h 2 0 1 5 ≤ h f ( x + h ) − f ( x ) ≤ h 2 0 1 5
⇒ h → 0 lim − h 2 0 1 5 ≤ h → 0 lim h f ( x + h ) − f ( x ) ≤ h → 0 lim h 2 0 1 5
⇒ 0 ≤ f ′ ( x ) ≤ 0 ⇒ f ′ ( x ) = 0
Hence f ( x ) is a constant function.
⇒ f ( 1 ) = f ( 2 ) = 2 0 1 6