Tuesday, June 7, 2016

real analysis - Suppose that the function f:[0,1]rightarrowmathbbR is continuous and that fleft(xright)>2



Suppose that the function f:[0,1]R is continuous and that f(x)>2 if 0x<1. Is it necessarily true that f(1)>2?



My attempt:



Yes, using the sequence definition of continuous.




Since f is continuous at x=1, so we can take xn1 as n, then f(xn)f(1) as n.



Furthermore, f(xn)>2 xn where 0xn<1.



f(1)>2.



Does this make sense?


Answer



You need to be careful with the distinction between and >. Specifically, just because a sequence has values that are >2, it doesn't mean that the limit is >2, just that it's 2.




In your case, f(x)=3x fulfills the criteria, but f(1)=2.


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