Thursday, 20 November 2014

functional analysis - Counterexample around Dini's Theorem




"Give an example of an increasing sequence (fn) of bounded continuous functions from (0,1] to R which converge pointwise but not uniformly to a bounded continuous function f and explain why Dini's Theorem does not apply in this case"





So clearly Dini's Theorem does not apply, as (0,1] is not a closed interval (or compact metric space), but I can't figure out an example.



My first thought is fn(x)=1xn, but this does not converge pointwise to a bounded continuous function, as x=1 is in the interval



My second thought is fn(x)=x1n. This is clearly an increasing sequence of bounded continuous functions (I think?) I believe this converges pointwise to f(x)=1 for all x(0,1], but I'm struggling to then show why this doesn't converge uniformly to f(x)=1



How would I do this? Or is then an easier/better example I could use?


Answer




Take fn(x)=e1nx and f=1.



Note that supx|fn(x)f(x)||fn(1n)1|=11e.


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