Paper 1, Section II, D

Analysis I | Part IA, 2012

Let ff be a continuous function from (0,1)(0,1) to (0,1)(0,1) such that f(x)<xf(x)<x for every 0<x<10<x<1. We write fnf^{n} for the nn-fold composition of ff with itself (so for example f2(x)=f(f(x)))\left.f^{2}(x)=f(f(x))\right).

(i) Prove that for every 0<x<10<x<1 we have fn(x)0f^{n}(x) \rightarrow 0 as nn \rightarrow \infty.

(ii) Must it be the case that for every ϵ>0\epsilon>0 there exists nn with the property that fn(x)<ϵf^{n}(x)<\epsilon for all 0<x<10<x<1 ? Justify your answer.

Now suppose that we remove the condition that ff be continuous.

(iii) Give an example to show that it need not be the case that for every 0<x<10<x<1 we have fn(x)0f^{n}(x) \rightarrow 0 as nn \rightarrow \infty.

(iv) Must it be the case that for some 0<x<10<x<1 we have fn(x)0f^{n}(x) \rightarrow 0 as nn \rightarrow \infty ? Justify your answer.

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